Download - J P Morniroli
-
7/30/2019 J P Morniroli
1/20
Journal of Microscopy, Vol. 232, Pt 1 20 08, pp. 726
Received 12 December 2007; accepted 22 February 2008
Contribution of electron precession to the study of perovskites
displaying small symmetry departures from the ideal cubic ABO3
perovskite: applications to the LaGaO3 and LSGM perovskites
J . - P . M O R N I R O L I
, G . J . A U C H T E R L O NI E, J . D R E N N A N& J . Z O U
Laboratoire de Metallurgie Physique et Genie des Materiaux, UMR CNRS 8517, USTL, ENSCL,
Batiment C6, Cite Scientifique, 59500 Villeneuve dAscq, France
Centre for Microscopy and Microanalysis
School of Engineering, The University of Queensland, 4072, Queensland, Australia
Key words. Electron precession, LaGaO3, lanthanum gallate, LSGM crystal
structure, perovskite.
Summary
Electron microscopy and electron diffraction are well adapted
to the study of the fine-grained, faulted pure and doped
LaGaO3 and LSGM perovskites in which the latter is useful for
fuel cell components. Because these perovskites display small
symmetry departures from an ideal cubic ABO3 perovskite,
many conventional electron diffraction patterns look similar
and cannot be indexed without ambiguity.Electronprecession
can easily overcome this difficulty mainly because theintensity of the diffracted beams on the precession patterns
is integrated over a large deviation domain around the exact
Bragg condition. This integrated intensity can be trusted and
taken into account to identify the ideal symmetry of the
precession patterns (the symmetry which takes into account
both the position and the intensity of the diffracted beams).
In the present case of the LaGaO3 and LSGM perovskites,
the determination of the ideal symmetry of the precession
patterns is based on the observation of weak superlattice
reflections typical of the symmetry departures. It allows an
easy and sure identification of any zone axes as well as the
correct attribution of hkl indices to each of the diffracted
beams. Examples of applications of this analysis to the
characterizations of twins andto theidentificationof thespace
groups are given. This contribution of electron precession can
be easily extended to any other perovskites or to any crystals
displaying small symmetry departures.
Correspondence to: J.-P. Morniroli. Tel: 33320436937; fax: 33320434040;
e-mail: [email protected]
Introduction
There are many new materials, which are either in the
market place or under development, that are based upon
high-temperature electrochemical processes. Some examples
of these materials are batteries (Kao et al., 1992), catalysts
(Lahousse et al., 1998), epitaxial substrates (Christen et al.,
1997), gas separation membranes (Balachandran et al.,
1998), high-temperature electrodes (Kawada et al., 2006),
oxygen pumps (Yuan & Kroger, 1969), oxygen sensors(Weissbart & Ruka, 1961), radioactive waste containment
(Ringwood etal.,1979)and solidoxidefuelcells(Minh, 1993).
With such diverse categories of materials and applications
one can see the importance of these materials to our modern
technological society (Stlen et al., 2006). Some of these new
materials are based upon the ABO3 perovskite-type structure
whereseveralinnertransition(rareearth)elementgallatesare
isostructural with the archetypal ABO3 perovskite, GdFeO3.
With A- and/or B-site doping, these perovskite-based oxides
transform from insulators into conductors. Rather than being
electron conductorsthese oxides are eithermixed electron/ion
conductors orion conductors. Because ions aremuchlarger in
sizethanelectrons,their migrationpathwaysbecome far more
critical because these ions must jostle past cations in a solid
lattice. It is therefore important to study the basic migration
mechanisms for electronic, ionic and mixed conduction in
these complex ABO3 perovskite-type oxides.
The ABO3 perovskite-type structure is both geometrically
andchemicallystablegivingittheadvantageofgreatchemical
flexibility. That is, the ability of the lattice to accommodate
many different types of dopant atoms with both wide-ranging
atomic sizes and chemical valencies. Thus, in these doped
ABO3 perovskites one must correctly identify the crystal
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society
-
7/30/2019 J P Morniroli
2/20
8 J . - P . M O R N I R O L I E T A L .
structure at bothroomand operatingtemperatures, especially
as at high temperatures, phase transitions occur which yield
the desired operating properties of these materials. Once the
crystal structure has been correctly identified, one can then
hopefully determine the anion conduction pathway through
the solid.The structure determination of these compounds using
X-ray patterns has been used routinely for many years;
for example see Wang et al. (1991). It indicates that some
perovskites adopt an ideal cubic aristotype structure but most
of them exhibit some small departures from this ideal cubic
symmetry leading to a lower symmetry to form orthorhombic,
rhombohedral, tetragonal or monoclinic structures.
However, as research moves into the nanoworld,
small and/or heavily faulted perovskite crystals are often
encountered that cannot be studied by X-ray or neutron
diffraction. The electron microscope is thus well adapted, not
only for imaging these fine-grained materials but also for
identifying or characterizing their crystallographic structureby means of electron diffraction. A major difficulty is then
encountered due to this symmetry lowering: that is many
zone axis diffraction patterns are very similar to each other
and cannot be indexed without ambiguity with conventional
electron diffraction. Nevertheless, the correct identification of
a zone axis pattern (ZAP) as well as the correct attribution
of hkl indices to each diffracted beam are required in many
material science fields, for example the characterization of
crystal defects (stacking faults, dislocations,twins. . .).Itisalso
required in electron crystallography in order to identify many
crystallographic features (the crystal system, the Bravais
lattice, the Laue class and the point and space groups).One solution to overcome this difficulty consists in
using convergent-beam electron diffraction or large-angle
convergent-beam electron diffraction which gives more
accurate andusefullinepatternsbutthesepatternscanbe very
complex and their interpretation is usually tedious and time
consuming. In addition, these methods require a specimen
of optimal thickness and, for the large-angle convergent-
beam electron diffraction technique, relatively large and non-
distorted crystals (Morniroli, 2002a). Recently, the electron
precession method, which was proposed by (Vincent &
Midgley, 1994) became commercially available and can be
fitted on most modern transmission electron microscopes. In
this technique (Fig. 1a), a parallel or a nearly parallel incident
beamisrapidlyrotatedbymeansofthepre-specimendeflection
coils of the microscope on the surface of a hollow cone whose
axis is directed along the optical axis and whose semi-angle
, in the range 0 to 3, is the precession angle. When a
[uvw] zone axis of the studied crystal is set as close as possible
along the optical axis, the diffraction pattern located in the
back focal plane is made of circles, which depending on the
precession angle, are more or less superimposed (Fig. 1b).
The transmitted circle gives the transmitted intensity as a
function of the orientation of the incident beam during the
precession movement whereas each hkl diffracted circle gives
the diffracted intensity as a function of the orientation of the
incident beam with respect to the corresponding (hkl) lattice
planes.
This circle pattern is transformed into a spot pattern by
means of the post-specimen deflection coils which act in asynchronizedway andin opposite directionwith respect to the
pre-specimen deflection coils. The precession patterns, thus
observed on the microscope screen (Fig. 1c) look similar to
selected-areaelectrondiffractionpatternsbuttheydisplayfour
main advantages:
(1)The diffracted intensity of each hkl spot is integrated over
thecorresponding hklcircle present in theback focal plane,
i.e. along a large deviation domain on each side of the
exact Bragg conditions. As a matter of fact, this integrated
intensity is directly connected with the area located under
the rocking curve (the diffracted intensity as a function
of the deviation from the exact Bragg condition). As a
result, the integrated intensities are not very sensitiveto a slight crystal misorientation and a [uvw] zone axis
precession pattern always looks well aligned even if the
[uvw] crystal zone axis is not exactly located parallel with
the incident beam. This also means that the intensities of
the diffracted beams can be taken into account and trusted
when analysing a precession pattern. Thus, the ideal
symmetry of the precession patterns becomes available.
This symmetry takes into account both the position and
the intensity of the diffraction beams present on a pattern.
It can be observed for both the zero-order Laue zone
(ZOLZ) and the whole pattern (WP)1 reflections and it
is connected with the Laue class (Morniroli & Steeds,1992). As will be illustrated in Section 4, very small
differences of intensities can be observed on precession
patterns, which can be used to identify the presence or
the absence of symmetry elements and to surely infer this
ideal symmetry. It is a crucial and critically important
advantagewith respect to selected area-electron diffraction
patterns or microdiffraction patterns whose diffracted
intensities are extremely sensitive to slight misorientations
and usually only provide the net symmetry (the
symmetry which only considers the position of the
reflections).
(2)The precession patterns display a larger number of
reflections in the ZOLZ and in the high-order Laue zones
(HOLZs) than a selected-area diffraction pattern and
this number increases with an increase in precession
angle. This important property is very useful in electron
crystallography (Morniroli & Redjaimia, 2007; Morniroli
et al., 2007).
1AWPdisplaystheZOLZandatleastoneHOLZ.Throughoutthispaper,thenotations
for the ZOLZ and WP ideal symmetries will be in accordance with (Morniroli &
Steeds, 1992); they are underlined and given between parentheses for the ZOLZ.
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
3/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 9
Fig. 1. Schematic description of the electron precession technique. (a) Electron ray-path in the column of a transmission electron microscope. For the
sake of simplicity, only one hkl diffracted beam is drawn and the column contains only one intermediate lens and no projector lens. (b) Circle pattern
observed in the back focal plane of the objective lens. (c) Spot pattern observed on the microscope screen.
(3)During the precession movement, the incident beam is
never aligned along the zone axis where the strongest
dynamical interactions occur. Thus, the precession
patterns are less dynamical than the conventional
diffraction patterns.
(4)A few-beam behaviour is encountered with large
precession angles. Because the multiple diffraction paths
to the forbidden reflections are unlikely to occur when
this few-beam behaviour prevails, forbidden reflections
disappear or become very weak on large-angle precession
patterns. This property allows the identification of the
kinematical forbidden reflections (Morniroli & Redjaimia,
2007).
The purpose of this paper is to demonstrate that these
remarkable and useful precession features can be used
to identify, without ambiguity, any [uvw] zone axes and
to assign the correct hkl indices to the diffracted beams
of perovskite crystals displaying some small symmetry
departures. To illustrate this possibility, LaGaO3 perovskite
specimens were selected and observed. LaGaO3 perovskite is a
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
4/20
1 0 J . - P . M O R N I R O L I E T A L .
Fig. 2. Description of the ideal cubic (a) and orthorhombic LaGaO3 (b) perovskites.
good example because both its room and high-temperatures
crystal structures exhibit a small departure from a cubic
symmetry to an orthorhombic and to a rhombohedral
symmetry, respectively. Some applications to the Sr and Mgdoped LaGaO3 perovskites (LSGM perovskites) will also be
given.
Description of the room temperature LaGaO3 crystal
structure
The ABO3 perovskite-type oxides consist of a trivalent
transition ion located in a A-site coupled with a trivalent
transition metal ion located in a B-site, which forms a BO 6octahedra (Glazer, 1972; Fig. 2a).
The arrangement of the BO6 octahedra in an ideal cubic
perovskite is shown in Fig. 2a. As reported by Marti et al.(1994), Slater etal. (1998)and Lerch etal.(2001),intheroom
temperature LaGaO3 perovskite, these octahedra are slightly
tilted and rotated (Fig. 2b). The corresponding structure is
then described by an orthorhombic unit cell with the space
group Pnma.
As a matter of fact, this pseudo-cubic LaGaO 3 perovskite
displays some very small symmetry departures from the ideal
cubicperovskite, whichmeans that its diffraction patterns will
be very close to cubic. To quantify this aspect, let us consider
a zone axis form of the ideal cubic perovskite. In the
general case,observedwhen u=v=wandnon-zero,thisform
contains48equivalent[uvw]directions(directionshavingthe
same parameter P[uvw] but different orientations because, in
the cubic crystal system, P[uvw] is not modified if the u, v, and
w signs are positive or negative and if the u, v and w indices
areinterchanged). Letus consider the case of the zone
axes. In the case of the Laue class m3m, it gives two types
of non-superimposable and mirror-related diffraction patterns
labelledA andB inFig. 3 where,forthesake of
simplicity, only the ZOLZ reflections are shown.
With the orthorhombic perovskite, the situation becomes
more complex because the 48 equivalent cubic
directions are transformed into six orthorhombic zone axis
forms : , , , , and
each of them containing eight equivalent directions
(in the orthorhombic crystal system, the parameter P[uvw] is
not modified if the u, v and w signs are positive or negative butthe u, v and w indices cannot be interchanged). As a result,
in the Laue class mmm, two sets of six different and mirror-
related diffraction patterns are obtained as shown in Fig. 3.
They differ from the cubic patterns by the presence of
weak extra reflections (to be more visible, they are magnified
10 times in Fig. 3),whichare located at twodifferent positions:
(1)In the middle of the small edge of the parallelograms
drawn with respect to the cubic reflections for the,
, and zone axis forms. Note that
the intensity of the extra reflections is different and typical
in each of these four patterns
(2)In the middle of both sides of the parallelograms as well
in the middle of the parallelograms for the two
and zone axis forms. The intensity of the extra
reflections is also typical of the zone axis.
How to correctly identify these patterns?
With conventional electron diffraction (selected-area electron
diffraction or microdiffraction), the diffracted intensities
are too strongly modified by dynamical effects (multiple
diffraction) and/or by thickness variations and crystal
misorientationsinthediffractedarea,sothatonlythepositions
of the reflections and the net symmetry can be trusted. This
means that a zone axis cannot be surely identified among the
four , , and or between the
two and zone axis forms.
This is no longer the case with electron precession because
the intensity and the ideal symmetry can be taken into
account. Thus, the observation of the intensity of these typical
additional reflections is the basis of the zone axis identification
described in the present paper.
To this aim, we describe, the pseudo-cubic LaGaO3perovskite with respect to the ideal cubic perovskite and,
in analogy with the ordered structures commonly observed
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
5/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 1 1
Fig. 3. Description of the diffraction patterns produced by: the 48 equivalent zone axes from the ideal cubic perovskite the corresponding
,,,, and zone axes from the orthorhombicperovskite.For the sake of simplicity,only theZOLZreflections
are shown. To increase their visibility, the weak extra reflections are magnified 10 times.
in metal alloys, we consider the corresponding diffraction
patterns as made of fundamental reflections common to
the cubic and pseudo-cubic perovskites and of superlattice
reflections typical of the new periodicities and thus typical ofthe pseudo-cubic perovskite.
Figure 4 illustrates this analogy. In order to make a direct
comparison between the ideal and pseudo-cubic perovskites
and to describe the ZAPs with the same [uvw] indices, both
structures must be described by means of comparable unit
cells.For this reason, theidealcubicperovskite is notdescribed
by its conventional primitive cubic unit cell but by means
of a multiple tetragonal unit cell (Fig. 4a) whose contour
(Fig. 4b and c) is close to the orthorhombic LaGaO 3 unit cell
(Fig. 4a, b and c). The corresponding reciprocal lattices are
alsogivenin Fig.4d and d. Both reciprocal lattices display the
samefundamentalreciprocal nodeswhereas the pseudo-cubic
one displays additional superlattice nodes, which occur when
h+ l is odd or when h+ l is even and k odd.Throughout the present text, the subscripts c, t, and o will
refer to the cubic, multiple tetragonal and orthorhombic unit
cells, respectively.
In Fig. 5, the main zone axis diffraction patterns
(those which correspond to the six c, twelve cand eight c ZAPs of the ideal cubic perovskite) are
displayed and arranged on a stereographic projection so that
their mutual orientations are preserved. These patterns were
kinematically simulated by means of the electron diffraction
software (Morniroli, 2002b). The patterns fromthe idealcubic
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
6/20
1 2 J . - P . M O R N I R O L I E T A L .
Fig. 4. Comparative descriptionof the ideal cubicand pseudo-cubic LaGaO3 perovskites. a, b, c, d Projection of thestructure (a), projection of thedirect
lattice (b), directlattice(c) andreciprocal lattice (d)of theidealcubic perovskite. a, b, c, d Projection of the structure (a), projection of thedirect lattice
(b), direct lattice (c) and reciprocal lattice (d) of the orthorhombic LaGaO3 perovskite.
perovskite (Fig. 5a) only display fundamental reflections.
One notices the absence of kinematical forbidden reflections
because the corresponding space group Pm3m does notcontain anyscrew axes orglideplanes. Asexpected, most ofthe
simulatedpatterns fromthe pseudo-cubic perovskites (Fig.5b)
display weak additional superlattice reflections. For the sake
of clarity, the intensity of these extra reflections is exaggerated
(their diameter is magnified 10 times). Some couples of
superlatticereflectionslocatedoneachsideofthemirrorsofthe
ideal cubic perovskite display a typical difference in intensity
connectedwith theloweringof symmetry (see, e.g. thecouples
of reflections (arrowed) on the [111]o, [111]o, [111]o and
[111]o ZAPs and located on each side of the pseudo-mirrors
m1 andm2). Some patterns also display kinematical forbidden
reflections connected with the glide planes and screw axes
of the Pnma space group of the orthorhombic perovskite.
With conventional electron diffraction (selected-area electron
diffraction or microdiffraction), the experimental observation
of these typical intensity differences between some couples of
superlattice reflections as wellas the identificationof forbidden
reflections is usually impossible due to multiple diffraction
paths connected with the strong dynamical behaviour of
electron diffraction. This is possible with electron precession
because the precession patterns are less dynamical and they
display, at least with large precession angle, a few-beam
behaviour.
Experimental procedures
Sample preparation for TEM observations
LaGaO3 perovskite specimens were prepared from powders of
99.99% pure La2O3 and Ga2O3. The powders were weighed
and mixed in a ball mill for 48 h in an alcohol slurry. They
were then calcined, ball milled for another 24 h and finally
sintered at 1200C in air for 12 h.
For LSGM, the raw powders capable of adsorbing H2O and
CO2 from the atmosphere were calcined at 1000C in air
and cooled in a dry, nitrogen atmosphere prior to weighing.
Powder milling was performed in a vibratory mill for 6 h
using stabilized zirconia in propan-2-ol. Milled powders were
dried in a vacuum oven at 6080C for 1012 h, and then
passed sequentially through 300-, 150- and 75-m stainless
steel sieves. After final sieving the powders were calcined at
the required temperature (10001200C) in air and then re-
milled. After passing through the 75-m sieve, the powder
was uniaxially pressed into either bar shapes for electrical
conductivity evaluation or pellets for phase assemblage and
microstructure characterization. The green specimens were
isostatically pressed then fired at 14001450C for 15 h in
air.
The resulting LaGaO3 and LSGM powders were pressed
into pellets at 200 kPa. Crushed pellets were mixed with a
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
7/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 1 3
Fig. 5. Schematic description of the main zone axis
patterns for the ideal cubic (a) and pseudo-cubic (b)
LaGaO3 perovskites.Forthecubicunitcell,thesubscript
c refersto theconventionalcubic unitcell.The subscript
o is used for the indices of the orthorhombic LaGaO 3
unit cell. Mirrors present on the patterns are indicated
by bold lines and pseudo-mirrors by dotted lines.
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
8/20
1 4 J . - P . M O R N I R O L I E T A L .
20-m aluminium powder with a ratio of about 10%
perovskite and 90% metal powder. A few tens of grams of
this powder mixture was then placed between two flat iron
sheets and laminated with a rolling mill at a pressure near the
maximum ofthe pressin orderto obtaina metalfoilof thickness
of about 50 m in which the crushed perovskite crystals areembedded.Three millimetre discs were thenpunched from the
foil with a Gatan disc punch (Gatan, Pleasanton, CA, USA).
ThesethinfoilswereionbeamthinnedonaGatan600DuoMill
at room temperature for 2 h with both the upper and lower
Ar+ ion beams operated at 4 kV, with a 10 mA ion current
per gun and an angle of incidence of 10 both above and
below the foil, until the foil waselectron optically transparent.
These specimens have a much better quality than the crushed
specimens deposited on a carbon film where the transparent
areas are usually very small because they result from a
cleavage mechanism which produces some rapid variations
of the specimen thickness. On the contrary, the ion thinning
of the Al/perovskite foils produces relatively large electron-optically transparent areas with relatively small thickness
variations of the embedded crystals. Because aluminium is a
very malleable metal, these specimens are not brittle and can
be observed many times. In addition, aluminium is relatively
transparent to electrons meaning that there is no risk of
confusion with the studied perovskite crystals. Aluminium
is also a very good thermal conductor well adapted to high
temperature experiments up to 500C.
Transmission electron microscopy
Room temperature experiments on the perovskite sampleswere performed with a Philips CM30 TEM operated at
300 kV and equipped with the electron precession Spinning
Star equipment from Nanomegas (Brussels, Belgium). The
specimens observed in the present study are usually heavily
faulted and contain many twins. In order to avoid artefacts
due to these defects and to probe only defect free areas, the
precession patterns were obtained in microdiffraction mode.
In this mode, the electron beam is a nearly parallel electron
beam produced by a 10-m C2 condenser aperture. This beam
is focused on the specimen with a probe diameter of between
10 and 50 nm. All the precession patterns were recorded on a
1k 1k Gatan CCD camera.
An electron micrograph of a LaGaO3 grain coming from a
thin area of the sample is shown in Fig. 6. It shows typical
contrast connected with approximately 1020 nm domain
sizes.
Experimental results
All the ZAPs considered in the present study only display the
ZOLZ and therefore only give the ZOLZ ideal symmetry. The
main zone axes of the ideal cubic perovskite are described, i.e.
the c, c and c ZAPs.
Fig. 6. Typical electron micrograph of a LaGaO3 grain.
Identification of the [uvw] zone axis from precession patterns
c zone axes
As shown in the theoretically simulated patterns in
Fig. 7, the six equivalent c zone axes of the ideal cubic
perovskite with ideal (4 mm) symmetry (Fig. 7a) give, in the
orthorhombic perovskite, two different patterns:
(1)The [010]o and [010]o patterns (type A) with (2 mm)
symmetry. They display additional superlattice reflections
located in the centre of the square drawn with respect to
the fundamental reflections. Typical kinematical forbiddenreflections are also located along the two remaining
mirrors m3 and m4 (Fig. 7b).
(2)The [101]o, [101]o, [101]o and [101]o patterns (type B)
with (2 mm) symmetry where the additional reflections
are located along one of the edges of the square. Some
kinematical forbidden reflections are located along its
mirror m2 (Fig. 7c).
There is no special difficulty to identify these two A and
B types of zone axes by electron precession (Fig. 7b and
c) even with conventional electron diffraction (Fig. 7b
and c) because the position of the superlattice reflections
on both types are very different and cannot be modified
by multiple diffraction. One notices that the kinematical
forbidden reflections are clearly visible on all these patterns.
Nevertheless, we can use electron precession to detect
these forbidden reflections. As indicated in Section 1, the
kinematical forbidden reflections can be identified by using
a large precession angle (3 was found to be a good value);
with these experimental conditions, a few-beam behaviour
is observed during the precession movement of the incident
beam so that the probability of having multiple diffraction
paths to the forbidden reflections is unlikely to occur. With
these conditions, a forbidden reflection disappears or becomes
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
9/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 1 5
Fig. 7. Electron diffraction patterns of the six c ZAPs for the pseudo-cubic perovskite. They are sorted into two types of patterns A and B.
a Kinematical simulated pattern, with (4 mm) symmetry, of the six equivalent c for the ideal cubic perovskite. b, b, b [010]o and [010]o
diffraction patterns with (2 mm) symmetry (type A). Kinematical simulated patterns (b), experimental precession pattern (b) and experimental selected-
area electron diffraction pattern (b ). The forbidden reflections are visible on the experimental patterns. c, c, c [101]o, [101]o, [101]o and [101]o
diffraction patterns with (2 mm) symmetry (type B). Kinematical simulated pattern (c), experimental precession pattern (c ) and experimental selected-
area electron diffraction pattern (c ). The forbidden reflections are visible on the experimental patterns. d, d Experimental precession pattern of type A
obtained with a 1 precession angle (d) and corresponding dynamical simulation performed with jEMS (d). The kinematical forbidden reflections (circled
reflections) are visible on both patterns. e, e Experimental precession pattern of type A obtained with a 3 precession angle (e) and corresponding
dynamicalsimulation performed withj EMS (e). Thekinematical forbiddenreflectionsare invisibleon both patterns.f, f Experimental precession pattern
of type B obtained with a 1 precession angle (f) and corresponding dynamical simulation performed with jEMS (f). The kinematical forbidden reflections
(circled reflections) are visible. g, g Experimental precession pattern of type B obtained with a 3 precession angle (g) and corresponding dynamical
simulation performed with jEMS (g). The kinematical forbidden reflections are invisible. Mirrors present on the simulated patterns are indicated by bold
lines.
very weak. This is what is observed on the patterns in
Fig. 7e and g, which correspond to the patterns with types
A and B. One notices that these patterns are in excellent
agreement with the corresponding dynamical simulations
(Fig. 7d, e, f and g) performed with the jEMS software
(Stadelmann, 1987, 2007).
c zone axes
The12equivalentc zoneaxesofthecubicperovskite
(Fig.8a), withsymmetry (2 mm),give fourdifferentdiffraction
patterns (Fig. 8be) in theorthorhombic LaGaO3 perovskites:
(1)The [001]o and [001]o patterns (type C) with (2 mm)
symmetry. The superlattice reflections are located on the
small edge of the rectangle drawn with respect to the
fundamental reflections. Some forbidden reflections are
located along the m2 mirror (Fig. 8b).
(2)The [111]o, [111]o, [111]o and [1 1 1]o patterns
(type D1) with (2) symmetry. Some superlattice
reflections are located along the diagonal d1 and
some forbidden reflections along the other diagonal d2(Fig. 8c).
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
10/20
1 6 J . - P . M O R N I R O L I E T A L .
Fig. 8. Electron diffraction patterns of the twelve c ZAPs for the pseudo-cubic perovskite. They are sorted into four types of patterns C, D1, D 2
and E. a Kinematical simulated patterns of the twelve equivalent c, with (2 mm) symmetry, for the ideal cubic perovskite. b, b [001]o and
[001]o diffraction patterns with (2 mm) symmetry (type C). Kinematical simulated pattern (b) and experimental precession pattern (b). The forbiddenreflections are visible on theexperimental pattern. c [111]o, [111]o, [111]o and [1 1 1]o kinematical simulated patterns with (2) symmetry (typeD 1). d
[111]o, [111]o, [11 1]o and [111]o kinematical simulated patterns with (2) symmetry (typeD 2).e [100]o and [100]o kinematical simulatedpatterns
with (2 mm) symmetry (type E). f Experimental precession pattern performed with a 1 precession angle. It is in agreement with the types D1, D2 or E.
g, g Experimental precession patterns performed with 1(g) and 3 (g) precession angles. The absence of kinematical forbidden reflections along the
d1 diagonal in figure g (circled reflections)provesthat this pattern belongs to the D2 type. h,h
Experimental precession patterns performed with 1(g)
and 3 (g) precessionangles.This pattern belongs to the type E because no forbiddenreflectionsare identified on the 3 precession pattern. The bold lines
indicate mirrors.
(3)The [111]o, [1 11]o, [11 1]o and [1 1 1]o patterns (type
D2) with (2) symmetry. The superlattice and forbidden
reflections also are along the d1 and d2 diagonals but these
patterns are mirror related with respect to the patterns of
type D1 (Fig. 8d).(4)The [100]o and [100]o patterns (type E) with (2 mm)
symmetry. Some superlattice reflections are located along
boththe d1 and d2 diagonals. Forbidden reflections are not
present on these zone axes (Fig. 8e).
Type C patterns areeasily distinguished from thethreeother
types D1, D2 and E because the position of the superlattice
reflections on these patterns is typical. Thus, the experimental
precession pattern in Fig. 8b belongs to this type. On the
other hand, the types D1, D2 and E are very difficult to be
distinguished from each other by conventional methods, or in
lowangleprecessionpatternsbecausetheforbiddenreflections
will appear by multiple diffraction (Fig. 8f). Actually, they can
be distinguished by large-angle electron precession as shown
ontheexamplesinFig.8g,g ,h,andh. Large-angle precession
patterns prove that there are no forbidden reflections
in Fig. 8h, whereas kinematical forbidden reflections are
identified along the d1 diagonal in Fig. 8g. Therefore, the
precession patterns in 8g and 8h belong to the types D2 and
E, respectively.
Another way to make the distinction among the three E,
D1 and D2 patterns consists of tilting the specimen along
the m1 and m2 mirrors or pseudo-mirrors until some more
typical patterns are observed and to compare them with
the corresponding simulated patterns. The two sets of
experimentalpatternsshownin Fig. 9a andb were obtained in
that way. Each set display some strong differences connected
withthe positionsor the intensityof thesuperlattice reflections
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
11/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 1 7
Fig. 9. Sets of electron precession patterns located along the m 1 and m2 mirrors around the c ZAPs. a, b Experimental precession patterns.
Some typical superlattice reflections are circled. c, d, e Corresponding simulated patterns located around the [100] o (or [100]o) (type E) (c), [111]o
(or [111]o, [111]o, [1 1 1]o) (type D1) (d) and [111] o (or [1 11]o, [11 1]o, [111]o) (type D2 (e). The arrows indicate some typical features. The bold anddotted lines indicate mirrors and pseudo-mirrors, respectively.
(see especially the circled superlattice reflections which prove
thepresenceoftwom 1 andm2 mirrorsinFig.9aandthelossof
these mirrors in Fig. 9b). The two sets are, in a unique way, in
perfect agreement with the corresponding simulated patterns
from type E and D1, respectively. Note that the simulated sets
of precession patterns also indicate a 2 mm,1 and1 WP ideal
symmetry for the types E, D1 and D2, respectively.
c ZAPs
The eight equivalent c ZAPs of the ideal cubic
perovskite (Fig. 10a) with (6 mm) symmetry give two types
of patterns in the orthorhombic perovskite:
(1)The [012]o, [012]o, [012]o and [01 2]o patterns (type F)
with (2 mm) symmetry (Fig. 10b).
(2)The [2 10]o, [2 10]o, [2 10]o and [210]o patterns (type G)
with (2 mm) symmetry (Fig. 10c).
Both types display forbidden reflections and very weak
superlattice reflections located at the same positions. They
also display the two same mirrors m1 and m4 meaning that
the four other m2, m3, m5 and m6 mirrors present in the
cubic perovskite are lost in the orthorhombic perovskite and
are only pseudo-mirrors. These two types of patterns are too
close to be easily identified even with electron precession.
To make the distinction between them, the solution consists
again in tilting the specimen along the m 1 mirror and the
two pseudo-mirrors m3 and m5 in order to reach some more
useful ZAPs like the ones shown in Fig. 10b, b , b and c,
c , c . The positions and the intensity of some superlattice
reflections present on these patterns is very typical of the types
F and G which can be identified without ambiguity in that
way (see, especially, the superlattice reflections indicated by
an arrow on the patterns b , b , c and c ). It is clear that
the experimental precession patterns shown in Fig. 10d and
d display superlattice reflections (marked with an arrow) in
agreement with type F patterns andin disagreement with type
G patterns. Note that the WP symmetry of the type F and G
patterns is m.
Identification of the otherc zone axes
Any otherc zone axes canalways be correctlyindexed
withrespecttothethreemainc,c andczone axes described above.
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
12/20
1 8 J . - P . M O R N I R O L I E T A L .
Fig. 10. Electron diffraction patterns of the eight c ZAPs for the pseudo-cubic perovskite. They are sorted into two types of patterns F and G.
a Kinematical simulated patterns of eightequivalent c ZAPs forthe ideal cubic perovskite. They display a (6 mm) symmetry.b [012]o, [012]o,[012]o and [01 2]o kinematical simulated diffraction patterns with (2 mm) symmetry (type F). c [210] o, [210]o, [2 10]o and [210]o kinematical
simulated diffraction patterns (type G) with (2 mm) symmetry. b, b , b Simulated diffraction patterns obtained after a 19.4 specimen tilt along the
m1, m3 and m5 mirror and pseudo-mirrors of the pattern of type F. c, c , c Simulated diffraction patterns obtained after a 19.4 specimen tilt along
the m1, m3 and m5 mirror and pseudo-mirrors of the pattern of type G. d, d, d , d - Experimental precession patterns in agreement with the simulated
patterns b, b, b and b (type F). The bold and dotted lines indicate mirrors and pseudo-mirrors, respectively.
Interpretation of a o ZAP
To fully interpret a [uvw] zone axis diffraction pattern, hkl
indices must be attributed to each of the diffracted spots. This
attribution is not always an easy task with the pseudo-cubic
LaGaO3 perovskite due to symmetry lowering. For example,
let us consider the case of the [010]o (or [010]o) pattern (type
A) (Fig. 11a). The (4 mm) symmetry (mirrors m1, m2, m3 andm4)whichwouldbeobservedonthispatternfortheidealcubic
perovskite is decreasedto a (2 mm)symmetry(mirrorsm2 and
m4)forthepseudo-cubicLaGaO 3 perovskite.Asaresult,subtle
and very weak differences of intensities affect some couples of
superlattice reflections located on each side of the two pseudo-
mirrors m1 andm3. They arevisible on the theoretical pattern
in Fig. 11a (see, e.g. the couples of reflections marked with
an arrow), but they are too weak to be surely distinguished
on the corresponding experimental precession pattern. This
means that it is nearly impossible to make an experimental
distinction between two [010]o patterns rotated by 90. To
remove this ambiguity, the solution is to observe more typical
ZAPs located around the studied pattern (a solution already
describedearlier to make the distinction between thecandc zone axes).Thisisthe case with thethree patterns
shown in Fig. 11bd located at about 18.5 from [010]. A
clearly visible difference of intensity is observed between somecouples of superlattice reflections (see the circled reflections in
Fig. 11) located on each side of the pseudo-mirrors m 1 and
m3. These patterns are in agreement in a unique way
with the [131]o, [131]o and [131]o dynamical simulations in
Fig. 11b, c d (or [131]o, [13 1]o, [1 3 1]o if the pattern in
Fig. 11a is interpreted as being the [010] ZAP). Therefore, the
hklindicesofthe[010]o ZAP(or[010]o ZAP)canbeattributed
without ambiguity (Fig. 11a and a).
A second example concerns the [101]o ZAP (or [101]o,
[101]o and [1 0 1]o) (type B) (Fig. 12a). This pattern displays
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
13/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 1 9
Fig. 11. Interpretation of the [010]o (or [010]o) zone axis pattern (type A). a, a [010]o (or [010]o) experimental (a) and simulated (a
) precession
patterns with (2 mm) symmetry. b, b [131]o (or [131]o) experimental (b) and simulated (b) precession patterns with (2) symmetry. c, c [131] o
(or [13 1]o) experimental (c) and simulated (c) precession patterns with (2) symmetry. d, d [131]o (or [1 3 1]o) experimental (d) and simulated (d
)
precession patterns with (2) symmetry. The bold and dotted lines indicate mirrors and pseudo-mirrors, respectively.
a (2 mm) symmetry with two mirrors m 1 and m3 whereas
it would display a (4 mm) symmetry (m 1, m2, m3 and m4mirrors) in the ideal cubic perovskite. As a matter of fact,
its WP symmetry is not (2 mm) but only m with a unique
mirror m1 meaning that two [101]o ZAPs rotated by 180
are different. This 180 ambiguity can be removed by tilting
the specimen around the [101]o and along the m1 mirror and
m3 pseudo-mirror in order to observe three more typical ZAPs
like the ones shown in Fig. 12bd. Again, a clear difference
of intensity among some couples of superlattice reflections
located on each sides of the pseudo-mirror m3 is visible (see
the circled and arrowed reflections in Fig. 12). Note that the
couples of superlattice reflections located on each side of the
m1 mirror display the same intensity. The comparison with
simulatedpatterns allows the unique attribution of the [313]o,
[201]o and [313]o indices to the experimental patterns b,
c and d, respectively. Thus, the correct hkl indices can be
assigned to the reflections of the [101]o pattern as shown in
Fig. 12a.
Applications
Identification of pertinent ZAPs for electron crystallography
The identification of both the Laue class and the possible
space groups of a crystal can be obtained, at microscopic
and nanoscopic levels, from observations of three features
available on conventional microdiffraction patterns:
(1)The net and ideal symmetries displayed by some specific
microdiffraction patterns provided the diffraction patterns
display at least one HOLZ. They are connected with the
crystal system and the Laue class.
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
14/20
2 0 J . - P . M O R N I R O L I E T A L .
Fig. 12. Interpretation of the [101] o (or [101]o, [101]o, [101]o) zone axis pattern (type B). a, a [101]o (or [101]o, [101]o, [101]o) experimental (a)
and simulated (a) precession patterns with (2 mm) symmetry. b, b [313]o (or [3 13]o, [31 3]o, [313]o) experimental (b) and simulated (b) precession
patterns with (2) symmetry. c, c [201]o (or [210]o, [201]o, [201]o) experimental (c) and simulated (c) precession patterns with (2 mm) symmetry. d,
d [313]o (or [313]o, [313]o, [3 13]o) experimental (d) and simulated (d) precession patterns with (2) symmetry. The bold and dotted lines indicate
mirrors and pseudo-mirrors, respectively.
(2)The shifts between the reflections located in the first-order
Laue zone (FOLZ) with respect to the ones located in the
ZOLZ. They are connected with the Bravais lattice.
(3)The periodicity differences between the reflections located
in the FOLZ and in the ZOLZ. They are connected with theglide planes.
A systematic method based on these features was proposed
by (Morniroli & Steeds, 1992) and was successfully applied
to various crystals (Redjaimia & Morniroli, 1994; Mateo
et al., 1997; Huve et al., 2000; Wei et al., 2000; Gomez-
Herrero et al., 2001; Ranjan et al., 2001; Meshi et al., 2002;
Quarez et al., 2003; Tarakina et al., 2003; Labidi et al., 2005;
Meshi et al., 2005).
Nevertheless, some experimental difficulties are
encountered with the ideal symmetry because its
identification requires a perfect alignment of the incident
beam along a zone axis. Other difficulties occur when the
studied crystal is not thin enough so that its patterns only
display a small number of reflections in the HOLZs. The
FOLZ/ZOLZ shifts and periodicity differences are then verydifficult to observe.
These difficulties are easily overcome with electron
precession mainly because the integrated intensities on the
precession patterns is directly connected with the ideal
symmetry and because the number of reflections present in
each of the Laue zones is larger than the one encountered on
conventional diffraction patterns.
InthepresentcaseoftheorthorhombicLaGaO 3,thismethod
requires one to observe the three following ZAPs: o(patterns with type E), o (patterns with type A) and
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
15/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 2 1
Fig. 13. Identification of the LaGaO3 space group from electron precession patterns. a [100] o zone axis precession pattern. Only the ZOLZ reflections
are visible and can be characterized by mean of a centred rectangular unit cell with sides parallel to the m1 and m2 mirrors. b, c Electron precession
patterns obtained when the crystal is tilted a few degrees away from [100] o along the m1 and m2 mirrors. The FOLZ reflections are characterized by
means of a rectangular unit cell and some FOLZ reflections are located on the m 1 and m2 mirrors (circled reflections). d Theoretical [100] o ZAP
obtained in the case of a primitive orthorhombic lattice (oP) with a diagonal n glide plane parallel to the (100) lattice planes. The corresponding partial
extinction symbol is Pn.. .
o (patterns with type C). As mentioned previously,
there is no difficulty to identify the o and oZAPs because the corresponding patterns display typical
superlattice reflections. This is no longer the case with the
o ZAP (type E) which can be easily confused with the
patterns with the types D1 or D2. We have also indicated
previously howto identify this zone axis by using a large-angle
precession pattern or by specific specimen tilts.
The pattern in Fig. 13a was identified in this way.
Nevertheless, its FOLZ is not visible because the FOLZ radius
is too large for the acceptance angle of the microscope. To
observe it, the specimen is tilted a few degrees away from the
zone axis along the mirror m1 and m2 until some reflections
located in the FOLZ are observed as shown in Fig. 13b and c.Two useful features are visible on these misoriented patterns:
(1)Some reflections are located on the mirrors m 1 and m2(circled reflections); they reveal the absence of shift of the
FOLZ reflections with respect to the ZOLZ reflections in
agreement with a primitive P Bravais lattice.
(2)More reflections areobservedin theFOLZ than in theZOLZ.
This feature can be easily quantified by drawing, in the
FOLZ and in the ZOLZ, the smallest rectangular unit cell
describing the2D lattice ofreflections. Inthe FOLZ,this unit
cell is a rectangle whose sides are parallel to the m 1 and
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
16/20
2 2 J . - P . M O R N I R O L I E T A L .
m2 mirrors whereas in the ZOLZ the unit cell is a centred
rectanglewhosesidesaretwotimeslarger.ThisFOLZ/ZOLZ
periodicity difference is connected with the presence of a
glide plane perpendicular to the [100] zone axis, i.e. along
the (100) lattice planes.
Comparison with the theoretical pattern in Fig. 13d(Morniroli & Steeds, 1992) indicates that these features
correspond, in a unique way, to a n glide plane parallel to
the (100) lattice planes and to a partial extinction symbolPn..
(Hahn, 2002).
The same experiments performedalongthe twoother [010]
and[001]zone axes allow the identification of the Pnma space
group for the LaGaO3 structure.
Identification of a twin
Due to symmetry lowering, twinsare very frequentlyobserved
in perovskites andwere describedby many authors in the case
of LaGaO3 (Wang etal.,1991;Yao etal., 1991;Fink-Finowickiet al., 1992; Bdikin et al., 1993; Wang & Lu, 2006; Wang &
Lu, 2007).
The identification of the twin law, i.e. the identification
of a rotation axis [uvw] and a rotation angle around it,
can be inferred from diffraction patterns (Morniroli & Gaillot,
2000). In principle, two couples of parallel [uvw]A//[uvw]Bdirections coming from the two crystals A and B located on
each side of the studied twin need to be identified. It could
be two couples of [uvw] zone axes or one couple of zone
axes [uvw] and one couple of parallel lattice planes (hkl). The
knowledgeofthecorrect[uvw]orhklindicesarecrucialforthis
identification.Let us consider the diffraction pattern in Fig. 14a. It was
obtained when the electron beam is focused on a LaGaO3twin and it is made of the superimposition of a [010]o ZAP
(typeA)anda[101] o ZAP(typeB).Intheprevioussections,we
indicate that 90 and 180 ambiguities exist for these [010]oand [101]o patterns, respectively, and that these ambiguities
can be removed by observing, on some ZAPs located around
the [010]o and [101]o, some typical superlattice reflections
located on each side of the pseudo-mirrors present on the
patterns.
Actually, the patterns in Figs 11 and 12 were obtained
with the incident beam located on each side of the studied
twin. Both figures are arranged in a coherent way so as to
preserve their mutual orientation. The correct [uvw] and hkl
indices of these patterns were demonstrated in the previous
sections. From these patterns, many couples of parallel [uvw]
ZAPs can be obtained, for example [010]oA//[101]oB and
[131]oA//[201]oB, which give the following twin law: a 120
rotation around the common rotation axis [210]. This result
is obtained from the mathematical calculation described by
Morniroli& Gaillot, 2000. A schematicdescriptionof this twin
with {121} twin plane is given in Fig. 14b. It is in agreementwith previous studies.
Fig. 14. Characterization of a LaGaO3 twin. a Electron precession
pattern obtained when the incidentbeam is located onthe twin. Itis made
of the superimposition of the [010] o and [101]o ZAPs. b Schematic
description of the twin. The two lattices A and B on each side of the twin
plane (121)oA//(121)oB are rotated bya 120 angle around the common
direction [210]o.
Another way to obtain the same twin law is to consider
the pattern in Fig. 14a, where in addition to the couple of
parallelZAPs[010]oA//[101]oB italsogives(101)oA//(020)oB.
It is pointed out, that the correct interpretation of the [010]and [101] ZAPs is required to identify the actual twin law.
Especially, the 90 and 180 ambiguity for the [010]o and
[101]o ZAPs should be taken into account. If not, wrong
twin laws like 90 around [101] or 180 around [111] are
obtained.
Identification of the crystal structure at microscopic
and nanoscopic level
The comparison between experimental precession patterns
and simulated patterns allows an unambiguous crystal
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
17/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 2 3
Fig.15. Identificationof the LSGMspace group. a Experimental precessionpatterns located around ac ZAP.b, c Simulateddiffraction patterns
located around the[010]o (or[010]o)(a)and [101]o (or[101]o,[101]o, [101]o) (b)for theorthorhombic Imma space group.d, e, f Simulated diffraction
patterns located around the [010]o (or [010]o)(d), [101]o (or [101]o,) (e) and [101] o (or [101]o) (f) for the monoclinic I112/b space group. The mirrorsare indicated by a bold line and the pseudo-mirrors by a dotted line.
identification, especially if the weak superlattice reflections
are taken into account. In the case of LaGaO3 there is no
special difficulty because most of the authors agree with
the orthorhombic structure. As a matter of fact, all the
experimental patterns from our specimens are in perfect
agreement with this structure.
This is no longer the case with the LSGM perovskite. At
room temperature, two different structures based on X-ray
and neutron diffraction experiments were proposed:
(1)An orthorhombic structure with space group Imma (Lerch
et al., 2001).(2)A monoclinic structure with space group I112/b (Slater
et al., 1998).
Both of them are very close and only differ by some very
slight changes and by different loss of symmetry elements
(there is less symmetry elements in the monoclinic structure
than in the orthorhombic).
The identification of the real LSGM space group can be
obtained from a careful examination of the superlattice
reflections present on somediffraction patterns located around
a c ZAP (Fig. 15a). In this figure, it is clear that all
the couples of reflections (circled and arrowed spots) located
on each side of the m1, m2, m3 and m4 mirrors of the ideal
cubic structure display a clear difference of intensity, which
proves that these patterns do not contain any mirror. This
absence ofmirror isin agreement in a unique way with the
corresponding simulated patterns located around the [101]m(or [101]m) ZAP from the monoclinic I112/b space group
(Fig. 15e). It is in disagreement with all the other ctheoreticalpatternsfrom the two possiblespacegroups I112/b
and Imma. As a result, the LSGM space group is identified as
I112/b.
Discussion
This analysis is based on a qualitative observation of the
symmetry elements present on the precession patterns and
the deduction of the ZOLZ and WP ideal symmetry. Table 1
summarizes these ideal symmetries for the main zone axes
of the cubic perovskite as well as the resulting symmetries
in the orthorhombic perovskite. A 180 ambiguity occurs
with the patterns displaying an m or 1 symmetry. A special
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
18/20
2 4 J . - P . M O R N I R O L I E T A L .
Table 1. ZOLZ and WP ideal symmetries for the main zone axis patterns of the cubic and the orthorhombic LaGaO3 perovskites.
Ideal cubic LaGaO3 cs c c
perovskite Laue class m-3 m
Ideal symmetry
ZOLZ (6 mm) (4 mm) (2 mm)WP 3 m 4 mm 2 mm
Orthorhombic Type F Type G Type A Type B Type C Type D1 Type D2 Type E
LaGaO3 perovskite [012]o [210]o [010]o [101]o [001]o [1-11]o [111]o [100]o
Laue class [01-2]o [-210]o [0-10]o [-10-1]o [00-1]o [11-1]o [-1-11] o [-100]o
mmm [0-12]o [-2-10] o [-101] o [-111]o [1-1-1] o
[0-1-2]o [2-10]o [10-1] o [-1-1-1] o [-11-1] o
Ideal symmetry
ZOLZ (2 mm) (2 mm) (2 mm) (2 mm) (2 mm) (2) (2) (2 mm)
WP m m 2 mm very m 2 mm 1 1 2 mm
close to (4 mm)
4 mm s
180 ambiguity 90 ambiguity 180 ambiguity 180 ambiguity 180 ambiguity
90 ambiguity is observed for the [010] and [010] zone axes
because their corresponding 2 mm symmetry is very close to
4 mm.
The ideal symmetry is easily observed on the experimental
precession patterns. This is a major advantage with respect
to the conventional electron diffraction where only the less
useful net symmetry is usually inferred with surety. On most
patterns, only the ZOLZ reflections are considered to identify
the ZOLZ ideal symmetry and the WP ideal symmetry was
deduced from specific specimen tilt experiments. Another
way to get this WP symmetry is to consider the HOLZ
reflections.This approach is also possible but it is usually morecomplex to perform especially with high-symmetry zone axes
which have a HOLZ radius too large to be accepted by the
microscope.
Using electron precession to identify the zone axis could be
applied to any zone, but it is pertinent to use high-symmetry
patterns like c, c or c because these
patterns are easily recognizable and these zone axes can be
used as a starting point to reach any other less symmetrical
zone axes. The transition from one high-symmetry ZAP to
another one less symmetrical is easily made by observing
the Kikuchi lines which are visible on the corresponding
convergent-beam electron diffraction patterns.
Some experimental difficulties are connected with twins,
which are very frequent in these materials. These twins are
only visible for some particular crystal orientations and the
contrast differences between the two crystals located on each
side of the twin is usually weak. As a result, twin free areas
are not easy to locate and have a small size of about 0.1 m
so that the analysis requires very accurate positioning of the
incident beam.
Other limitations are connected with the possible tilt angles
of the specimen holder. The double tilt specimen holder used
in thepresent study allows45 and30 and tilt angles,
respectively. The required zone axes should be located within
these angular domains.
We indicate that the kinematical forbidden reflections
can be identified on large-angle precession patterns (about
3) because a few-beam behaviour prevails, which strongly
decreases the possibility of multiple diffraction. Nevertheless,
most of theprecessionpatterns given in thepresent paper were
obtained with a precession angle of about 1. The alignments
are then easy to perform even for very small spot sizes down
to 50 nm and the resulting patterns display a high quality.
With large precession angle (the maximum value is about 3
with our precession equipment) the perfect scan and descanalignments are more difficult to achieve and the resulting spot
size is larger due to spherical aberration so that it could be
difficult to focus the beam on a twin free area. Then, the
quality of large-angle precession patterns is usually poorer.
A microscope equipped with a spherical aberration corrector
should allow one to obtain larger precession angles without
decreasing the spot size.
Most of the simulated patterns given in the present paper
result from kinematical calculations, where the intensity of
the diffracted beams is connected with the square modulus
of the structure factor. These calculations are well adapted to
the weak superlattice reflections, which have a kinematical
behaviour especially at a large precession angle. The latestversion of the jEMS software from Pierre Stadelmann allows
one to simulate dynamical precession patterns. They were
particularlyusefultointerprettheeffectoftheprecessionangle
on the forbidden reflections on Fig. 7.
Conclusions
Electron precession is very useful for the analysis of these
pseudo-cubic perovskites. This property is mainly due to the
integrated intensity of the diffracted beams meaning that the
diffracted intensities can be taken into account and trusted.
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
19/20
C O N T R I B U T I O N O F E L E C T R O N P R E C E S S I O N 2 5
The more useful ideal symmetry is available instead of the
net symmetry. Very small differences of intensity are also
detectable and are of very great help when dealing with the
identification of the presence or the absence of mirrors on
the precession patterns. The possibility of identifying, on large
precession patterns, the kinematical forbidden reflections dueto glide plane or screw axes is also very useful.
A method to identify any zone axis without ambiguity
is described and some applications of this method to the
characterization of twins and to the identification of the space
group are given.
This method is general and can be easily extended to other
perovskites or crystals displaying small symmetry departures
from a high symmetry.
Acknowledgements
We thank Justin Kimpton (RMIT) for supplying the LaGaO3
and LSGM; the French Embassy and the Australian Scienceand Technology (FEAST) program for J.-P.M. to travel to UQ
(October 2003, February 2006) and forG.J.A. to travel to Lille
(July 2005, July 2006) and The University of Queensland for
a UQ Travel grant for G.J.A. to Lille (July 2007).
References
Balachandran,U., Ma, B.,Maiya, P.S. et al.(1998) Development of mixed-
conducting oxides for gas separation. Solid State Ion. 108, 363370.
Bdikin, I.K., Shmytko, I.M., Balbashov, A.M. & Kazansky, A.V. (1993)
Twinning of LaGaO3 single-crystals. J. Appl. Crystallogr. 26, 7176.
Christen, H.M., Boatner, L.A., Budai, J.D., Chisholm, M.F., Gerber, C.
& Urbanik, M. (1997) Semiconducting epitaxial films of metastableSrRu0.5Sn0.5O3 grown by pulsed laser deposition. Appl. Phys. Lett. 70,
2147-2149.
Fink-Finowicki, J., Berkowski, M. & Pajaczkowska, A. (1992) Twinning
structure of LaGaO3 grown by the Czochralski method. J. Mater. Sci.
27, 107110.
Glazer, A.M. (1972) Classification of tilted octahedra in perovskites. Acta.
Crystallogr B B 28, 33843392.
Gomez-Herrero, A., Landa-Canovas,A.R., Johnson, A.W.S. & Otero-Diaz,
L.C.(2001) Transmission electron microscopystudy of Y1xxCr2S4,
x similar to 1/3 phase. J. Alloys Compd. 323, 8690.
Hahn, T. (2002) International tables for crystallography. Brief Teaching
Edition of Volume A, Space-Group symmetry, 5th edition. Published
for the International Union of Crystallography by Kluwer Academic
Publishers, Dordrecht, Boston.
Huve, M., Vannier, R.N. & Mairesse, G. (2000) EDS and TEM study of the
familyof compoundswitha structurebasedon [Bi12O14] columns in
the Bi2O3-MoO3 binary system. J. Solid State Chem. 149, 276283.
Kao, W.H., Haberichter, S.L. & Bullock, K.R. (1992) Corrosion-resistant
coating for a positive lead-acid-battery electrode. J. Electrochemical Soc.
139, L105L107.
Kawada, T., Sase, M., Kudo, M. et al. (2006) Microscopic observation
of oxygen reaction pathway on high temperature electrode materials.
Solid State Ion. 177, 30813086.
Labidi, O., Roussel, P., Huve, M., Drache, M., Conflant, P. & Wignacourt,
J. P. (2005) Stabilization of a new polymorph in P-
substituted Pb2BiVO6: single crystal structure of Pb2Bi(V0.84P0.16)O6
and conductionpropertiesof related materials.J. Solid State Chem. 178,
22472255.
Lahousse, C., Bernier, A., Grange, P., Delmon, B., Papaefthimiou, P.,
Ioannides, T. & Verykios, X. (1998) Evaluation of g-MnO2 as a VOC
removal catalyst: comparison with a noble metal catalyst. J. Catalysis
178, 214225.
Lerch, M., Boysen, H. & Hansen, T. (2001) High-temperature neutron
scattering investigation of pure and doped lanthanum gallate. J. Phys.
Chem. Solids. 62, 445455.
Marti, W., Fischer, P., Altorfer, F., Scheel, H. & Tadin, M. (1994) Crystal
structures and phase transitions of orthorhombic and rhombohedral
RGaO3 (R= La, Pr, Nd) investigated by neutron powder diffraction. J.
Phys. Condens. Matter6, 127135.
Mateo, A., Llanes, L., Anglada, M., Redjaimia, A. & Metauer, G. (1997)
Characterization of the intermetallic G-phase in an AISI 329 duplex
stainless steel. J. Mater. Sci. 32, 45334540.
Meshi, L., Zenou, V.Y., Ezersky, V., Munitz, A. & Talianker, M. (2002)
Identification of the structure of a new Al-U-Fe phase by electron
microdiffraction technique. J. Alloys Compd. 347, 178183.Meshi, L., Zenou, V., Ezersky, V., Munitz, A. & Talianker, M. (2005)
Tetragonal phase in Al-rich region of U-Fe-Al system. J. Alloys Compd.
402, 8488.
Minh, N.Q. (1993) Ceramic fuel-cells. J. Am. Ceramic Soc. 76, 563588.
Morniroli, J.-P. (2002a) Large-Angle Convergent Beam Diffraction.
Applications to Crystal Defects, SF, Paris.
Morniroli,J.-P. (2002b) Electron Diffraction, a Software to Simulate Electron
Diffraction Patterns. USTL & ENSCL, Lille.
Morniroli, J.P. & Gaillot, F. (2000) Trace analyses from LACBEDpatterns.
Ultramicroscopy 83, 227243.
Morniroli,J.P.& Redjaimia, A.(2007)Electronprecessionmicrodiffraction
as a useful tool for the identification of the space group. J.
Microsc.(Oxford) 227, 157171.
Morniroli,J.-P. & Steeds, J.W.(1992)Microdiffraction as a toolfor crystal-structure identification and determination. Ultramicroscopy 45, 219
239.
Morniroli, J.P., Redjaimia, A. & Nicolopoulos, S. (2007) Contribution
of electron precession to the identification of the space group from
microdiffraction patterns. Ultramicroscopy 107, 514522.
Quarez, E., Huve, M., Abraham, F. & Mentre, O.
(2003) From the mixed valent 6H-Ba3Ru25.5+NaO9
to the 6H-Ba3(Ru1.69C0.31)(Na0.95Ru0.05)O8.69 oxycarbonate
compound. Solid State Sci. 5, 951963.
Ranjan, R., Pandey, D., Schuddinck, W., Richard, O., De Meulenaere, P.,
VanLanduyt,J.&VanTendeloo,G.(2001)Evolutionofcrystallographic
phasesin(Sr 1xCax)TiO3 withcomposition(x).J. Solid State Chem. 162,
2028.
Redjaimia, A. & Morniroli, J.-P. (1994) Application of microdiffraction to
crystal-structure identification. Ultramicroscopy 53, 305317.
Ringwood, A.E., Kesson, S.E., Ware, N.G., Hibberson, W. & Major,
A. (1979) Immobilization of high-level nuclear-reactor wastes in
SYNROC. Nature 278, 219223.
Slater, P.R., Irvine, J.T.S., Ishihara, T. & Takita, Y. (1998) High-
temperature powder neutron diffraction study of the oxide ion
conductor La0.9Sr0.1Ga0.8Mg0.2O2.85. J. Solid State Chem. 139, 135
143.
Stadelmann, P.A. (1987) EMS a software package for electron-
diffraction analysis and HREM image simulation in materials science.
Ultramicroscopy. 21, 131145.
C 2008 The AuthorsJournal compilation C 2008 The Royal Microscopical Society, Journal of Microscopy, 232, 726
-
7/30/2019 J P Morniroli
20/20
2 6 J . - P . M O R N I R O L I E T A L .
Stadelmann, P.A. (2007)jEMS a Software Package to Simulate Dynamical
Electron Precession Patterns. EPFL, Lausanne.
Stlen, S., Bakken, E. & Mohn, C.E. (2006) Oxygen-deficient perovskites:
linkingstructure, energeticsand iontransport. Phys.Chem. Chem.Phys.
8, 429447.
Tarakina, N.V., Tyutyunnik, A.P., Zubkov, V.G., DYachkova,
T.V., Zainulin, Y.G., Hannerz, H. & Svensson, G. (2003) High
temperature/high pressure synthesis and crystal structure of the new
corundum related compound Zn4Nb2O9. Solid State Sci. 5, 459463.
Vincent, R. & Midgley, P.A. (1994) Double conical beam-rocking
system for measurement of integrated electron-diffraction intensities.
Ultramicroscopy 53, 271282.
Wang, W.L. & Lu, H.Y. (2006) Phase-transformation-induced twinning
in orthorhombic LaGaO3: {121} and [010] twins. J. Am. Ceramic Soc.
89, 281291.
Wang,W.L.& Lu,H.Y.(2007) rotationtwins in an orthorhombic
LaGaO3 perovskite. J. Am. Ceramic Soc. 90, 264271.
Wang, Y., Liu, X., Yao, G.-D., Liebermann, R. & Dudley, M. (1991) High
temperature transmission electron microscopy and X-ray diffraction
studiesoftwinningandthephasetransitionat145CinLaGaO3. Mater.
Sci. Eng. A 132, 1321.
Wei, Q., Wanderka, N., Schubert-Bischoff, P., Macht, M.P. & Friedrich,
S. (2000) Crystallizationphases of theZr 41Ti14Cu12.5Ni10Be22.5 alloy
after slow solidification. J. Mater. Res. 15, 17291734.
Weissbart,J. & Ruka, R. (1961)Oxygengauge. Rev. Sci. Instrum. 32, 593.
Yao, G.D., Dudley, M., Wang, Y., Liu, X. & Liebermann, R.C. (1991)
Synchrotron radiation topography studies of the phase-transition in
LaGaO3 crystals. Nucl. Instrum. Methods Phys. Res. B 56(7), 405408.
Yuan, D. & Kroger, F.A. (1969) Stabilized zirconia as an oxygen pump. J.
Electrochem. Soc. 116, 594600.