hannes krüger 1 , v á clav pet říč ek 2 , volker kahlenberg 1
DESCRIPTION
A phase transition in Ca 2 Fe 2 O 5 with coexistence of a 3d and a (3+1)-dimensional phase. Hannes Krüger 1 , V á clav Pet říč ek 2 , Volker Kahlenberg 1. 1 Institute of Mineralogy and Petrography, University of Innsbruck - PowerPoint PPT PresentationTRANSCRIPT
Hannes Krüger1, Václav Petříček2, Volker Kahlenberg1
1Institute of Mineralogy and Petrography, University of Innsbruck2Institute of Physics, Academy of Sciences of the Czech Republic, Prague
A phase transition in Ca2Fe2O5 with coexistence of a 3d and
a (3+1)-dimensional phase
Ca2Fe
2O
5
(Srebrodolskite)C
2F
x=0
Imma(00)s00above 710°C
Ca2FeAlO
5
(Brownmillerite)C
4AF
x=0.5
Ca2Al
2O
5
C2A
x=1
The Ca2(Fe
1-xAl
x)
2O
5 solid solution series
x=0.28
x=0.7Pnma I2mb
ambient conditions elevated pressure
Taylor (1997)Kahlenberg et al. (2000)Redhammer et al. (2004)Fukuda & Ando (2002)Berastegui et al. (1999)Krüger & Kahlenberg (2005)
ca
c
b
Basic building units of brownmillerite-type structures
1) Layers of corner-sharing FeO
6 octahedra
Ca2Fe
2O
5 at room
conditions. Pnma
a
cBasic building units of brownmillerite-type structures2) zweier single chains of distorted FeO
4 tetrahedra
Ca2Fe
2O
5 at room conditions. Pnma
R
L
R/Ldisorderedas in Imma
Pnma
b
a
L
R
c*
b*c*
b*
(1kl)
(0kl)
827(5)°CMain Reflections indexed witha=5.493(1), b=15.037(3),c=5.651(1)ÅSpace group: Imma
Satellite reflections indexed withq=(0,0,0.588(2))(3+1)-dimensional Superspace group: Imma(00)s00
The superspace mirrorplane
Imma(00)s00
glide vector ½ along x4
Positions of O3-O3' and Fe2-Fe2'should not be occupied in any physical space section of the (3+1)-dimensional model
Fe2 and O3 have to be defined on½ of the period in x
4
Fe2 Fe2'
O3' O3
Lambert et al. (2002)
x4 x
4
x1
x1
x1-x
4 sections of the 4-dimensional F
obs synthesis
Fe2, x2=0.25, x
3=0.569 O3, x
2=0.25, x
3=0.626
L L R L R L R R L R
„t-map“ (physical space section) at t=0, x2=0.25, calculated from
4-dimensional Fobs
synthesis
-5.9
-5.8
-5.7
-5.6
-5.5
-5.4
-5.3
-5.2
-5.1
600 620 640 660 680 700 720 2.8
2.9
3
3.1
3.2
3.3
3.4
3.5heating (y1)cooling (y2)
DTA measurement Ca2Fe
2O
5, 5K/min, 100 µl Pt-crucible, 95.39mg sample
Temperature [°C]
He
at F
low
[µV
]
(1kl)
c*
b*
Reconstructed reciprocal space, measured at 697(5)°CCa
2Fe
2O
5
Reflectionsh+k+l ≠ 2n
andSatellites
are present!
This is observed fora temperature range of (~20K)
Facts
• Pnma Pnma + Imma(00)s00 Imma(00)s00
• only one thermal effect in DTA
• can be indexed with P cell (continuous with t) + 1 q-vector
• temperature gradient in sample <7K
• all attempts in primitive superspace groups failed
Exclusive sets of Reflections
Pnma hkl0: h+k+l ≠ 2nImma(00)s00 hklm: m ≠ 0
0
2
4
6
8
10
12
14
16
640 660 680 700 720 740 760 780 800 0
1
2
3
4
5
6
Temperature (C)
P-Reflections heatingP-Reflections cooling
Satellites heatingSatellites cooling
aver
age
I/(
I) o
f 113
P-r
efle
ctio
ns, (
hkl0
):h+
k+l≠
2naverage I/
(I) of 182 satellite reflections, (hklm,m
≠0):h+k+
l=2n
688°C 709°C
Refinement in Jana2000 with a „two-phase model“
Integration: I-centred lattice, with q=(0,0,)+ hkl0: h+k+l ≠ 2n
multiphase refinement (via twinning option)
both phases are required to have the samenumber of dimensions
calculation of combined structure factor:
0
10
20
30
40
50
60
70
80
90
100
688 690 692 694 696 698 700 702 704 706 708
Vol
ume
frac
tion
[%]
Temperature [C]
Volume fractions of Pnma and Imma(00g)s00 structures
fraction Pnma (heating)fraction Imma (heating)fraction Pnma (cooling)
Conclusions
Pnma and Imma(00)s00 phases of Ca2Fe
2O
5 are coexisting in a
range of 20K
This can be handled as a special case of multiphase refinement in Jana2000
To do:
experiments at higher temperatures (up to 1400°C)Satellite darkfield HT TEMSubstitution Fe/Al
Acknowledgements
Waltraud WertlEduardo Hernandez BocanegraKaren FrieseJuan Manuel Perez-MatoMichael Czank
Thank you for your attention
0.596
0.598
0.6
0.602
0.604
0.606
0.608
0.61
0.612
0.614
0.616
680 700 720 740 760 780 800 820 840
valu
e of
gam
ma
IPDS-II sample Temperature (C)
temperature dependence of q-vector
heatingcooling
Pnma
I2mb
b
a
L
R
R
R
Refinement
2 Crenel-functions, occupation of Fe2, O3 (2 parameters)
Harmonic modulation functions (positional modulation):Fe1 in x (1 par.)Ca in x (2 par.)O1 in x, y (3 par.)O2 in x (2 par.)
Equivalent reflections used: 934 (360 main, 574 satellites)Parameters refined: 43R
w (all; main; sat): 0.055; 0.053; 0.059
Acta Crystallogr. B61, 656-662
300 400 500 600 700 800 900 1000-35
-30
-25
-20
He
at
Flo
w [
mW
]
T [°C]
DTA-Experiment Ca2Fe
2O
5
T1
T2
2 thermal effects:
T1=443°C 0.2 J/g
Néel temperature Takeda et al. (1968)
T2=688°C 0.5 J/g
structural phase transitionImma or I2mb Berastegui et al. (1999)incommensurate? Redhammer et al. (2004)
Experimental setup (single crystal X-ray diffraction)
Diffractometer: Stoe IPDS-II, 2 circleHeating method: Heated N
2-stream
Temperature: 827(5)°CCrystal size: 0.18x0.18x0.06 [mm]Absorption Correction: integration, using 8 indexed facesRadiation: Mo K
max
(°): 29.31
Measured Reflections
Main: 2208Satellite: 3932R
int: 0.057