FT-RAMAN STUDY OF THREE SYNTHETIC SOLID
SOLUTIONS FORMED BY ORTHORHOMBIC SULFA TES:
CELESTITE-BARYTES, BARYTES-ANGLESITE AND
CELESTITE-ANGLESITE
Key words: FT-Raman, solid solutions, celestite, barytes, &nglesilc
1.M. Alia"', H.G.M. Edwardsb, S. L6pez-Andr�s', P. Gonzalez-Martfn".
F J. Garcla-Navarro" and H.R. Mansourb
• E. U.f. T.A. - Dpfa, de Quimica Fisica. Uni�rrsjdod de Cos/illo-Lo Manc/IU, J 3071 Cludad Real, Spain � Chem/slry and FonlUic Sdencu, Uniwrsiry of Bradford, Bradford BD7 I DP. Uniled Kingdom • DpfO. de Crislolografia y Miner-a/ogia, Fae. de Geo{ogia, Universidad Campiu/r:rlst, 2807 J Madrid, Spain
ABSTRACT
In the present work. three different solid solutions whose end-members are •
the orthorhombic sulrales celestite (SrSOd. barylc5 (Ba804) and anglesitt
(PbSO�) are studied using FT-Raman speelroscopy. SuI rate lllion symmetric
internal modes have bull examilled in detail by means of band-shape analysis and
component fitting procedures_ The symmetric stretching mode ",(AI) changes its
wavenumber position linearly with the cationic composition of the samples which
further confinns the ideal character of the solid solutions studied. The
• .... ulhoe" for corrnponckn«. E-mait: [email protected]:s
corresponding full-width at half-height is strongly increased in the central
components of the different solid solutions which can be understood as an effect
of the positionaJ disorder induced by random cationic substitution. Similar results
are observed in the symmetric bending mode, VJ(E). The study of the low
frequency spectral region pennits one to differentiate traslational modes of the
sulfate anion, which changes its wavenumber position when the alkali-earth metal
ion changes from rotational modes. This permits the tentative band assigrunent of
the anglesite rotational Raman bands al 134 and 152 cm-I, which were previously
not assigned.
INTRODUCTION
The study of those solid solutions fonned by sparingly soluble orthorhombic
sui fates (namely 8aSO. barytes, srSO. celestite, and PbSO. anglesite) has
received considerable attention in recent ye�I�. Geochemical, mineralogical,
industrial and environmental reasons justify this interest (see S� and references
Iherein).
Although mechanistic\ Idnelie' and structural' studies dealing with the three
binary solid solutions fonned in the system BaSO.-SrSO.-PbSO. have been
published, the vibrational dynamics of such solid solutions has not been studied,
to the best of our knowledge. This lack of data contrasts with the numerous
available references on Raman and infrared studies of the corresponding simple
sulfatesl9•
The present work reports the analysis of the three binary solid solutions
fonned in the system BaSO.-SrSO.-PbSO •. The samples obtained by precipitation
are studied by means of powder X-ray diffraction and FT-Raman spectroscopy.
The experimental results allow the calculation of the unit cell dimensions of the
samples and 10 study the ideality of the solid solutions obtained. The FT-Raman
spectra are analyzed in detail, including band-fitting of the low frequency spectral
region.
EXPERlMENTAL Samples were prepared by the method of Komicker et al.', adding dropwise
an aqueous solution with the appropriate quantities of the cations (as nitntes, 0.25
M) to 6 M sulfuric acid a' 60-65 "C with vigorous stirring. The stoichiometry of
the precipitates was detC'nnined by dissolution of ca. 0.1 g (accurately weighed to
±<I.I mg) in lOO mL of 0.02 M NaJEDTA at pH 10.8. The ucess of chdating
agent was subsequently detennined with Cal+. Triplicate analyses were run for
each sample. The observed stoichiometries of the cations �re always within ±2% of their nominal value.
X.ray diffraction was carried out using a Philips PW 1710 automatic
diffractometer with Cu K... radiation (wavelength .. 1.54056 A), automatic
divergence slit and graphite monochromator. Scanning rate (from 29 - 5· to 70")
was 0.5· min" using Si as an external standard.
FT·Raman spectra were excited al 1064 nm using an Nd:YAG laser and a
Broker IFS66 optical bench with a FRA 106 Raman JKcessory. Laser power was
selal ta. lOO mW and 1000 scans were accumulated with a resolution of 2 cm".
Powdered samples were lightly pressed in the Broker powder holder and mounted
with I gO" scattering geometry. The mathematical (curve fitting) treatment of the
spectn was carried out using the commercial software GRAMSJJ2- (Galactic
Industries). Smoothing procedures or baseline correction routines were nol
applied in this work. All the figures in the paper are prodoced from integrated
inte�ty·nonnalized spectra.
RESULTS
Figure I shows the evolution of the unil--cell paramelers against the chemical
composition of the samples. Well·fitted linear plots are obtained in all the cases (see Table 1), which dcmonstrales that the solid solutions studied obey Vegard's
law. This observation can be understood as a first index of ideality. Table 2
sumnw-jzes the unit--cell dimensions in the end·members of the solid solutions
(pure phases). As can be observed, these are very close 10 the last reported
• • . · . . • . • I";0o;;0o<I • I"'9"l
• i t · t ·
I •
I •
3 3 • •
• •
• • " .. .. • " .. .. ..
• -•
Il'I>,Sr,""SO� � .. � ......
)' -•
• t I 3
· •
r .a
I ! la
--•
,. • •
• • " .. .. • a
• • • • • -..... _-
FIG. J. Evolution oflhe unit·ccll parameters against the chemical composition of
the samples.
TABLEt
Linear Fitting of the Unit Cell Parameters (a, b, c: A ; V: A') against the Mole i Fraction x. s.e. -standard error; s.t.e. - standard error of the estimate. I ,
Sr.Ba(I·.tSO. " Paramet l/1tercept s.e. Slope s.e. s.e.t.
" a 8.8991 0.0396 -0.4871 0.0669 -0.9245 0.0702 b 5.4642 0.0060 -0.0958 0.0101 -0.9534 0.0106 , 7.1505 0.0045 -0.2856 0.0075 -0.9969 0.0079 V 346.68 0.32 -38.608 0.538 -0.9991 0.56
U.,Pb(I."SO. " Poramel 1/1/treept S.e. Slap< s.e. S.t.t.
" a 8.4664 0.0043 0.431\ 0.0073 0.9987 0.0076 b 5.3970 0.0055 0.065\ 0.0093 0.9192 0.0098 , 6.9542 0.0025 0.2051 0.0042 0.9981 0.0044 V 317.66 0.25 30.156 0.424 0.9991 0.44
Sr,P!>tl .• ,sO. " Paramet Intercept S.t. Slap< s.e. s.t.e.
" a 8.4720 0.0050 -0.102& O.OOSS -0.9708 0.0089 b 5.)942 0.0044 -0.0380 0.0075 -0.8596 0.0079 , 6.9584 0.0017 -0.0886 0.0029 -0.9952 0.0031 V 317.98 0.32 -10.032 0.536 -0.9874 0.5626
TABLE 2
Unit-c.ell Parameters of the End-members. All data in Angstrom (A).
B.SO. SrSO. PbSO. Obstrvtd Reffl I) Observed Ref[J J) Observed Re!{J J]
a 8.909 8.879 8.390 8.354 8.475 8.472 • 5.449 5.454 5.351 5.346 5.376 5.397 , 7.156 7.154 6.874 6.867 6.962 6.955 V 347.4 346.4 308.6 306.7 317.2 318.0
structure refmement" which is remarkable taking into accoWl! the IlIture of the
precipitation of the solid solutions studied here. Figure 2 shows the FT ·Raman spectral region com::sponding to the sulfate
ion symmetric stretching V)(AI) fundamental in several samples of the solid
solutions. These are fairly symmetric bands whose Raman wavenumbers change
linearly with the chemical composition of the samples (Figure 3, left).
The behavior of the corresponding half·widths (measured IS the full·width at
half·height, FWHH) (Figure 3, right) deserves some comments. The solid
solutions which contain lead (BaxPbl .• S04 and Sr.Pbl .• SO.) have a bimodal
tendency with a maximum near the central points being observed, similar to that
reported in other solid solutions't.Il. However, the system barytes/celestite
(Ba.Sr, .• SO.) shifts from this behavior and the FWI1I1 changes linearly between
both extremes. This observation could result from the geochtmical similarity
between barium and strontium ions; furthennore, their influence on the sulfate
anion vibrational dynamics is very close, as can be deduced from the reported
stretching and bending force constants of sulfate in barytes, celestite, and
anglesitel4.
Figure 4 shows the symmetric bending mode V}(E) in samples of the different
solid solutions. As in the ",(AI) fundamental, the difference behYeen the
celestilelbarytes solid solution and those which contain lead is evident. In Figure
S, left. the evolution or the band group center of mass is plotted against the
chemical composition. As can be observed. this parameter does not change along
the range or substitution in the celestite/baryles solid solution. 110wever, the shift
between the components or this doubly degenerate rWldamental is very sensitive
to the changes in the chemical composition. as shown in Figure S, right. As
ahady reported for other solid solutions'2.11,the shift between the components of
a degenerate fundamental tends to decrease in the central members of a solid
solution as an effect of the local disorder. Thus, the observed shift decreases
abruptly in the first members of the series, increasing later, because anglesite
shows the greatest shift between the components (11.4 cm-I)_ A similar, although
attenuated, effect can be observed in the celestite/barytes solid solution.
••
•• Sr,a·c • ..rS°.j
•• .-, .. ; �
" . " ••
,�
.. , .. ,� .. .. - * ,. * m � m
'" IS;.Pbc':�O� '" "
, � ! •• '-, I'·
,. ,�
, .. .. • • .. - - � * m M , . ••
'" 1B.·p�(�.;a ."
, " •
-
,�
'L--��"'; _ '1120 ."'. 1lI0II MO .. '" .. MII fOIl RI --,_.
FIG. 2. SulCate anion symmetric .stretching ",(A,) spectral region in the end
members and several samples of the solid solutions. The roolc fractions, x, of the
corresponding cation are 003, O.S and 0.7.
••
••
� ... ,
I" ••
",
.. r----�� 1 .. ,· ....... °1 ••
� ...
i� ••
•• • I......-=""�---..:::
. .. .. <1 . .. ... _ ... •• --, ....
FIG. 4. Sulfatc anior. symmetr;e bending �£) spectral region in the end
members and several samples orthc solid solutions. The mole fractions. x, of the
cornspoooing cation are 0.3, O.S and 0.7.
..
". • • • • • •
• • • ".
• , • E '5O i •
• · • • •
'" • E , • • • • • > ... • • •
• • ...
• Sr-Ba • • ... • Sr-Pb •
... Ba-Pb i
-• '" '" .. .. " .
% 01 the second cation
"
• Sr-Ba
• Sr-Pb • •
" •
... Ba-Pb •
• • • .. •
, • E • - . •
• • • � • • • •
• • •
• •
• • •
, •
• • " .. " .. ".
% ollh ... tond c:aUon
G. 5. (Top) Wavenumber of the sulfate &rOOD symmetric bending band against chemK:aJ cofq)Osition of the samples. (Bottom) Band full-widlh at half-heighl.
-
-
om l
r� 0"
-
om ,. o�
om
�.-• I "-,IS
ON
0-�
.-o.
0-
� ... • 1-·
_.
_.
_M -
tce .. ·w.csrSOJI
le
�I�" m
IBary1"IOaSOJI
m
[An8, .. lCa IPbSO.J1
'. '.
I, I , I \ "Iil
I �\ i j , \! 1 ,
: "._ ", ! t ' I . '/. ' ' \ ' • I, \ / i " :\1' , . J \'" --" ' <. ' !. ;
," '.
� i
" ! 1 rI
1J� 1, f , ,
\ I.i\
! ,
.. 0 --,-�
,.
"
"
"
FIG. 6. Band-fitted klw frequency spe<:tnlJ region down to the experimental limit
of 70 cm" in the end-membm (single p�) of the solid solutions stud;ed.
Components represented in broken line correspond to rotational (R) modes.
'e 0
� < • • • � «
250
Pb Ba 5, 200 •
• 0 • • •
0 • 0 150 0 • •
• • • 0 • • 0 • • • • •
100 0 • 0 • •
5O L--�--�-___ � ____ ---' 0.120 0.125 0.130 0135 0.140 0.145 0.150
h.!l·U2
FIG. 7. Wavenumber of the low ficquency fitted components against the square
root of the inverse cationic mass. Open symbols correspond to rotational (R)
mod ..
Figure 6 sho� the low frequency spectral region down 10 the experimental
limit of 70 cm" in the cnd-members (single phases) of the solid solul;ons studied.
EJclemal modes can be conveniently divided into translations en and rotations
(R). The fonner imply mainly the cation/anion interactions whereas the rotations
arc characteristic libralions of the anion. "I he R modes appear at wavcnumbers
which arc nearly independent oflhe cation present in the structure (see Figure 7).
lbe decrease in the number of components observed in barytes and, more clearly.
in anglesite is due to the experimental limit (70 cm") of the FT-Raman
spectrometer which precludes the observation or the lower wavenumber spectrum. As the T modes dt(rease their frequency when the mass of the cation
increases, observed in Figures 6 and 7, !;Cl/era] externa! modes of the samples
studied, mainly in angles;te, must appear below the limit of observation. In any
case, all these external modes are very weak in the FT-Raman spectraI3.U.l6. The
cationic substitution promotes the broadening of the T components which further
hinders the rigorous analysis of this spectral region. However, the spectral study
of those solid solutions which contain lead has enabled us to assign as rotations
the anglesite Raman bands at 134 and 152 cm·l, which were previously nOI
assigned", since their wavenwnber position and half-widths remain very
approJ[imateiy constant despite the cationic substitution.
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