J. Phys. Chem. 1995,99, 12975-12979
12975
Crystal Structure and Spectroscopic Characterization of M A s ~ O(M
~ = Pb, Ca). Two
Simple Salts with As06 Groups
Enrique R. Losilla? Miguel A. G. ArandaJ F. Javier Ramirez,’ and Sebastian Bruque*”
Departamento de Quimica Inorghnica and Departamento de Quimica Fisica, Facultad de Ciencias,
Universidad de Mhlaga, 29071 Mhlaga, Spain
Received: April 13, 1995; In Final Form: June 13, 1 9 9 9
The crystal structures of MAs206 (M = Pb, Ca) have been redetermined _using X-ray powder diffraction data
by the Rietveld method. These compounds are trigonal (space oup P31m) with a = 4.875(1) 8, and c =
5.504( 1) 8, for M = Pb, and a = 4.826( 1) 8, and c = 5.082( 1) for M = Ca. The agreement factors were
RWP= 11.8% and RF = 3.5% for M = Pb and RWP= 10.3% and RF = 1.4% for M = Ca. The diffraction
peaks show a very strong broadening anisotropic peak shape that has been fitted and results in a remarkable
improvement of the description of the structures. This series of compounds has arsenic atoms in a
pseudooctahedral environment of oxygens. As06 groups are very poorly spectroscopically characterized;
hence, we also report the IR and Raman data. The results of the vibrational study are in agreement with the
crystal structure, and an initial assignment of the spectra is proposed. We observe a displacement of the
intense bands for the As04 groups at ~ 8 8 cm-’
0
(IR active) and e 8 4 0 cm-I (Raman active) to e 7 4 0 and
~ 7 5 cm-’,
0
respectively, for the As06 groups.
K
Introduction
Metal phosphates and arsenates are currently of interest for
their chemical and physical properties and resulting applications
as catalysts, nonlinear optical materials, ion exchangers, and
ionic and electronic conductors.I Phosphates adopt a wide
variety of stoichiometries and structure types and are being
carefully studied. The phosphate groups may be isolated or
condensed in different ways, but they are always in a fourcoordinated oxygen environment (pseudotetrahedral geometry).
Metal arsenates have been studied much less. This may be due
to the possible toxicity of some derivatives, although the greater
flexibility of the arsenate groups can result in a wider chemistry.
Metal arsenates usually crystallize in the same structure types
as the corresponding phosphates. It is also possible to isomorphously replace arsenic by phosphorus in many cases. However,
some arsenates have different structures from those of the
analogous phosphates.
Although pentavalent arsenic atoms in the arsenate groups
are usually surrounded by four oxygen atoms, some compounds
with arsenic atoms in six-coordinated oxygen environments
(pseudooctahedral geometry) are known. The oxygen-toarsenic(V) bond distances, in four-coordinated arsenate groups,
range between approximately 1.64 to 1.70 A. The 0-As bond
distances in six-coordinated arsenate groups range between
approximately 1.80 and 1.90 A. As06 pseudooctahedral groups
are found in As~05?H ~ A s ~ OBaH&s4014$
I~?
and NaHAs~06.~
However, in these compounds, the As06 octahedra are combined
with As04 tetrahedra. There is one structure type with only
six-coordinated arsenic atoms, Mr1As2O6(M = Ca, Sr, Cd, Hg,
Pb, C O ) . ~
M**AS206compounds belong to the PbSb206 structure type.637
They crystallize in a primitive trigonal unit cell of dimensions
a FZ 4.8 8, and c x 5.0-5.5 A, depending upon the metal cation.
CaAszO6 has recently been studied using powder diffraction8
The compound is hexagonal, and its powder X-ray diffraction
I
Departamento de Quimica InorgLnica.
* Departamento de Quimica Fisica.
@
Abstract published in Advance ACS Abstracts, August 1, 1995.
0022-365419512099-12975$09.0010
pattem does not present any systematic absences, rendering eight
feasible trigonal space groups. The highest symmetry space
groups are Pglm (No. 162) centrosymetric and P j l m (No. 157)
and p312 (No. 149) noncentrosymetrics. Stefanidis et aL8chose
the space group P312, although the y coordinate for the oxygen
atoms at the general 6(1) position_ was very close to zero, y =
0.008( 15). For a model in the P31m space group, the y value
of the oxygen is zero, fixed by the symmetry. These authors
tested P31m, but although RF was lowered, the average
interatomic distances were not as satisfactory as in the P312
space group. Their model in P312 gave a poor fit (RF= 0.09),
and all isotropic temperature factors were negative.
As part of our study of metal phosphates and arsenates, we
describe the synthesis, crystal structure, and spectroscopic
characterization of lead@) and calcium metaarsenates, M.4~206
(M = Pb, Ca). The framework of PbAszO6 is different from
that of the analogous pho~phate.~
We decided to carry out a
new refinement of the structure of CaAsz06like that proposed
by Stefanidis et aL8
Experimental Section
Lead(I1) metaarsenate may be obtained by thermal decomposition of a-Pb(HAs04)2*H20 in the temperature range 270335 OC,Io but this method results in a powder with a low-quality
X-ray diffraction pattem because the sample is poorly crystalline. For this reason, we carried out the synthesis by a direct
method. PbAszO6 was obtained by refluxing lead tetraacetate
and arsenic acid.
The water amount is very important in the refluxing process,
as the arsenic acid concentration must be higher than 6 M to
avoid the formation of PbHAs04 and Pb(HAs04)2.H20 phases.
However, the As:Pb molar ratio must be close to 45 to avoid
the formation of other unidentified products. A typical procedure is given. Pb(CHsC00)4 (1.5 g) was dissolved in 19 mL
of hot glacial acetic acid in a round-bottom flask. Then, 13.5
mL of H3As.04 (80% wlw) was slowly added under constant
stimng, giving an amorphous white precipitate. The reaction
mixture was refluxed vigorously for 1 week, and the solid was
filtered off and washed with deionized water, to remove the
0 1995 American Chemical Society
Losilla et al.
12976 J. Phys. Chem., Vol. 99, No. 34, 1995
excess arsenic acid, and finally with acetone. Under these
conditions, a single phase of trigonal lead metaarsenate is
obtained. CaAs2O6 was synthesized following the method
reported by Stefanidis et a1.8
Powder X-ray diffraction profiles of the sample were recorded
on a Siemens D501 automated diffractometer at room temperature using graphite-monochromatedCu K a radiation. The data
were collected in the Bragg-Brentano ( 0 / 2 0 ) geometry mode
(reflection mode). The powder pattem for indexing lead(I1)
metaarsenate was collected in the 2 0 range 19-70' by counting
for 2 s/0.02" steps. The diffraction profiles used to refine the
structure for PbAszO6 were recorded between 19 and 130" in
2 0 , in 0.03" steps, counting for 16 s per point. For CaAs2O6,
the pattem was scanned in steps of 0.03" over the angular range
15-130" in 2 0 counting for 14 s per steps. The data were
transferred to a VAX 8530 computer for Rietveld" analysis by
the GSAS programI2using a pseudo-Voight peak shape function
and a refined background function.
Infrared spectra were recorded on a Perkin-Elmer 883
spectrometer using a dry KBr pellet containing 2% sample.
Raman spectra were recorded on a Jobin Yvon Ramanor UlooO
spectrometer using an excitation radiation wavelength at 5 14.5
nm generated by a Spectra Physics argon ion laser working at
300-500 mW. To increase the signal-to-noise ratio, a minimum
of 10 scans were accumulated for the Raman spectra, resulting
in a resolution of 2 cm-I.
Pb.41106
I
I
I
I
I
I
I
I
X "
V
0
I
I
I
I
0 3
0 4
0 5
2 - T h e t a . deg
I
0 6
I
I
O B
0 7
0 9
1
I
1 0
1 1
I
I
1 2
XlOE
1 3
2
Figure 1. Final observed (points), calculated (full line),and difference
X-ray profiles for PbAs206. Reflection marks are shown.
TABLE 1: Final Profde and Structural Parametgs for
PbAs206 and CaAs206 ( I t a h ) in Space Group P31m
(No. 162), with Esd's in Parentheses
Refinement Data
no. of pts
in refinement
3334
no. of
allowed reflctns
83
71
no. of
variables
18
20
3834
RWP
11.8
10.3
*
I
E0l I I
Results and Discussion
Structural Study of MAszOs (M = Pb, Ca). The X-ray
powder diffraction pattem of PbAszO6 was autoindexed by using
the program TRFiOR13 from the position of 14 reflections. The
result was a primitive trigonal cell, which in the hexagonal
setting has the following parameters: a = 4.8749(7) A, c =
5.5035(14) A, and V = 113.3 A3. Figures of meritt were MI4
= 8314 and F I =~46 (0.018, 17).15 This cell is very similar to
that of CaASzO6 [a = 4.824(1) A, c = 5.080( 1) A, p31.21.' We
have used the published coordinates of calcium metaarsenate8
as starting model for a Rietveld refinement of the structure of
lead(I1) metaarsenate. First, we refined the overall parameters,
background, histogram scale factor, zero shift error, unit cell
parameters, and peak shape parameters, for the Pseudo-Voight
function. After refining the overall parameters, the peak shape
was not satisfactorily described. a careful examination of the
powder pattem of PbAszO6 revealed that the (hM))peaks were
sharper than the (hkl) peaks with 1 0. This is clear evidence
of anisotropic peak broadening (along the c axis), which can
be modeled inside GSAS. The refinement was improved when
we used a Lorentzian Scherrer broadening including the
anisotropic broadening coefficients, RWPfell by 3.0%, and RF
fell by 2.0%.
At this stage, we tested P312 and P j l m space groups. The
models in both space groups gave similar RWP,Rp, and bond
distances but a slightly better RF for the last one. Hence, we
chose P31m. The final refinement involved 18 parameters and
converged to RWP= 11.8%, Rp = 8.6%, and RF = 3.5%. The
results of the refinement are given in Table 1, and the final
observed, calculated, and difference profiles are given in Figure
1.
The crystal structure of CaAs2O6 was refined using the
coordinates of Stefanidis et aL8 as the starting model but in the
P31m space group instead of P312. First, we refined the overall
parameters, background, histogram scale factor, zero shift error,
unit cell parameters, and peak shape parameters, for the pseudoVoight function. The X-ray powder diffraction pattem of
calcium metaarsenate also showed a strong anisotropic peak
I
R P
R E
8.6
7.0
3.5
1.4
Cell Data
a, A
c,
4.8698(12)
4.8258(12)
A
5.4837(2)
5.0824(7)
v,A 3
Z
ecalc.
glcm'
112.621(8)
102.501(7)
1
1
6.68
4.63
Atomic Parameters
B, A2
atom
sympos
M
la
0.OOO
0.000
0.000
0.70(3)
As
2d
113
213
112
0
6k
0.3822(14)
0.3766(6)
0.OOO
0.000
0.3126(16)
0.2969(5)
0.04(4)
0.13(2)
0.3(1)
0.20(7)
Y
X
Z
0.40(4)
TABLE 2: Bond Distances (A) and Angles (deg) for
MAs206 (M = Pb, Ca)
bond
Pb
Ca
angle
Pb
Ca
M-0 ( ~ 6 )2.530(7) 2.362(3) 0 - M - 0 ( ~ 6 ) 79.1(3) 83.6(1)
0 - M - 0 ( ~ 3 ) 180.0
180.0
0 - M - 0 ( ~ 3 ) 100.9(3) 96.4(1)
AS-0 ( ~ 6 1.833(5)
)
1.833(2) 0 - A S - 0 ( ~ 6 ) 91.7(4) 91.4(1)
0 - A S - 0 ( ~ 3 ) 79.9(4) 81.1(1)
0 - A S - 0 ( ~ 3 ) 98.2(5) 97.2(2)
0 - A S - 0 ( ~ 3 )167.1(4) 168.7(2)
M-0-As ( ~ 2 )127.6(2) 127.6(1)
AS-0-AS ( X 1) 100.1(4) 98.9(1)
broadening as in the lead analogue. In the final refinement, all
of the variables, including the anisotropic peak broadening
parameters and isotropic temperature factors, were refined
independently,giving RWP= 10.3% and RF= 1.4%. The results
of the refinements are given in Table 2 , and the Rietveld plot
is showed in Figure 2.
Stefanidis et aL8 did not take into account the anisotropic
peak broadening in the X-ray powder pattem of CaAs2O6. This
may be the source of the problems present in their refinement.
The adequate fit of the anisotropy has allowed us to refine the
crystal structures of MAs206 (M = Pb, Ca) at a much greater
level of precision.
Characterization of MAs206 (M = Pb, Ca)
;:I
1
x
I
I
J. Phys. Chem., Vol. 99, No. 34, 1995 12977
I
1
0
u
I
I
0 2
2-Theta.
0 4
I
0 6
1
0 8
I
1 0
deq
I
I
1 2
XlOE
2
Figure 2. Rietveld plot for CaAS206 as in Figure 1.
I
1000
I
I
I
800
1
l
1
20
500
v (cm.1)
Figure 5. IR spectra for (a) PbAsz06 and (b) CaAs206.
t
Figure 3. Ortep view along the [110] direction of the MAs206 structure
(c-axis vertical). Two @SO,), and one Pb layers are shown.
b
240 0
360.0
b
4 8 a o
600.0
720 0
Wavenumber (cm.1)
l
Figure 6. Raman spectra for (a) PbAszO6 and (b) CaAS206.
Figure 4. Ortep view along the c direction of the MAs206 structure.
Two (AsO~),and the intermediate Pb layers are shown. Two hexagonal
As6024 cycles are depicted. Small circles are the arsenic atoms and
the largest ones are the oxygen atoms.
MAs206 (M = Pb, Ca) belong to the PbSbzO6 structure
t ~ p e . ~The
. ~ structure is three-dimensional and built of As06
and PbO6 groups. The arsenate groups are condensed in twodimensional sheets (AsO3), in the ab plane and joined along
the c-axis direction through the M2+ cations (Figure 3). The
arsenate sheets are formed by As06 groups with six As-0 bond
distances of 1.83 8, for both M = Pb and Ca compounds. These
pseudooctahedra share three edges. By sharing two edges, a
As6024 cycle with hexagonal symmetry is generated (Figure
4). These As6024 cycles are linked together in the ab plane by
sharing a third edge. M cations are situated between the arsenate
layers, bonded to three oxygens of a As6024 cycle in the upper
sheet, and to three oxygens of another group in the downer sheet.
This coordination enerates a MO6 group with six M-0 bond
distances of 2.53 for M = Pb and 2.36 8, for M = Ca.
Vibrational Study of MAszOs. The IR spectra of PbAs206
and CaAs206are depicted in Figure 5 and the Raman spectra
in Figure 6. The unit cell of the PbSbzOetype structure is
shown in Figure 7. MAs206 (M = Pb, Ca) crystallize in the
trigonal P?lm space group, and hence, they belong to the D3d
factor group. The unit cell is primitive and contains one formula
unit. As deduced from the crystallographic study, the As atoms
are placed in D3 sites and the M cations are located in D3d sites.
The structure depicted in the Figure 7 shows two different types
of bonds, As-0 and Pb-0. However, the atomic bond
distances listed in Table 2 indicate that As-0 bonds are quite
a bit shorter than the M - 0 ones. This is a consequence of the
highest covalent character of the As-0 bonds. On this basis,
the vibrational analysis has been carried out considering the
A s ~ 0 6 ~group
as a single covalent unit linked to the metallic
1
Losilla et al.
12978 J. Phys. Chem., Vol. 99, No. 34, 1995
TABLE 4: Experimental Frequencies: Relative Intensities,"
and Proposed Assignments" for the Vibrational Spectra of
PbAs20aand C~ASZOS
CaAszOd
PbAszOd
raman
infrared
raman
infrared
freq
int
760
745
sh
vs
freq
720
598
545
513
413
w
m
360
w
262
m
370
306
TABLE 3: Correlation among the Internal Symmetry of
the AszOa2-Group Its Site Symmetry, and the Factor Group
of a Unit Cell of MAs9Or
Internal
Site
Crystal
D2h
CS
D3d
Ag\
cations by electrostatic forces. This approximation is, in our
opinion, the clearest way to assign the infrared and Raman bands
of the MAs206 compounds.
First, we consider the structure of M.4~206as built of metal
cations in the corners of the unit cell at (O,O,O)with the covalent
As*O6*- unit in the center at (l/2,I/2,l/2). Under this approximation,I6there are six lattice vibrations, which have been classified
according to the symmetry species of the factor group D3d as
2A2, 2E,. The lattice vibrations are A2,, 4-E,,, both infrared
active, in the far-infrared region, and Raman inactive. Their
usual low frequencies do not permit us to observe them. The
acoustic modes are A2,, E,,, and their frequencies strongly
depend upon the wave vector, k, and are zero for k = 0;
therefore, they can only be observed by inelastic neutron
scattering.
Second, in addition to the lattice modes, there are the socalled optical intemal modes. These are the molecular vibrations
of the covalent unit As2O6. From the crystal structure of
MAs206, we c-an infer a D2h symmetry for As206 in the unit
cell, and the 18 normal vibrations of this atomic group are
distributed as follows: 4A,
lA,
~ B I , ~ B I , , 2B2,
3B2,
1B3, 3 B3,,.
All of these vibration can be described in terms of the D3d
symmetry by means the correlation diagram shown in Table 3,
which relates intemal and site symmetry of the As206 group
with the factor group, D3d. However, and in order to take into
account the highest covalent character of the As-0 bond, we
will employ the D2h description for the As206 moiety in further
+
+
+
+
+
int
775
763
sh
vs
m
s
s
539
428
w
w
399
w
290
m
m
w
freq
int
750
617
595
m
s
s
385
m
assignment
v(As0)
v(As0)
v(As0M)
v(As0)
v(As0M)
v(As0M)
G(As0As)
G(As0As)
G(As0M)
G(As0M)
G(As0M)
vibrational analysis and assignments. As can be seen from
Table 3, each As206 normal vibration gives rise to at least one
Raman-active and one infrared-active vibration that should
appear with similar frequencies. In addition, we could observe
the A,, vibrations, which are silent in the D2h symmetry, in both
infrared and Raman spectra.
A vibrational analysis for the As-0 stretching vibrations
classifies them into the symmetry species of the D2h group as
2Ag lBIg B1, B2, B2,, 3B3,.
So, there are four Raman-active AS-0) modes and four
infrared ones. They have to be assigned to the highest frequency
bands in each spectrum, and if the crystal were purely ionic,
their frequencies would be almost identical for both Pb and Ca
compounds. The comparison between the frequencies listed in
Table 4, for both compounds, shows nonnegligible shifts for
these bands. The frequencies also confirm that all vibrations
have a mixed character involving the As-0 and M - 0 bonds.
This fact confirms that the M-0 bonds have some covalent
character and thus supports our initial hypothesis. The unambiguous assignment of the different vibrations to the symmetry
species can only be carried out with data from polarized infrared
and Raman spectra on single crystals or oriented aggregates
(film). We could not grow single crystals, and all the attempts
to make films have been unsuccessful so far.
On the basis of the above discussion, we have approximately
distinguished between stretching and bending vibrations, the
former being assigned to the highest frequencies. We have also
distinguished between As-0 and As-0-M modes, as a way
to point out greater involvement of the metallic ion in the
vibrations. As can be observed in Table 4, half of the measured
frequencies shift around 16-17 cm-' upon metal exchange,
while the rest of the frequencies shift significantly more, thus
indicating greater M - 0 character. The proposed assignments
are consistent with the fact that in both stretching and bending
vibrations, the highest frequencies have the smallest shifts as a
consequence of the biggest molecular weight of Pb in comparison with As and Ca. It is also evident from Table 4 that
similar vibrations have higher frequencies for CaAszO6 than for
the Pb analogue. This is very easily explained because Ca
cations are much more light than Pb cations, and hence, the
frequencies are displaced to higher values.
Metal arsenates generally have arsenic atoms in a tetrahedral
environment of oxygens, As04. The As-0 bond distances are
well established by many crystallographic studies and range
between 1.64 and 1.70 A. The vibrational bands of this atomic
group are also well described in standard
For arsenic
+
+
freq
a In cm-I.
s = strong; m = medium; w = weak; v = very; sh =
shoulder. Y = stretching; 6 = bending.
Figure 7. Unit cell view of the PbSbzO6-type structure.
+
int
+
+
+
+
+
+
Characterization of MAs206 (M = Pb, Ca)
atoms in a tetrahedral environment, we should expect a set of
four bands. Two are actives in the IR spectrum at ~ 8 8 and
0
460 cm-I, and the other two are actives in the Raman spectrum
at Z840 and 350 cm-I. The number of bands may increase
due to a lowering of symmetry, and the positions are slightly
displaced due to the presence of cations in the structure.
However, to the best of our knowledge, there are no welldescribed spectroscopic studies of metal arsenates with arsenic
atoms in a octahedral environment of oxygens, AsO6. There
are some crystallographicstudies, and the As-0 bond distances
range between 1.80 and 1.90 A. As the coordination number
increases, the bond distances increase, and this should be evident
in the IR and Raman spectra. MA& compounds are the best
candidates for studying this effect, as there is only one
crystallographic independent arsenic atom in a pseudooctahedral
environment.
Our study clearly shows the displacement of the IR intense
band at ~ 8 8 cm-'
0
for the As04 group to ~ 7 4 cm-'
0
for the
As06 group. In a similar way, the Raman intense band at ~ 8 4 0
cm-' for the As04 group is displaced to ~ 7 5 cm-'
0
for the
As06 group. Longer bond distances are associated with lower
force constants for these bonds, resulting in lower vibrational
frequencies. However, as we have shown by replacing Pb by
Ca, the effects of the cations cannot be neglected.
Finally, from Table 4, it is evident that the vibration bands
for Pb.4~206and CaAs206 active in the IR spectrum are inactive
in the Raman spectrum and vice versa. Hence, the exclusion
principle is followed and the crystals must be centrosymmetric
( P s l m ) , in full agreement with our crystallographic study and
in disagreement with the Stefanidis et a1.* result (P3 12) for
CaAs2O6.
J. Phys. Chem., Vol. 99, No. 34, 1995 12979
Acknowledgment. We thank DGICYT (Ministerio de Educacidn y Ciencia) for financial support, Research Project PB9311245, and Junta Andalucia, for Research Group Grant 6107/
93.
References and Notes
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(3) Jost, K. H. Worzala, H.; Thilo, E. Acta Crystallogr. 1966,21, 808.
(4) Blum, D.; Durif, A.; Guitel, J. C. Acta Crystallogr. 1977, B33,
3222.
(5) Dung, N. H.; Tanar, J. Acta. Crystallogr. 1978, 834, 3727.
(6) Magneli, A. Ark. Kemi, Mineral. Geol. 1941, ISB, 1.
(7) Wells, A. F. Structural Inorganic Chemistry, 4th ed.; Oxford
University: Oxford, 1975; pp 721 -722.
(8) Stefanidis, T.; Nord, A. G.; Kierkegaard, P. Z . Krystallogr. 1985,
173, 313.
(9) Jost, K. H. Acta. Crystallogr. 1964, 17, 1539.
(10) Frydrych, Von R.; Lohoff, K. Z.Anorg.Al1g. Chem. 1971,38,221225.
(11) (a) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. (b) The
Rietveld Method Young, R. A., Ed.; Oxford University: Oxford, 1993.
(12) Larson, A. C.; Von Dreele, R. B. Los Alamos National Laboratory,
Report LA-UR-86-748, 1987.
(13) Werner, P. E. Z. Krystallogr. 1969, 120, 375.
(14) Wolff, P. M. J . Appl. Crystallogr. 1968, 1, 108.
(15) Smith, G. S.; Snyder, R. L. J . Appl. Crystallogr. 1979, 12, 60.
(16) Tunell, G. Infrared and Raman Spectra of Crystals; Academic:
London, 1972.
(17) Nakamoto, K. Infrared Spectra of Inorganic and Coordination
Compounds, 4th ed.; Wiley-Interscience: New York, 1986; p 138.
(18) Gadsden, J. K. Infrared Spectra of Minerals and Related Inorganic
Compounds; Butterworth: London, 1975; p 26.
Jp95 1056W