High temperature crystal chemistry and thermal expansion of synthetic powellite (CaMoO4): A high temperature X-ray diffraction (HT-XRD) study

Article abstract of DOI:10.1016/j.jpcs.2005.11.009

In this communication, the high temperature crystal chemistry of scheelite type (space group: I4₁/a) CaMoO₄ (powellite) is reported. The details of the high temperature crystal structures were obtained by high temperature X-ray diffractometer (HT-XRD) data in the temperature range of 25-1000 °C. The lattice thermal expansions of this compound show the an-isotropic behavior, with the coefficient of thermal expansion along c-axis (α_c) is almost double of that along the a-axis (α_a). The average values of thermal expansion coefficient for CaMoO₄ are: α_a=13.5×10^-6 and α_c=22.8×10^-6 °C. The typical thermal expansion behavior is basically controlled by the temperature-induced expansion of the Ca-O bonds, i.e. by the thermal expansion of the CaO₈ polyhedra. The MoO₄ polyhedra acts as typical rigid unit, with virtually no expansion. The preliminary data analysis for CaWO₄ indicates almost similar values of thermal expansion coefficients, viz. α_a=12.7×10^-6 and α_c=21. 4×10^-6/°C (for CaWO₄) in the same temperature range. The further details are explained in this manuscript.

Full text of DOI:10.1016/j.jpcs.2005.11.009

Journal of Physics and Chemistry of Solids 67 (2006) 774–781
www.elsevier.com/locate/jpcs
High temperature crystal chemistry and thermal expansion of synthetic
powellite (CaMoO4): A high temperature X-ray diffraction (HT-XRD) study
*
S.N. Achary, S.J. Patwe, M.D. Mathews, A.K. Tyagi
Applied Chemistry Division, Solid State Chemistry Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
Received 15 April 2005; accepted 21 November 2005
Abstract
In this communication, the high temperature crystal chemistry of scheelite type (space group: I41/a) CaMoO4 (powellite) is reported. The
details of the high temperature crystal structures were obtained by high temperature X-ray diffractometer (HT-XRD) data in the temperature range
of 25–1000 8C. The lattice thermal expansions of this compound show the an-isotropic behavior, with the coefficient of thermal expansion along c-
axis (ac) is almost double of that along the a-axis (aa). The average values of thermal expansion coefficient for CaMoO4 are: aaZ13.5!10K6 and
acZ22.8!10K6 8C. The typical thermal expansion behavior is basically controlled by the temperature-induced expansion of the Ca–O bonds, i.e.
by the thermal expansion of the CaO8 polyhedra. The MoO4 polyhedra acts as typical rigid unit, with virtually no expansion. The preliminary data
analysis for CaWO4 indicates almost similar values of thermal expansion coefficients, viz. aaZ12.7!10K6 and acZ21.4!10K6/8C (for CaWO4)
in the same temperature range. The further details are explained in this manuscript.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds; C. X-ray diffraction; D. Crystal Structure; D. Thermal expansion
1. Introduction
ions are surrounded by four oxygen atoms forming tetrahedral
unit [13,14]. The crystal structure can be related well with the
fluorite type structure where the cations get rearranged due to
difference in ionic radii.
Scheelite type ABO4 compounds have been of interest
because of their luminescent, scintillators and laser host
applications [1]. Studies of these materials under extreme,
i.e. non-ambient conditions are of further interest due to the
possibility of formation of new compounds [2–6]. In particular,
several reports on the formation of new compounds under high
temperature and high-pressure conditions are available in
literature. Systematic studies on the high-pressure show the
presence a sequence of scheelite, fergusonite and post-
fergusonite phases or wolframite phases [6–10]. Abraham
has explained the diversity in crystal chemistry of ABO4 type
molybdates with nature of A2C cation in several of his reports
[11]. It is well known that the tungstates or molybdates of the
cations of larger ionic radius, like alkaline-earth metal ions
except Mg2C, form scheelite type tetragonal lattice [12]. In this
structure, the A2C exist as a bisdisphenoid polyhedron with
eight oxygen atoms, whereas, the tungsten and molybdenum
We have been studying the crystal chemistry of various
materials under non-ambient (high temperature and/or high
pressure) conditions [15–19]. The high temperature crystal
chemistry and in particular thermal expansion of strategically
important materials bear special attraction, owing to their
applications at high temperatures. In this communication, the
high temperature behavior of CaWO4 and CaMoO4 are being
reported in details. Although, there are several reports dealing
variation of crystal structure of such compounds under high
pressure [4–10,20], only few reports dealing with thermal
expansions of these materials are available in literature [21,22].
The thermal expansion of a chemical bond arises from the
anharmonicity in the potential energy curves. The mean bond
length increases due to the increase of the amplitude of
vibration of the atoms with the temperature, which is reflected
in the expansion of the material. The crystal structure of a given
compound often provides an explanation for its thermal
expansion behavior. Sleight and his group have studied large
groups of framework materials in phosphates, tungstates,
molybdates, etc. and have arrived at a close correlation
between the structure and thermal expansion behavior
[24–29]. In addition, the packing density of the lattice also
*
Corresponding author. Tel.: C91 22 25595330; fax: C91 22
25505151/25519613.
E-mail addresses: sachary@apsara.barc.ernet.in (S.N. Achary), aktyagi@
magnum.barc.ernet.in (A.K. Tyagi).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2005.11.009

S.N. Achary et al. / Journal of Physics and Chemistry of Solids 67 (2006) 774–781
775
plays an important role in the lattice expansion [27]. The strong
covalency of the chemical bonds in general leads to rigid
polyhedral unit and tilting of such polyhedra often lead to
anomalous expansion behavior [27–29]. Khan [30] has used
the thermal expansion of a chemical bond to calculate the
lattice expansion of several complex materials. Thermal
expansion of CaMoO4 and several other scheelite type
materials have been studied by Deshpande and Suryanarayana
[23], in the temperature range of 25–350 8C. The typical values
of average thermal expansion coefficients along a and c-axes
(aa and ac) are: 10.71!10K6 and 16.17!10K6 in the
temperature range of 30–350 8. The typical thermal expansion
coefficient (aa and ac) of the CaWO4 and CaMoO4 at ambient
behavior of the WO4 and MoO4 tetrahedra in other structure
types.
2. Experimental
The CaWO4 and CaMoO4 compounds were prepared by
heating appropriate amounts of dried CaCO3 and WO3 or
MoO3 at 1200 8C. The stoichiometric compositions were
initially heated at 900 8C for tunsgates and 700 8C to avoid
loss of WO3 and MoO3 prior to reaction. The phase purity of
product obtained was confirmed by powder XRD patterns
recorded on a Philips PW1710 model powder X-ray
diffractometer. The XRD patterns of the samples at higher
temperatures were recorded in static air on a Philips X-Pert Pro
diffractometer, with Anton Parr high temperature attachment.
The well ground sample was mounted on a platinum strip,
which served the purpose of both sample holder as well as
heater. The XRD patterns were recorded in the 2q range of
10–70 8C with step and step time as 0.028 and 0.8 s. The
diffracted beam was monochromatized using a curved graphite
monochromator. The sample was heated to a desired
temperature at the rate of 20 8C/min and held for 5 min for
equilibration and then XRD data were collected. The
temperature was controlled with Eurotherm temperature
programmer, with an accuracy of G1 8C.
temperature are reported as: 6.35!10K6 and 12.38!10K6
;
7.67!10K6 and 11.88!10K6 [23], respectively. In a recent
study, Sarantopoulou et al. [22] have reported the volume
thermal expansion of CaMoO4 estimated from the Debye
model and high temperature and high pressure Raman studies.
Bayer [31] has reported the average axial thermal expansion for
CaWO4 as 13.7!10K6 and 21.5!10K6/8C (in the temperature
range of 20–1020 8C) and 11.5!10K6 and 19.2!10K6/8C (in
the temperature range of 20–520 8C). In this study, the authors
have determined the unit cell parameters by indexing the
observed reflections at different temperatures. In a wider range
of temperature (K273 to 1000 8C), Senyshyn et al. have
estimated various thermo-physical properties of CaWO4 by
lattice dynamic simulation [32]. The simulated values of
thermal expansion coefficients were appreciably good agree-
ment with experimental value up to about 550 8C. Besides,
these authors have also observed an anisotropic thermal
expansion for CaWO4. The high-pressure crystal chemistry
of these materials showing strong an-isotropic compression,
has been explained by Hazen et al. [33]. Errandonea et al.
[4,34] also reported similar anisotropic compressibility for
CaWO4, SrWO4 and LiYF4 (all scheelite type) under high
pressure. In these studies the authors have compared the high-
pressure structural transformation of several scheelite or zircon
type ABO4 compounds. It is well known that the scheelite type
tungstates and molybdates bear a close similarity under high
temperature as well as high pressure. To the best of our
knowledge, in addition to these reports no other studies dealing
with experimental results for axial thermal expansion
coefficient of CaMoO4 are available in the literature. Hence,
we have carried out the high temperature XRD studies on
CaMoO4 to elucidate the high temperature crystal chemistry of
these compounds. For a comparison, a parallel study on
CaWO4 is also carried out.
3. Results and discussion
The phase purity of the prepared CaMoO4 and CaWO4
samples is confirmed by comparing the observed powder
XRD pattern with the reported PDF 29-0351 and 41-1431
data, respectively. Further characterization of the samples
was carried out by the Rietveld refinement of the observed
powder diffraction profile using Fullprof software package
[38]. The starting model for the refinement of room
temperature phase was taken from the reported crystal
structure of CaWO4 (Ca: (4b: 0, 1/4, 5/8), Mo: (4a: 0, 1/4,
1/8) and O: (16f: x, y, z), ZZ4 and space group I41/a, no.
88) [13,14]. All the observed diffraction patterns were
Earlier, a series of framework type phosphates have also
been studied and the close correlation of inter-polyhedral angle
(M–O–P) and thermal expansion behavior was established
[16,19,27]. In all these compounds, the thermal expansion of
the MO4 and PO4 is almost negligible, but the tilting of
polyhedra governs the thermal expansion behavior. Sleight and
his group have shown that the thermal expansion of the WO4 or
MoO4 tetrahedra is almost negligible in several of the
tungstates and molybdates having framework structure
[26,28,29,32–37]. This study will be an extrapolation of the
Fig. 1. The Rietveld refinement plot of CaMoO4 at 25 8C: vertical marks
indicate the Bragg positions; lower vertical marks indicate platinum base metal
reflections.

776
S.N. Achary et al. / Journal of Physics and Chemistry of Solids 67 (2006) 774–781
Fig. 2. The profile refinement plot of CaWO4 at 25 8C: Vertical marks indicate
the Bragg positions; lower vertical marks indicate platinum base metal
reflections.
Fig. 4. The Rietveld refinement plot of CaMoO4 at 1000 8C: vertical marks
indicate the Bragg positions; lower vertical marks indicate platinum base metal
reflections.
subjected to biphasic refinements due to the presence of
platinum reflections, which appeared due to the platinum
sample holder. The full crystal structure refinement was
carried out only for CaMoO4. Initially, the scale, back-
ground and unit cell parameters were refined for both
phases together. The profile was fitted using Pseudo-Voigt
profile function. The typical half-width, mixing parameters,
preferred orientation and the asymmetry parameters were
also refined. Subsequently the position coordinates of
oxygen atoms and isotropic thermal parameters of all the
atoms are refined. In the case of CaWO4, the unit cell
parameters are determined by Le Bail fit using the Fullprof-
2K software package. In this case the scale, background and
half-width parameters along with the unit cell parameters
are refined. The goodness of the refinement was observed
from the residuals (R-values) of the refinements.
The typical observed and calculated XRD patterns along
with the difference plots for CaMoO4 for the Rietveld
refinements at ambient temperature are shown in Fig. 1. A
similar observed and calculated X-ray diffraction pattern of
CaWO4 for the Le Bail profile refinement data is shown in
Fig. 2. The refined unit cell parameters for CaMoO4 at ambient
˚
temperature are: aZ5.2261(1) and cZ11.4329(3) A,
3
VZ312.25(1) A (RpZ12.8%, RwpZ18.4%, c2Z1.2, and
˚
RBZ4.2%) and those for the CaWO4 at ambient temperature
3
˚
˚
are: aZ5.2422(1) and cZ11.3722(2) A, VZ312.53(1) A
(RpZ11.5%, RwpZ17.4%, c2Z1.5, and RBZ4.6%). These
values are in good agreement with the reported data
[11,13,31,32]. The crystal structure of CaMoO4 can be
explained by the CaO8 polyhedra and MoO4 tetrahedra as the
building blocks. The CaO8 polyhedra share four of its edges
with four other CaO8 polyhedra extending in the c-directions.
Fig. 5. The Profile refinement plot of CaWO4 at 1000 8C: vertical marks
indicate the Bragg positions; lower vertical marks indicate platinum base metal
reflections.
Fig. 3. Crystal structure of CaMoO4 at room temperature (25 8C).

S.N. Achary et al. / Journal of Physics and Chemistry of Solids 67 (2006) 774–781
777
Table 1
Unit cell parameters of CaMoO4 and CaWO4 at various temperatures
Temperature (8C)
CaMoO4
CaWO4
3
3
˚
a (A)
˚
c (A)
˚
V (A)
˚
a (A)
˚
c (A)
˚
V (A)
25
5.2261(1)
5.2305(1)
5.2365(1)
5.2429(1)
5.2496(1)
5.2562(1)
5.2635(1)
5.2710(1)
5.2789(1)
5.2866(1)
5.2950(1)
11.4329(3)
11.4496(3)
11.4711(3)
11.4936(3)
11.5175(3)
11.5425(3)
11.5685(3)
11.5963(3)
11.6244(3)
11.6555(3)
11.6869(3)
312.25(1)
313.24(1)
314.59(1)
315.93(1)
317.40(1)
318.90(1)
320.50(1)
322.18(1)
323.89(1)
325.75(1)
327.66(1)
5.2422(1)
5.2464(1)
5.2519(1)
5.2583(1)
5.2647(1)
5.2712(1)
5.2780(1)
5.2851(1)
5.2923(1)
5.2996(1)
5.3074(1)
11.3723(2)
11.3869(2)
11.4069(3)
11.4289(3)
11.4510(3)
11.4754(3)
11.4999(3)
11.5258(3)
11.5528(3)
11.5797(3)
11.6101(4)
312.52(1)
313.42(1)
314.63(1)
316.00(1)
317.39(1)
318.85(1)
320.36(1)
321.94(1)
323.57(1)
325.23(1)
327.04(1)
100
200
300
400
500
600
700
800
900
1000
3
˚
˚
Each oxygen atom of the CaO8 polyhedra is connected to one
Mo atoms. Thus, each oxygen atom is coordinated with two of
Ca and one Mo atoms. A typical three-dimensional represen-
tation of CaMoO4 unit cell is shown in Fig. 3. In other words, a
cubic close packing of Ca2C and MoO4 or WO4 tetrahedral
units in an ordered manner can explain the scheelite structure.
The powder XRD patterns of CaMoO4 recorded at various
other temperatures are refined by similar Rietveld refinement
procedures, with the starting model based on the refined results
of the previous temperature. The typical observed unit cell
parameters for CaMoO4 at 1000 8C are: aZ5.2950(1) and
cZ11.6869(4) A, VZ327.66(1) A (RpZ15.9%, RwpZ22.1%,
c2Z1.5, and RBZ5.4%). The typical observed powder XRD
pattern along with calculated and difference plot at 1000 8C is
shown in Fig. 4. A similar profile refinement plot for CaWO4 at
1000 8C is shown in Fig. 5. The refined unit cell parameters for
˚
CaWO4 at 1000 8C are: aZ5.3074(1) and cZ11.6101(3) A,
VZ327.04(1) A (RpZ16.1%, RwpZ21.8%, c2Z1.5, and
3
˚
RBZ5.4%). The observed unit cell parameters at other
temperatures are given in Table 1. The variations of unit cell
parameters for CaWO4 and CaMoO4 with temperature are
shown in Fig. 6. In the temperature range of 25–1000 8C, the
Fig. 6. Variation of unit cell parameters (a and c/2) of CaMoO4 and CaWO4
with (accuracy G0.0001 and G0.0003 for a and c/2, respectively) temperature.
For comparison of reported data on these compounds are also shown. CaMoO4
(a: &; c/2: C), CaWO4 (a: :; c/2; ;) (present data); CaWO4 (a and c/2: B)
[31]; CaWO4 (a and c/2: C) [32]; CaMoO4 (a and c/2: ,) [23]. Ambient
temperature a and c/2 data for CaMoO4 and CaWO4: 6 [12] and 7 [33].
Fig. 7. Variation of percent linear thermal expansion of CaMoO4 and CaWO4
with temperature (observed by dilatometry). For comparison the values
obtained from HT-XRD data are shown. The average relative expansion aav is
defined as haaviT ZðhlaviT Khlav 25
i Þ=hlavi25Þ, where hlaviT Zð2aCcÞ=3; a and c
are the observed unit cell parameters (of CaMoO4 and CaWo4).

778
S.N. Achary et al. / Journal of Physics and Chemistry of Solids 67 (2006) 774–781
linear thermal expansion of well-sintered pellet by dilatometer.
The percentage linear thermal expansion for both compound
recorded in the temperature range of 25–850 8C are shown in
Fig. 7. The mean coefficient of linear thermal expansion
observed for CaMoO4 and CaWO4 are 15.9!10K6 and 14.1!
10K6/8C, respectively. For a comparison of the present HT-
XRD and dilatometry data, the relative average thermal
expansion calculated from the HTXRD are also included in
Fig. 7.
A comparison of the present thermal expansion behavior
with those reported by Deshpande and Suryanarayana [23]
reveals a close agreement in the common temperature range.
However, it needs to be mentioned here that the thermal
expansion values reported by Sarantopoulou et al. [22] at
higher temperature are relatively higher than the present case.
Besides, the present observed experimental values for the
a-axis at different temperature are also in good agreement with
theoretical values of Senyshyn et al. [32] in the complete range
of temperatures. But, the calculated values of Senyshyn et al.
[32] for the c-axis show a little higher value above 500 8C.
It can be mentioned here the c/a for CaMoO4 and CaWO4 at
ambient temperature is 2.188 and 2.169, respectively. But the
unit cell volumes of the molybdates are generally smaller than
analogues tungstates. The higher c/a ratio in molybdates can be
explained by the higher cation–cation electrostatic repulsion in
CaMoO4 than that in CaWO4 [12]. The variation of c/a with
temperature shows that the c/a increases with temperature, i.e.
the c-axis has a higher rate of expansion than the a-axis. At
1000 8C, the c/a for CaMoO4 and CaWO4 increases to 2.207
and 2.188, respectively. The typical variation of the c/a with
temperature is shown in Fig. 8.
Fig. 8. Variation of c/a for CaMoO4 and CaWO4 with temperature.
linear thermal expansion coefficients along a- and c-axes for
CaMoO4 are: 13.5!10K6 and 22.8!10K6/8C, respectively. In
the same temperature range, the linear thermal expansion
coefficients for CaWO4 along a- and c-axes are: 12.7!10K6
and 21.4!10K6/8C, respectively. The volume thermal expan-
sion coefficient are found to be 50.6!10K6 and 47.5!10K6/8C
for CaMoO4 and CaWO4, respectively. These results show that
the average thermal expansion for CaWO4 is relatively lower
than that of CaMoO4. Earlier, Bayer [31] has reported the
lattice thermal expansion for CaWO4 as: aaZ13.7!10K6 and
acZ21.5!10K6/8C in the similar temperature range, which
are in close agreement with the present observed values. These
values are further substantiated by the measurement of average
The refined structural parameters for CaMoO4 at various
temperatures are summarized in Table 2. The typical inter-
atomic distances for Ca–O and Mo–O and Ca–Mo bond are
summarized in Table 3. The typical variations of Ca–O and
Mo–O distances with temperature are shown in Fig. 9. The
variation of XRD patterns and other crystallographic data as a
function of temperature implies a systematic expansion of the
crystal structure, without any discontinuity. A comparison of
various inter-atomic distances (Table 3 and Fig. 9) indicates a
Table 2
Typical refined structural parameters for CaMoO4
Temperature
Oxygen position coordinatesa
B(A2)
Rp, Rwp, c2 and RB
(8C)
x
y
z
Ca
Mo
O
25
0.146(2)
0.147(2)
0.141(2)
0.144(2)
0.144(2)
0.144(2)
0.141(2)
0.140(2)
0.144(2)
0.140(2)
0.135(2)
0.006(1)
0.008(1)
0.004(1)
0.006(1)
0.006(1)
0.006(1)
0.007(1)
0.003(2)
0.007(2)
0.005(2)
0.2083(7)
0.2081(7)
0.2085(7)
0.2068(7)
0.2069(7)
0.2075(8)
0.2071(8)
0.2044(8)
0.2056(8)
0.2044(8)
0.2000(10)
1.0(2)
1.2(2)
1.8(2)
1.5(2)
1.6(2)
2.3(2)
1.9(2)
2.6(2)
2.8(2)
2.7(3)
3.8(3)
0.5(1)
0.4(1)
0.8(1)
07(1)
1.8(4)
2.2(4)
2.7(4)
2.7(4)
2.6(4)
2.8(4)
2.8(4)
3.3(4)
3.6(4)
4.1(4)
5.7(5)
12.8, 18.4, 1.2, 4.2
12.6, 18.1, 1.2, 3.9
13.0, 18.9, 1.2, 4.3
13.9, 19.6, 1.3, 4.8
13.8, 19.9, 1.3, 4.2
14.0, 20.8, 1.3, 4.3
14.0, 20.0, 1.3, 4.6
14.6, 21.0, 1.4, 4.7
14.3, 20.0, 1.3, 4.4
14.9, 21.3, 1.4, 4.4
15.9, 22.1, 1.5, 5.4
100
200
300
400
500
600
700
800
900
1000
0.9(1)
1.1(1)
0.6(1)
1.2(1)
1.4(1)
1.4(1)
1.7(1)
K0.004(2)
a
Oxygen, 16f (x, y, z); Ca, 4b (0, 1/4, 5/8) and Mo, 4a (0, 1/4, 1/8) (space group I41/a, no. 88 and ZZ4).

S.N. Achary et al. / Journal of Physics and Chemistry of Solids 67 (2006) 774–781
779
Table 3
The typical inter-atomic distances in CaMoO4 at various temperatures
˚
Ca–O1 (A)
˚
Ca–O1 (A)
˚
Mo–O (A)
˚
Ca–Mo (A)
Temperature (8C)
Ca–Mo, Ca–Ca,
˚
Mo–Mo (A)
25
2.450(8)
2.476(8)
2.444(8)
2.469(8)
2.474(8)
2.474(8)
2.482(8)
2.495(9)
2.501(8)
2.512(10)
2.520(11)
2.474(8)
2.495(8)
2.495(8)
2.486(8)
2.487(8)
2.495(8)
2.509(8)
2.498(9)
2.509(9)
2.510(10)
2.497(11)
1.765(8)
1.762(8)
1.765(8)
1.759(8)
1.763(8)
1.767(8)
1.758(9)
1.757(9)
1.760(9)
1.756(9)
1.759(10)
3.695(0)
3.699(0)
3.703(0)
3.707(0)
3.712(0)
3.717(0)
3.723(0)
3.727(0)
3.733(0)
3.738(0)
3.744(0)
3.873(0)
3.877(0)
3.883(0)
3.890(0)
3.896(0)
3.903(0)
3.910(0)
3.918(0)
3.934(0)
3.934(0)
3.943(0)
100
200
300
400
500
600
700
800
900
1000
significantly larger expansion of Ca–O bonds compared to the
Mo–O bond. A comparison of various inter-atomic distances in
CaMoO4 (Table 3) shows that the two Ca–O bonds increase
appreciably with increasing temperature. The typical Ca–O
bond lengths at ambient temperature are: 2.450(8) and
about 0.019 and 0.00, respectively, observed in this tempera-
ture ranges. Hazen and Prewitt [35] have reported the average
thermal expansion of Ca–O bond in eight coordinated Ca2C
polyhedra as about 0.016, which is in fairly good agreement
with the present data. A close view of the refined structural
parameters, in particular the oxygen position coordinates for
CaMoO4 at various temperatures (Table 2) indicates a very
insignificant change. Such insignificant change in oxygen
position coordinates has also been observed by Hazen et al.
[33] in their high-pressure structural studies on CaMoO4. Such
behavior can be expected for the scheelite type lattice, as the
structure is made up from the hard (MO4)2K anions where MZ
W or Mo and they are surrounded by the Ca2C ions to balance
the charge. As with temperature or pressure the (MO4)2K ions
˚
˚
2.474(8) A and those at 1000 8C are: 2.52(1) and 2.50(1) A.
However, the typical Mo–O bond lengths do not show any
variation in the studied temperature range and the average Mo–
O bond length in the temperature range of 25–1000 8C is
˚
1.76(1) A. Besides, the M–M (MZCa and Mo) distances show
a gradual increase from room temperature to 1000 8C. The
average Ca–O bonds vary linearly and the typical relation of
the chemical bond with temperature is shown in Fig. 9. The
average thermal expansion of the Ca–O and Mo–O bonds is
˚
Fig. 9. Variation of Ca–O and Mo–O distances (A) in CaMoO4 with
temperature. dhCa–Oiav (A)Z2.461(2)C5.13(3)!10K5 (T), where TZtempera-
Fig. 10. Variation of CaO8 and MoO8 polyhedral volume with temperature.
3
˚
˚
ture in 8C.
VCaO8v (A) Z26.5(5)C0.0018(1) (T), where TZtemperature in 8C.

780
S.N. Achary et al. / Journal of Physics and Chemistry of Solids 67 (2006) 774–781
Fig. 11. Typical environment of CaO8 polyhedra. (a) Indicates close packed CaO8 polyhedra; and (b) connection of MoO4 with CaO8 with polyhedra.
do not show any deviation, the oxygen positions do not change
significantly with either temperature or pressure. Errandonea et
al. [34] have also observed similar features for other scheelite
type materials at high pressure.
The high pressure studies on CaWO4 and CaMoO4 [4,32]
shows a systematic decreases in c/a with pressure as against the
systematic increase with temperature of the present investi-
gation. Since, the less rigid (higher ionic) Ca–O bond has
significant expansion or compression compared to the Mo–O or
W–O bonds, the structural evolution under pressure or
temperature is basically governed by the CaO8 polyhedron.
The crystal chemistry of CaWO4 and CaMoO4 under pressure
shows a similar linear compression of the Ca–O bond with
pressure, whereas the Mo–O or W–O bonds remain almost
invariant with pressure. Besides, under pressure the structure
transforms to various other structural modifications, like
fergusonite or post-fergusonite or Wolframite due to the
increase in the packing density as well as co-ordination number
of cations. However, the expansion of lattice is the only effect
observed in the temperature driven structural evolution, in the
present study.
It is well known that the polyhedra with higher charge
cations, like, P5C, Si4C, Mo6C, W6C remains as rigid under
pressure or temperature [33]. Hence, the thermal expansion of
the unit cell of scheelite type CaMoO4 is basically governed by
the thermal expansion of the Ca–O bond and in turn with the
expansion of the CaO8 polyhedral volumes. The variation of
the CaO8 and MoO4 polyhedral volume with temperature is
shown in Fig. 10. The MoO4 polyhedral volume remains
almost unchanged over the temperature range used in this
study. Such behaviors were earlier observed in high tempera-
ture structural studies of silica and several of tungstates,
phosphates [19,26,27,32–36]. A closer insight to the unit cell of
CaMoO4 (Fig. 3) and the variation of the CaO8 polyhedra and
MoO4 tetrahedra volume indicates the c-axis should have
higher expansion as CaO8 polyhedra are closely connected
along the c-axis. Typical CaO8 polyhedra along with its
neighboring polyhedra are shown in Fig. 11(a) and (b). As it
can be seen in Fig. 11(a), a CaO8 polyhedra share four of its
edges with four other CaO8 polyhedra and extend in
c-direction. The increase in Ca–O bond causes the increase
in Ca–Ca separation, which contributes significantly towards
the expansion of c-axis. Similarly, Fig. 11(b) shows the sharing
of eight corners with eight MoO4 tetrahedra. The typical Ca–
4. Conclusion
In this study, we have determined the thermal expansion
behavior of the scheelite type CaWO4 and CaMoO4, by high
temperature XRD studies in the temperature range of 25–
1000 8C. The linear thermal expansion coefficients along a and
c-axes for CaMoO4 are: 13.5!10K6 and 22.8!10K6/8C,
respectively, and those for CaWO4 are: 12.7!10K6 and
21.4!10K6/8C, respectively. The volume thermal expansion
coefficients for CaMoO4 and CaWO4 are found to be 50.6!
10K6 and 47.5!10K6/8C, respectively. The difference in the
thermal expansion of Ca–O and Mo–O or W–O bond cause the
difference in the thermal expansion along the two axes. The
detailed structural evolution shows a significant expansion of
CaO8 polyhedra compared to MoO4 polyhedra. These studies
indicate the coefficient of thermal expansion of Ca–O bond is
relatively higher than that reported by Hazen and Prewitt.
˚
Mo distance in ab plane is about 3.69 A and that perpendicular
˚
to this plane is about 3.87 A at ambient temperature. The
increase in these two bonds again points towards the difference
in a and c-axes expansion behavior. Thus, the expansion along
a-axis or b-axis is suppressed by the presence of the rigid
MoO4 tetrahedra. The similar reason for the lower thermal
expansion of zircon and zircon type lattices are also well
known. Often, the presence of the highly rigid or covalent
polyhedra suppresses the expansion of the more ionic
polyhedra.
References
A comparison of the present results obtained for the CaWO4
and CaMoO4 under high temperature with the reported
structural variation under pressure [4,32] of these compounds
shows a fairly close but inverse relation with each other.
[1] A.A. Kaminskii, Laser Crystals, second ed., Springer, Berlin, 1990.
¨
[2] D. Errandonea, M. Somayazulu, D. Hausermann, Phys. Status Solid (b)
231 (2002) R1–R3.

S.N. Achary et al. / Journal of Physics and Chemistry of Solids 67 (2006) 774–781
781
[3] A. Grzechnik, W.A. Crichton, M. Hanfland, S.V. Smaalen, J. Phys.:
Condens. Matter 15 (2003) 7261.
¨
[4] D. Errandonea, F.J. Manjon, M. Somayazulu, D. Hausermann, J. Solid
[22] E. Sarantopoulou, C. Raptis, S. Ves, D. Christofilos, G.A. Kourouklis,
J. Phys.: Condens. Matter 14 (2002) 8925.
[23] V.T. Deshpande, S.V. Suryanarayana, J. Phys. Chem. Solids 30 (1969)
2484.
State Chem. 177 (2004) 1087.
[5] X. Wang, I. Loa, K. Syassen, M. Hanfland, B. Ferrand, Phys. Rev. B 70
(2004) 064109.
[24] J.S.O. Evans, T.A. Mary, T. Vogt, M.A. Subramanian, A.W. Sleight,
Chem. Mater. 8 (1996) 2809.
[6] A. Sen, S.L. Chaplot, R. Mittal, Phys. Rev. B 68 (2003) 134105.
[7] A. Jayaraman, B. Batlogg, L.G. VanUitert, Phys. Rev. B 28 (1983) 4774.
[8] A. Jayaraman, B. Batlogg, L.G. VanUitert, Phys. Rev. B 31 (1985) 5423.
[9] A. Jayaraman, B. Batlogg, L.G. VanUitert, J. Raman Spectrosc. 29 (1998)
305.
[25] J.S.O. Evans, T.A. Mary, A.W. Sleight, J. Solid State Chem. 137 (1998)
148.
[26] J.S.O. Evans, T.A. Mary, A.W. Sleight, J. Solid State Chem. 133 (1997)
580.
[27] T.G. Amos, A.W. Sleight, J. Solid State Chem. 160 (2001) 230.
[28] J.Z. Tao, A.W. Sleight, J. Solid State Chem. 173 (2003) 45.
[29] J.S.O. Evans, T.A. Mary, A.W. Sleight, Phyisca B 241–243 (1998) 311.
[30] A.A. Khan, Acta Crystallogr., Sect. A 32 (1976) 11.
[31] G. Bayer, J. Less-Common Met. 26 (1971) 255.
[32] A. Senyshyn, H. Kraus, V.B. Mikhailik, V. Yakovyna, Phys. Rev. B 70
(2004) 214306.
[10] V. Panchal, N. Garg, Sangeeta, S.M. Sharma, Solid State Commun. 130
(2004) 203.
[11] S.C. Abraham, J.M. Reddy, J. Chem. Phys. 43 (1965) 2533.
[12] A.W. Sleight, Acta Crystallogr., Sect. B 28 (1972) 2899.
[13] A. Zalkin, D.H. Templeton, J. Chem. Phys. 40 (1964) 501.
[14] M.I. Kay, B.C. Frazer, I. Almodovar, 40 (1964) 504.
[15] S.N. Achary, G.D. Mukherjee, A.K. Tyagi, S.N. Vaidya, J. Mater. Sci. 37
(2002) 2501.
[33] R.M. Hazen, L.W. Finger, J.W.E. Mariathasan, J. Phys. Chem. Solids 46
(1985) 253.
[16] S.N. Achary, G.D. Mukherjee, A.K. Tyagi, B.K. Godwal, Phys. Rev. B 66
(2002) 184106.
[34] D. Errandonea, J. Pellicer-Porres, F.J. Manjon, A. Segura, Ch. Ferrer, R.S.
Kumar, O. Tschauner, P. Rodriguez-Hernandez, J. Lopez-Solano, S.
Radescu, A. Mujica, Munoz, G. Aquilanti, Cond-Mat/0504133, 2005.
[35] R.M. Hazen, C.T. Prewitt, Am. Miner. 62 (1977) 309.
[36] A.K.A. Pryde, K.D. Hammonds, M.T. Dove, V. Heine, J.D. Gale,
M.C Warren, J. Phys.: Condens. Matter 8 (1996) 10973.
[37] R. Mittal, S.L. Chaplot, H. Schober, T.A. Mary, Phys. Rev. Lett. 86
(2001) 4692.
[17] S.N. Achary, O.D. Jayakumar, A.K. Tyagi, S.K. Kulshreshtha, J. Solid
State Chem. 176 (2003) 37.
[18] G.D. Mukherjee, V. Vijaykumar, S.N. Achary, A.K. Tyagi, B.K. Godwal,
J. Phys.: Condens. Matter 16 (2004) 7321.
[19] S.N. Achary, A.K. Tyagi, J. Solid State Chem. 177 (2004) 3918.
¨
[20] D. Errandonea, M. Somayazulu, D. Hausermann, Phys. Status Solidi 235
(2003) 162.
[38] J. Rodriguez-Caravajal, Fullprof 2000: a program for Rietveld, profile
matching and integrated intensity refinements for X-ray and neutron data,
Version 1.6, 2000.
[21] D. Christofilos, G.A. Kourouklis, S. Ves, J. Phys. Chem. Solid 56 (1995)
1125.