A novel route to synthesize cubic ZrW2-xMoxO8 (x=0-1.3) solid solutions and their negative thermal expansion properties

Article abstract of DOI:10.1016/j.jssc.2007.09.011

Cubic ZrW_2-xMo_xO₈ (c-ZrW_2-xMo_xO₈) (x=0-1.3) solid solutions were prepared by a novel polymorphous precursor transition route. X-ray diffraction (XRD) analysis reveals that the solid solutions are single phase with α- and β-ZrW₂O₈ structure for 0≤x≤0.8 and 0.9≤x≤1.3, respectively. The optimum synthesis conditions of ZrWMoO₈ are obtained from differential scanning calorimetry-thermal gravimetric analysis (DSC-TGA), XRD and mass loss-temperature/time curves. Following the above experience, the stoichiometric solid solutions of c-ZrW_2-xMo_xO₈ (x=0-1) are obtained within 1 wt% of mass loss. The relationships of lattice parameters (a), phase transition temperatures (T_c) and instantaneous coefficients of thermal expansion (α_i) against the content x of Mo are discussed based on the variation of order degree parameters of ZrW_2-xMo_xO₈.

Full text of DOI:10.1016/j.jssc.2007.09.011

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Journal of Solid State Chemistry 180 (2007) 3160–3165
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A novel route to synthesize cubic ZrW2ꢀxMoxO8 (x ¼ 0–1.3) solid
solutions and their negative thermal expansion properties
Ã
Ruiqi Zhaoa, Xiaojing Yanga, Huiliang Wanga, Jingsa Hana, Hui Mab, Xinhua Zhaoa,
aCollege of Chemistry, Beijing Normal University, 100875, PR China
bAnalyzing and Testing Center, Beijing Normal University, 100875, PR China
Received 27 June 2007; received in revised form 5 September 2007; accepted 10 September 2007
Abstract
Cubic ZrW2ꢀxMoxO8 (c-ZrW2ꢀxMoxO8) (x ¼ 0–1.3) solid solutions were prepared by a novel polymorphous precursor transition
route. X-ray diffraction (XRD) analysis reveals that the solid solutions are single phase with a- and b-ZrW2O8 structure for 0pxp0.8
and 0.9pxp1.3, respectively. The optimum synthesis conditions of ZrWMoO8 are obtained from differential scanning
calorimetry–thermal gravimetric analysis (DSC–TGA), XRD and mass loss-temperature/time curves. Following the above experience,
the stoichiometric solid solutions of c-ZrW2ꢀxMoxO8 (x ¼ 0–1) are obtained within 1 wt% of mass loss. The relationships of lattice
parameters (a), phase transition temperatures (Tc) and instantaneous coefficients of thermal expansion (ai) against the content x of Mo
are discussed based on the variation of order degree parameters of ZrW2ꢀxMoxO8.
r 2007 Published by Elsevier Inc.
Keywords: ZrW2ꢀxMoxO8; Polymorphous precursor transition; Synthesis; Negative thermal expansion (NTE)
1. Introduction
working temperature range. Efforts have been performed
to shift Tc either below or above the working temperature
range by adjusting the composition of the compound
without changing its NTE property. Among the modifica-
tions, cubic ZrW2ꢀxMoxO8 (c-ZrW2ꢀxMoxO8) have been
investigated widely [4–8] and Tc is found decreasing
with increasing Mo [5,8]. As a representative compound,
c-ZrWMoO8, whose Tc is as low as ꢀ3 1C [7], could be used
as an applicable material or as an additive of composite for
practical uses above room temperature.
Although there are a variety of methods to synthesize
c-ZrW2O8 [5,9–15], only two methods have been reported
to prepare c-ZrW2ꢀxMoxO8 up to date, such as the
combustion route [11] and the low-temperature method
[4–6]. However both methods are either incontrollable or
time-consuming to obtain phase-pure c-ZrWMoO8. There-
fore it is very important to find a novel method to
synthesize the solid solution conveniently.
Cubic zirconium tungstate (c-ZrW2O8) has received
considerable interests because of its isotropic negative
thermal expansion (NTE) over a wide temperature range
(ꢀ273 to 777 1C) [1]. Due to the large NTE coefficient,
c-ZrW2O8 is considered to have potential applications in
many fields, especially in those requiring precisely con-
trollable positive, negative or near-zero coefficients of
thermal expansion (CTE) composites, such as optical,
electronic, engineering and dental-filling applications.
However, an order–disorder phase transition of c-ZrW2O8
occurs at 155 1C [2], around which there are different
CTEs (a is about ꢀ9 ꢁ 10ꢀ6 1Cꢀ1 for a-ZrW2O8 and
ꢀ6 ꢁ 10ꢀ6 1Cꢀ1 for b-ZrW2O8 [3]). The abrupt change of
thermal expansion is disadvantageous for applications if
the phase transition temperature (Tc) is included in the
In this work, c-ZrWMoO8 as well as c-ZrW2ꢀxMoxO8
(x=0–1.3) solid solutions were synthesized by a novel
polymorphous precursor transition route and the crystal-
lographic property, phase transition temperatures and
Ã
Corresponding author. Fax: +86 10 58802075.
E-mail address: xinhuaz@bnu.edu.cn (X. Zhao).
0022-4596/$ - see front matter r 2007 Published by Elsevier Inc.
doi:10.1016/j.jssc.2007.09.011

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3161
NTE properties of the synthesized solid solutions were
investigated systematically.
2.2. Measurements
Powder X-ray diffraction (XRD) measurement of samples
was carried out using a Philips X-Pert MPD diffractometer.
The data were collected from 101 to 901 (2y), using a step size
of 0.01671 (2y) and the counting time of 20 s per step under
40 kV and 40 mA with CuKa radiation. The reflections (2y/1)
were calibrated using the line-pair method and indexed using
Treor 90 program suited in PowderX software [17]. The
lattice parameters were deduced using Unitcell program [18]
from the XRD data, which were calibrated with SiO2 as the
internal standard.
Differential scanning calorimetry–thermal gravimetric
analysis (DSC–TGA) was performed using SDT Q600
(TA-Instruments) with air flow at a heating rate of
20 1C minꢀ1, and an open Pt crucible was used as the
container. Isothermal mass loss-temperature/time depen-
dence was measured by annealing the pellet of sample
either at different temperature for 1 h or in different time
at 940 1C.
Thermal expansion of ceramic pellets was measured
along the ceramic diametrical direction by thermal
mechanical analysis (TMA Q400, TA-Instruments) using
a macro-expansion quartz probe. The data were collected
at a heating rate of 3 1C minꢀ1 from room temperature
to 300 1C under a constant force of 0.5 N. The tempera-
tures of the minimums of the instantaneous CTE (ai) versus
temperature (T) plot were taken as the order–disorder
phase transition temperatures of ZrW2ꢀxMoxO8 [2].
2. Experimental section
2.1. Synthesis of c-ZrW2ꢀxMoxO8 (x=0–1.3) solid
solutions
Starting materials ZrOCl2 ꢂ 8H2O, (NH4)10W12O41
ꢂ
5H2O and (NH4)6Mo7O24 ꢂ 4H2O are analytical-grade
reagents. c-ZrW2ꢀxMoxO8 (x=0–1.3) were prepared using
the procedure described previously [16]. Briefly, the white
precipitates were obtained by dropping the solutions of
Zr (IV) and Mo (VI) simultaneously into the slurry of
W (VI) stoichiometrically for the preparation of ZrW2ꢀx
MoxO8 (x ¼ 0–1.3). The precipitates were dried together
with the mother liquor at 100 1C and subsequently ground
in agate mortar to obtain homogeneous powders. Then
the powders were sintered to be precursors at 600 1C
for 3 h.
After the precursors (ꢃ0.6 g) were dry-pressed to
form cylindrical pellets with a diameter of 10 mm using
a steel mould under a pressure of 4 MPa, each pellet
was put into a Pt crucible capped with a Pt foil and
annealed in a preheated furnace at different temperatures
for 1 h. c-ZrW2ꢀxMoxO8 (x ¼ 0–1.3) were obtained by
quenching the heating pellets in ambient atmosphere
immediately.
Fig. 1. XRD patterns of (a) the precursor and the precursor calcined for 1 h at (b) 870 1C (indexed with trigonal system), (c) 905 1C, (d) 913 1C ((indexed
J
with cubic system), (e) 1150 1C (K and
denote the reflections of WO3 and ZrO2, respectively).

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Calibration of the instrument was performed using an
aluminum standard.
3. Results and discussion
3.1. Reaction and transformation of c-ZrW2ꢀxMoxO8
(x ¼ 0–1.3) solid solutions
As shown in Fig. 1, the XRD pattern of ZrWMoO8
precursor (Fig. 1(a)) is identical with that of the precursor
calcined at 870 1C (Fig. 1(b)). Both of them are indexed
˚
with the trigonal crystal system with a ¼ 9.875 A,
˚
c ¼ 17.538 A, which is very similar to that of the ZrW2O8
trigonal polymorph with the lattice parameters a ¼
˚
˚
9.8101(1) A, c ¼ 17.602(2) A [19].
The comparison of XRD patterns between Fig. 1(c) and
1(d) shows that some reflections of c-ZrWMoO8 appear
besides those of trigonal ZrWMoO8 (t-ZrWMoO8) when
the precursor is calcined at 905 1C and single c-ZrWMoO8
crystallizes completely at 913 1C, which can be indexed
with the disordered b-ZrW2O8 structure with lattice
Fig. 3. The mass loss dependence on the temperature of the ZrWMoO8
pellet: (a) calcined at different working temperatures for 1 h and
(b) holding at 940 1C for different time.
˚
˚
transformation of the cubic phase. Thus, the key procedure
of preparing pure c-ZrWMoO8 is to optimize heating
temperature and holding time to reduce the sublimation of
MoO3 farthest. The time-dependence of mass loss is
proportional to the holding time linearly as shown in
Fig. 3(b). There is only ca. 1 wt% of continuous mass loss
in the temperature range of 870–950 1C as DSC–TGA
curves indicated, and less than 1 wt% mass loss within 2 h
as the isothermal mass loss temperature/time curves shown.
Consequently, phase-pure c-ZrWMoO8 is prepared be-
neath 1.8 mol% deviation of the W/Mo ratio between 913
and 950 1C within 2 h.
A contrastive experiment was performed by calcining the
mixture of ZrO2, MoO3 and WO3 in 910–950 1C for 1 h,
but pure c-ZrWMoO8 can’t be obtained. Increasing
sintering temperature to 980 1C and prolonging the holding
time to 3 h, nominal cubic ZrWMoO8 accompanied with
trace ZrO2 and WO3 can be formed. Therefore, it can be
concluded that the formation of the polymorphous
precursor stabilizes MoO3 from sublimating and is
beneficial to prepare the stoichiometrically cubic solid
solutions.
parameter a ¼ 9.1406(2) A (reference to a ¼ 9.1400(5) A
[11]). According to XRD analysis, the acute endothermic
peak in the DSC curve, as shown in Fig. 2, is attributed to
the transition from trigonal precursor to c-ZrWMoO8.
Increasing the sintering temperature above 1050 1C,
ZrWMoO8 decomposes to ZrO2, WO3 and MoO3, which
is revealed from the coexistence of the reflections of
c-ZrWMoO8, ZrO2 (JCPDS Card no. 83-0951) and WO3
(JCPDS Card no. 86-1451) as shown in Fig. 1(e). Both the
results of the variable temperature DSC–TGA determina-
tion (Fig. 2) and isothermal mass-loss measurement at
different temperature (Fig. 3(a)) indicate that the mass loss
increases drastically when temperature exceeds 1000 1C,
accompanied with a continuously endothermic process.
Considering the absence diffractions of MoO3 in Fig. 1(e),
the mass loss is attributed to the sublimation of MoO3.
Preventing the sublimation, namely preventing the
decomposition of ZrWMoO8, must be beneficial to the
Fig. 4 shows the XRD patterns of the precursors
(x ¼ 0–1.3), which can be classified into two kinds of
substances. For x ¼ 0.2–1.3, most reflections can be
assigned to t-ZrW2ꢀxMoxO8 and therefore the process
of preparing c-ZrW2ꢀxMoxO8 is similar to that of
c-ZrWMoO8. However the precursors with low x contain
large amounts of amorphous phase, although there are
some distinct reflections in the XRD patterns. The
amorphous precursors convert to the mixture of ZrO2,
WO3 and MoO3 rather than t-ZrW2ꢀxMoxO8 after
calcination for 3 h in 700–900 1C. Further increasing the
temperature up to 1160 1C and holding the annealing time
for 1 h, the precursor of ZrW1.9Mo0.1O8 is also converted
to the cubic phase instead of to the trigonal phase.
Fig. 2. DSC–TGA curves for the precursor of ZrWMoO8.

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Fig. 6. The relations of (a) lattice parameter and (b) order degree of
J
c-ZrW2ꢀxMoxO8 versus the content x of Mo. (K): the ordered; and ( ):
disordered structure type. (n): order degree (m represents the extrapolated
order degree from linear equation displayed in Fig. 8).
Fig. 4. The XRD patterns of the precursors (x ¼ 0–1.3).
Fig. 5. The XRD patterns of c-ZrW2ꢀxMoxO8. The content x of Mo and
the preparation conditions are displayed on the pattern. The indexes of
reflections are indexed with the a-ZrW2O8 structure.
Fig. 7. The dependence of instantaneous coefficient of thermal expansion
(ai) of c-ZrW2ꢀxMoxO8 on temperature.
3.2. Lattice parameters and the order degrees of c-
ZrW2ꢀxMoxO8 (x ¼ 0–1.3) solid solutions
The process of transition from trigonal precursors to
pure c-ZrW2ꢀxMoxO8 is analogous to that of c-ZrWMoO8.
The optimal couple of temperature and time emerges
through experiments and the phase-pure solid solutions of
nominal c-ZrW2ꢀxMoxO8 (x ¼ 0–1.3) are synthesized. The
XRD patterns and the synthesis conditions are displayed in
Fig. 5. In these conditions, mass loss is less than 1 wt%
except for the Mo-rich solid solutions (x41). We note that
there is a close relation between the synthesis temperature
and the content x of Mo in solid solutions. Compared
with c-ZrW2O8 [20], the synthesis temperature of pure
c-ZrW2ꢀxMoxO8 decreases markedly with the introduction
of Mo. Beyond x ¼ 1, the synthesis temperatures reverse to
higher temperature and result in more mass loss. For those
with x41.3, no pure c-ZrW2ꢀxMoxO8 is obtained by the
present method.
From the patterns shown in Fig. 5, c-ZrW2ꢀxMoxO8 can
be indexed with the ordered a-ZrW2O8 structure (S.G.:
P213) for x ¼ 0–0.8 and disordered b-ZrW2O8 structure
ðS:G: : Pa3Þ for x ¼ 0.9–1.3 [1,2]. As shown in Fig. 6, the
lattice parameters a of a-ZrW2ꢀxMoxO8 (x ¼ 0–0.8) shrink
markedly with the introduction of Mo. Meanwhile, the
relative intensity of reflection 310 gradually decreases,
as shown in Fig. 5, implying that the order degree (o) of
a-ZrW2ꢀxMoxO8 (x ¼ 0–0.8) is reduced with increase x
[6,21,22].
According to Bragg–Williams approximation [23],
T
2o
¼
(1)
Tc lnð1 þ o=1 ꢀ oÞ

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rapidly and the order degree parameter o which can be
deduced from Eq. (1) decreases synchronously. By the
same reasoning, a-ZrW2ꢀxMoxO8 at room temperature
with different ai would also be rationalized.
The dependence of Tc on the levels x of Mo is fitted
linearly. Tc of ZrWMoO8 is obtained by extrapolating the
fitted line on ꢀ4.2 1C, which is consistent with ꢀ3 1C [7].
The downtrend of Tc depending on x can be understood
from the viewpoint that there are different bond strengths
for Mo–O and W–O, respectively. With more introduction
of weaker bond Mo–O, the reversal of adjacent MO4
tetrahedrons becomes easier [2].
4. Conclusions
A series of c-ZrW2ꢀxMoxO8 (x ¼ 0–1.3) solid solutions
were successfully synthesized by the polymorphous pre-
cursor transition route. The optimum combination of
annealing temperature and holding time are summed up
from experiments. Under these conditions, stoichiometric
solid solutions are synthesized within 1 wt% of mass
loss. At room temperature, c-ZrW2ꢀxMoxO8 adopt the
a-(x ¼ 0–0.8) and b-(x ¼ 0.9–1.3) ZrW2O8 structures,
respectively. The lattice parameters (a) and phase transi-
tion temperatures (Tc) of a-ZrW2ꢀxMoxO8 decrease with
increasing x. As temperature approaches Tc, the instanta-
neous CTE (ai) of a-ZrW2ꢀxMoxO8 decrease rapidly with
increasing temperature. This trend agrees well with the ai
dependence on the content x of a-ZrW2ꢀxMoxO8 at the
same temperatures. However, ai vary slightly at the
temperatures far away from Tc, where the order degree
of c-ZrW2ꢀxMoxO8 closes to 1 or zero.
Fig. 8. The dependence of phase transition temperatures Tc on the content
x of c-ZrW2ꢀxMoxO8.
the order degree (o) of a given a-ZrW2ꢀxMoxO8 at room
temperature (supposing T ¼ 293 K) can be deduced from
its phase transition temperature (Tc, which is correlated
with content x of Mo), which is determined by TMA curves
as shown in Fig. 7. Based on the experimental results, the
dependence of o on x is established in Fig. 6, which is fitted
well with polynomial function and the value of content x
with o ¼ 0 is extrapolated to be 0.87, which is consistent
with the value deduced from linear equation displayed in
Fig. 8. The similar trends of both lattice parameter and
order degree parameter on content x appear in Fig. 6
obviously and a correlation between them emerges.
For x ¼ 0.9–1.3, ZrW2ꢀxMoxO8 is indexed with the
b-ZrW2O8 structure and the lattice parameters are
˚
˚
a ¼ 9.1417 A and 9.1406 A for x ¼ 0.9 and 1, respectively,
Acknowledgments
in which the latter value is comparable to the reported
˚
lattice parameter of ZrWMoO8 (9.1400(5) A) [11]. Over the
This work was supported by a grant from the National
Science Foundation of China: NSFC 20471010 and the
Foundation of Beijing Key Discipline of Inorganic
Chemistry. And we are also grateful to the laboratory of
TA Instruments-Waters LLC in Beijing for their help in
DSC–TGA.
content x ¼ 1, little change of lattice parameters are
observed for nominal ZrW2ꢀxMoxO8 (x ¼ 1.1, 1.2, 1.3).
Apparently, the real concentration of Mo in the presented
ZrW2ꢀxMoxO8 does not exceed 1. Considering the higher
synthesis temperature, the deviation from the nominal
composition is explicable.
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