Zr1-xLnxW2O8-x/2 (Ln=Eu, Er, Yb): Solid solutions of negative thermal expansion-synthesis, characterization and limited solid solubility

Article abstract of DOI:10.1016/j.jssc.2006.12.008

Zr_1-xLn_xW₂O_8-x/2 solid solutions (Ln=Eu, Er, Yb) of different substitution fractions x have been synthesized. Their X-ray diffraction (XRD) patterns have been indexed and lattice parameters calculated based on the α-ZrW₂O₈ structure. The coefficients of thermal expansion (CTEs) of these solid solutions were estimated to be -10.3×10^-6 K^-1 in temperature range of 30-100 °C. The solubility of lanthanide ions in these solid solutions decreases linearly with the increase in the radius of substituted lanthanide ions. Based on the concentration dependence of phase transition temperatures, a novel method for determination of solubility of the lanthanide ions in Zr_1-xLn_xW₂O_8-x/2 solid solutions has been developed. This method seems to be more sensitive as compared with that based on XRD technique.

Full text of DOI:10.1016/j.jssc.2006.12.008

ARTICLE IN PRESS
Journal of Solid State Chemistry 180 (2007) 852–857
www.elsevier.com/locate/jssc
Zr1ꢀxLnxW2O8ꢀx/2 (Ln ¼ Eu, Er, Yb): Solid solutions
of negative thermal expansion-synthesis, characterization
and limited solid solubility
Ã
Hai-Hua Lia, Jing-Sa Hana, Hui Mab, Ling Huanga, Xin-Hua Zhaoa,
aDepartment of Chemistry, Beijing Normal University, Beijing 100875, PR China
bAnalysis and Test Center, Beijing Normal University, Beijing 100875, PR China
Received 18 September 2006; received in revised form 28 November 2006; accepted 4 December 2006
Available online 15 December 2006
Abstract
Zr1ꢀxLnxW2O8ꢀx/2 solid solutions (Ln ¼ Eu, Er, Yb) of different substitution fractions x have been synthesized. Their X-ray
diffraction (XRD) patterns have been indexed and lattice parameters calculated based on the a-ZrW2O8 structure. The coefficients of
thermal expansion (CTEs) of these solid solutions were estimated to be ꢀ10.3 ꢁ 10ꢀ6 Kꢀ1 in temperature range of 30–100 1C. The
solubility of lanthanide ions in these solid solutions decreases linearly with the increase in the radius of substituted lanthanide ions. Based
on the concentration dependence of phase transition temperatures, a novel method for determination of solubility of the lanthanide ions
in Zr1ꢀxLnxW2O8ꢀx/2 solid solutions has been developed. This method seems to be more sensitive as compared with that based on XRD
technique.
r 2006 Elsevier Inc. All rights reserved.
Keywords: Solid solution; Solid solubility; Negative thermal expansion; Zr1ꢀxLnxW2O8ꢀx/2
1. Introduction
Ln3+ (Ln ¼ Yb, Er and Eu). It has been found that the
solubility of lanthanide cations in Zr1ꢀxLnxW2O8ꢀx/2 solid
solutions increases as the radius of the lanthanide cation
decreases. In the meantime, an approach to determine the
solubility limit of sparingly soluble Ln3+ ions has been
developed.
Substantial interest of both scientists and engineers has
been directed towards the cubic ZrW2O8-type compounds
due to their isotropic negative thermal expansion (NTE)
property over a wide temperature range (from 272.9 to
777 1C and 1107 to 1257 1C, respectively) [1–4]. Special
efforts in recent years have been devoted to improve their
thermal as well as mechanical performance through the
study of ZrW2ꢀxMoxO8 solid solutions [5–12]. Similar
effects have been observed upon substitution of Zr4+ by
lower valent cations such as Sc3+, Y3+, In3+ and Lu3+
[13,14] despite the fact that the solid solubility of these
doping ions is limited.
In this paper, we report synthesis and DSC, X-ray
diffraction (XRD)-characterization of a series of solid
solutions with cubic ZrW2O8 structure, in which the crystal
sites of Zr4+ are substituted partially by lanthanide cations
2. Experimental section
The raw materials ZrO(NO3)2 ꢂ 2H2O, Yb2O3 (99.99%),
Er2O3 (99.99%) and 5(NH4)2 ꢂ 12WO3 ꢂ 5H2O used for
samples preparation were commercially available reagents
from Beijing Chemical Reagents Company and Tianjin
Juneng Chemical Company Ltd., respectively. The num-
bers of hydrated water in the raw materials have been
checked through gravimetric determination. The samples
were prepared by dropwise addition of [(1ꢀx0) Zr4++x0
Ln3+] aqueous mixture of assigned ionic concentration x0
into the aqueous slurry consisted of given amount of W
(VI) in deionized water. The white powder obtained by
drying the reactant mixture at 100 1C was then annealed at
,
Ã
Corresponding author. Fax: +86 10 6220 7971.
E-mail address: xinhauz@bnu.edu.cn (X.-H. Zhao).
0022-4596/$ - see front matter r 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2006.12.008

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853
600 1C for 3 h, and used as the precursors of the
Zr1ꢀxLnxW2O8ꢀx/2 solid solutions. After grinding, press-
ing, the precursor pellet was sintered at high temperature
for 5 h according to the heating condition listed in Table 1
and then quenched in ambient atmosphere immediately.
Both DSC and XRD measurements were carried out by
routine procedure described previously [11]. Briefly, room
temperature and variable temperature XRD data were
collected from 101 to 1001 (2y) in ambient atmosphere on a
Philips X’-Pert MPD diffractometer and a TCU 2000 unit
is equipped to control the temperature in temperature-
variable measurements.
The lattice parameters of cubic Zr1ꢀxLnxW2O8ꢀx/2 at
different temperatures were deduced using Unitcell pro-
gram [15] from the variable temperature XRD data. The
XRD data calibrated with SiO2 (JCPDS-PDF: 33-1161,
Quartz) as internal standard were used to calculate the
lattice parameters of cubic Zr1ꢀxLnxW2O8ꢀx/2 at room
temperature. The sample used in measurements has been
dehydrated completely, so the possible effect of trace water
contained in sample was eliminated [3].
and the appearance of the exothermic peak near 860 1C is
most likely associated with the decomposition process of
ZrW2O8 into the binary oxides ZrO2 and WO3.
A close inspection of these XRD patterns in Fig. 2
reveals that at 1120 1C only the peaks corresponding to
WO3 (JCPDS-PDF: 83-0951) and ZrO2 (JCPDS-PDF: 86-
1451) occur, but a weak reflection peak at 211 (2y) appears
at 1130 1C and it approaches to saturation at 1150 1C. This
diffraction peak growing with temperature seems to be
most likely associated with 210 reflection of cubic-
Zr0.96Yb0.04W2O7.98 (as arrow shown). Hence, it is reason-
able to assign the DSC endothermic peaks at 1128–1145 1C
to the reaction responsible for cubic-Zr1ꢀxLnxW2O8ꢀx/2
formation from its parent oxides. Therefore, the tempera-
ture between the two endothermic peaks can be selected as
the reaction temperature (see Table 1).
3.2. Lattice parameters and thermal expansion
characteristics of Yb, Er, and Eu-substituted solid solutions
DSC curves in temperature range of 30–1300 1C were
measured with heating rate of 10 1C/min on DTA-404 PC
instrument (Netzsch) under air-atmosphere and an empty
Pt crucible was used as the reference.
The XRD patterns of the Zr1ꢀxLnxW2O8ꢀx/2 with
Ln ¼ Yb, Er and Eu at room temperature are presented
in Fig. 3.
3. Results and discussion
3.1. Synthesis and thermal analysis of Zr1ꢀxLnxW2O8ꢀx/2
solid solutions
The preparation conditions for Zr1ꢀxLnxW2O8ꢀx/2 solid
solutions (Ln ¼ Yb, Er and Eu) of different substitution
fractions x were selected based on their DSC patterns
combining with XRD measurement. Typical DSC-curves
of Zr1ꢀxYbxW2O8ꢀx/2 solid solutions with x ¼ 0, 0.01,
0.02, 0.03, 0.04 are displayed in Fig. 1, which indicates that
all solid solutions show a similar DSC pattern character-
ized by a single exothermic peak and two endothermic
peaks, regardless of their lanthanide substitution fraction
x. Considering the phase diagram of the ZrO2–WO3 system
reported previously [16], the endothermic peak near
1258 1C can be assigned to the melting point of ZrW2O8
Fig. 1. DSC curves of Zr1ꢀxYbxW2O8ꢀx/2 solid solutions (x ¼ 0, 0.01,
0.02, 0.03, 0.04).
Table 1
Heating conditions for preparing the solid solutions
The component of
Yb (mol%)
Optimal heating
system
The component of Er
(mol%)
Optimal heating
system
The component of Eu
(mol%)
Optimal heating
system
0
1200 1C 2 h and then
1160 1C 3 h
0
1200 1C 2 h and then
1160 1C 3 h
0
1200 1C 2 h and then
1160 1C 3 h
1
1200 1C 1 h and then
1160 1C 4 h
1
1200 1C 1 h and then
1160 1C 4 h
1
1200 1C 2 h and then
1160 1C 3 h
2, 3
1180 1C 1 h and then
1150 1C 4 h
1170 1C 1 h and then
1150 1C 4 h
2
1180 1C 1 h and then
1155 1C 4 h
1170 1C 1 h and then
1155 1C 4 h
2, 3
—
1200 1C 2 h and then
1160 1C 3 h
—
4, 5, 6, 8
3, 4, 5, 6

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Fig. 4. Dependence of lattice parameters of Zr1ꢀxLnxW2O8ꢀx/2 on the
concentration x0 of Ln3+ in the system at 25 1C.
Fig. 2. XRD patterns of Zr0.96Yb0.04W2O7.98 solid solution at different
temperatures.
lattice parameters become invariant above a ‘‘threshold’’
xth: specifically xthX0.044 for Zr1ꢀxYbxW2O8ꢀx/2 and
xthX0.033 for Zr1ꢀxErxW2O8ꢀx/2
. Undoubtedly, the
‘‘threshold’’ is associated with a partial solubility of the
lanthanide ion and the relationship between the lattice
parameter and the concentration x of Ln3+ in the solid
solutions can be described by following equations:
For Zr1ꢀxYbxW2O8ꢀx=2; a ¼ 9:1567 ꢀ 0:173x,
0oxp0:044; R : 0:9994; SD : 1:1402 ꢁ 10ꢀ4
.
ð1Þ
For Zr1ꢀxErxW2O8ꢀx=2; a ¼ 9:1564 ꢀ 0:112x;
(2)
0oxp0:033; R : 0:9923; SD : 2:2134 ꢁ 10ꢀ4
:
It should be noted, however, that the ‘‘threshold’’ xth of
Zr1ꢀxEuxW2O8ꢀx/2 is a relative low value, which is difficult
to be measured with sufficient accuracy by the traditional
method based on XRD.
The lattice parameters evaluated for Zr1ꢀxLnxW2O8ꢀx/2
solid solutions studied at different temperatures, like their
parent ZrW2O8, show clearly the NTE property, i.e., their
average coefficient of thermal expansion (CTE)
(a ¼ ðaT ꢀ aT Þ=ðaT ðT ꢀ T0ÞÞ) evaluated from the slope
Fig. 3. XRD patterns of ZrW2O8 and Zr0.98Ln0.02W2O7.99 (Ln ¼ Yb, Er,
Eu) at room temperature.
It can be seen that all solid solutions studied show the
similar XRD patterns. We can index these XRD patterns
using PowderX software [17] with a cubic unit cell
corresponding to the ZrW2O8 crystal structure. As we
know the polymorphs of cubic ZrW2O8 transforms from
the order form (denoted a-ZrW2O8 with space group: P213)
to the disorder form (denoted b-ZrW2O8 with space group:
0
0
of lattice parameter-temperature plots exhibits a invariant
CTE of approximately ꢀ10.3 ꢁ 10ꢀ6 Kꢀ1 in temperature
range of 30–100 1C, but it decreases to a limited value
about ꢀ5.1 ꢁ 10ꢀ6 Kꢀ1 at temperature above 150 1C. The
behaviors of Zr1ꢀxYbxW2O8ꢀx/2 presented in Fig. 5
services as a typical sample. Of interest to note that the
slope of lattice parameters-temperature plots at tempera-
ture below ꢃ100 1C is almost the same for all
Zr1ꢀxLnxW2O8ꢀx/2 solid solutions studied, but above this
temperature a slope change is detected upon increasing
temperature. Moreover, the plots for all Zr1ꢀxLnxW2O8ꢀx/2
solid solutions studied converge together in spite of
differences in the nature and the concentrations of the
lanthanide ions involved.
¯
Pa3) at 428 K [2]. Therefore, according to the reflection
conditions, the presence of reflection 310 enables us to
identify these solid solutions as adopting the ordered a-
ZrW2O8 structure.
The dependence of the lattice parameters on Ln3+ ionic
concentration x0 in aqueous mixture is displayed in Fig. 4,
which indicates clearly that the lattice parameters
of Zr1ꢀxLnxW2O8ꢀx/2 solid solutions decrease linearly
with increasing x0 as predicted by Vegard’s rule, but the

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855
Fig.
5. Temperature
dependence of lattice
parameters
of
0
Zr1ꢀxYbxW2O8ꢀx/2 solid solutions.
Fig. 6. Dependence of the relative order parameter ZT on the temperature
in Zr1ꢀxLnxW2O8ꢀx/2
.
The phase transition temperature (Tc) was
0
determined by extrapolating ZT0ꢃT curve to ZT ¼ 0.
3.3. Relative order degree parameter, phase transition
temperature and solid solubility
For cubic ZrW2O8, the order–disorder phase transition
is thought to be related with the orientation of WO4
groups. [2] To quantitatively describe this order–disorder
phase transition in Zr1ꢀxLnxW2O8ꢀx/2 solid solutions, we
0
introduce a characteristic parameter: ZT , which corre-
sponds to the degree of order in the WO4 orientations at a
given temperature T, which, following Yamamura et al.
[18] can be determined from the integrated intensity of
reflection 3 1 0 normalized by that of fundamental reflec-
tion 210, I310/I210
:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
½ðI310=I210ÞZr
½ðI310=I210ÞZrW O ꢄ298 K
ꢄT
.
M W O
x
1ꢀx
2
8ꢀy
0
ZT
¼
Fig. 7. Phase transition temperature (Tc) of Zr1ꢀxLnxW2O8ꢀx/2 against
2
8
the concentration (x) of Ln3+
.
0
The relative order parameter, ZT for Zr1ꢀxLnxW2O8ꢀx/2
For Z1ꢀxYbxW2O8ꢀx=2; TcðꢅCÞ ¼ 165:0 ꢀ 1485:7x,
0oxp0:042; R : 0:9942; SD : 3:3594,
(x ¼ 0, 0.01, 0.02, 0.03, 0.04 for Yb3+; x ¼ 0.01, 0.02, 0.03
for Er3+ and x ¼ 0.005, 0.01, 0.02, 0.03 for Eu) were
calculated from the refined integrated peak intensity using
Philips Profile Fitting program, [19] and their values as a
function of temperature T are displayed in Fig. 6. It is
ð3Þ
ð4Þ
ð5Þ
For Z1ꢀxErxW2O8ꢀx=2; TcðꢅCÞ ¼ 166:0 ꢀ 1300x,
0oxp0:031; R : 0:9918; SD : 2:6458,
0
evident that ZT decrease upon increasing t0emperature.
For Z1ꢀxEuxW2O8ꢀx=2; TcðꢅCÞ ¼ 168:3 ꢀ 600x,
0oxp0:016; R : 0:9820; SD : 0:8165.
For Zr1ꢀxLnxW2O8ꢀx/2 (Ln ¼ Yb, Er), the solubility
limits determined by Tcꢃx plots are in good agreement
with those determined from lattice parameter, aꢃx plots.
Therefore, it can be deduced that Tc may be used as an
additional parameter, which affords increased sensitivity to
ascertain the composition of solid solutions, especially for
those ions with very limited solubility.
We define the temperature, at which ZT approaches to
zero, as the phase transition temperature, Tc. This
characteristic te0mperature can be deduced simply by
0
extrapolating ZT ꢃT curves to ZT ¼ 0. It can be seen in
Fig. 7 that the phase transition temperatures, Tc’s obtained
in this way are sensitive to changes in both the nature and
doping level x of the Ln3+ ions. Interestingly, a remarkable
decrease in Tc is observed for x values below 1.6 mol% in
Zr1ꢀxEuxW2O8ꢀx/2 (see Fig. 7), above which a further
increase in doping level x leads the Tcꢃx plot to level off
indicating the solid solubility limit has been reached. The
linear relations between Tc’s and concentration x of Ln3+
can be described quantitatively by equations:
The limited solid solubility for selected partial solid
solutions of Zr1ꢀxLnxW2O8ꢀx/2 as a function of the radii of
corresponding lanthanide ions is displayed in Fig. 8. This

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856
correlation demonstrates that the increase in solid solubi-
lity follows the lanthanide contraction trend in a way that
is opposite to that observed for lattice parameter, i.e., the
Zr1ꢀxLnxW2O8ꢀx/2 solid solutions containing smaller
lanthanide ions exhibit a less pronounced lattice parameter
contraction, but shows a higher limited solid solubility.
Similar trends have been reported also for
Zr1ꢀxMxW2O8ꢀy solid solutions with other metal ions
M ¼ Sc3+, In3+, Y3+ and Lu3+ [13,14]. This behavior
seems to be understandable in terms of substitution-
induced distortion of the local environment around W–O
site due to charge compensation upon replacing Zr4+ by a
trivalent lanthanide ion [18].
in both cases implies that variation of the phase transition
temperature seems to be associated with the change in
relative order degree of the solid solution induced by tri-
valence ion substitution, which leads to a decrease in the
number of the orientationally ordered WO4 pairs [18]. By
the same reasoning, the ionic radius-dependence of relative
0
order parameters, Z25
solutions would also be rationalized.
of Zr1ꢀxLnxW2O8ꢀx/2 solid
ꢅ
C
4. Conclusion
Zr1ꢀxLnxW2O8ꢀx/2 partial solid solutions with Ln ¼ Yb,
Er and Eu have been synthesized. The XRD patterns of
these solid solutions have been indexed based on the cubic
a-ZrW2O8 structural and their lattice parameters were
found to be described by Vegard’s rule. The CTE of these
solid solutions was estimated to be ꢀ10.3 ꢁ 10ꢀ6 Kꢀ1 in
temperature range of 30–100 1C. The gradually change in
CTE observed at temperature above 100 1C implies that a
phase transformation occurs in a higher temperature
region. The limited solubility of lanthanide ions in
Zr1ꢀxLnxW2O8ꢀx/2 solid solutions at sintering temperature
was found to decrease with an increase in their radius.
Specifically; the limited solubility is estimated to be about
Finally, it should noted that the relative order para-
0
meters at 25 1C, Z25
of Zr1ꢀxLnxW2O8ꢀx/2 solid solu-
ꢅ
C
tions also decrease linearly with the increase in substitution
fraction x (Fig. 9) as that observed for phase transition
temperature Tc (Fig. 7). The similar x-dependence observed
4.4, 3.3 and 1.6 mol% for Yb3+, Er3+ and Eu3+
,
respectively. In the meantime, a novel method for
evaluating limited solubility of lanthanide ions in
Zr1ꢀxEuxW2O8ꢀx/2 solid solutions has been developed
based on the x-dependence of the phase transition
temperatures Tc.
Acknowledgments
Fig. 8. Lattice parameters of saturated solid solutions and limited
solubility of Zr1ꢀxLnxW2O8ꢀx/2 as a function of the radius of substituted
Ln3+ ions. The inserted star represents pure ZrW2O8 as the reference
point.
Supports for this research from the National Science
Foundation of China under Grant NSFC 20471010 and
Foundation of Beijing Key Discipline of Inorganic
Chemistry of Beijing Education Committee are gratefully
acknowledged.
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