Further contraction of ZrW2O8 [9]

Full text of DOI:10.1021/ja992569+

10432
J. Am. Chem. Soc. 1999, 121, 10432-10433
Further Contraction of ZrW2O8
N. Duan, U. Kameswari, and A. W. Sleight*
Department of Chemistry, Oregon State UniVersity
153 Gilbert Hall, CorVallis, Oregon 97331-4003
ReceiVed July 20, 1999
Insertion of water into the framework of cubic ZrW2O8 is found
to result in a volume contraction of 10% without shortening of
Zr-O or W-O bond lengths. The contraction is related to twisting
motions of the WO4 tetrahedra and ZrO6 octahedra, which are
linked by corner sharing (Figure 1). Presumably, hydrogen
bonding forces are responsible for pulling in the framework.
Neutron diffraction data obtained on ZrW2O8 at 1443 K show
that its unusual negative thermal expansion has continued from
1050 K at about the same magnitude as below 1050 K. Analysis
of these data shows that some interatomic distances are increasing
with increasing temperature as the overall structure is contracting.
This suggests a contribution to negative thermal expansion not
previously considered.
Figure 1. Expanded (a) and contracted (b) structures of ZrW2O8 shown
as WO4 tetrahedra and ZrO6 octahedra. A slight twisting of the ZrO6
octahedra causes the unit cell contraction.
Zirconium tungstate, ZrW2O8, is apparently thermodynamically
stable only from about 1380 to 1530 K.1 It can, however, be
quenched from high temperature to room temperature. It is then
kinetically stable to about 1050 K. A negative thermal expansion
coefficient (∆l/l) for ZrW2O8 has been reported from 0.3 to 1050
K.2,3 The magnitude of this coefficient is large at room temper-
ature, being -9.1 × 10-6 K-1 for ZrW2O8 compared to +6.5 ×
10-6 K-1 for R-Al2O3. This behavior for ZrW2O8 is especially
remarkable and useful considering its cubic crystal structure. This
thermal contraction has been attributed to transverse vibrational
modes of O in Zr-O-W linkages.2-4 Heat capacity measure-
ments, inelastic neutron scattering experiments, and analysis of
the thermal expansion data have provided experimental evidence
for these low-energy modes.5-7 No structural information on
ZrW2O8 has been previously reported in the temperature range
where it is thermodynamically stable.
Figure 2. Unit cell edge of cubic ZrW2O8 vs temperature. The broken
line connects the low-temperature data to a single point at 1443 K.
Neutron diffraction data8 obtained at 1443 K show that the
negative thermal expansion of ZrW2O8 continues up to this
temperature (Figure 2). Above a phase transition at 428 K, the
pair of WO4 tetrahedra may be considered to have merged into a
W2O8 group with a bridging oxygen (Figure 3).3 The O3 position
is 50% occupied, and ZrW2O8 has been shown to possess high
* Address correspondence to this author.
(1) Chang, L. L. Y.; Scroger, M. G.; Phillips, B. J. Am. Ceram. Soc. 1967,
50, 211.
(2) Mary, T. A.; Evans, J. S. O.; Vogt, T.; Sleight, A. W. Science 1996,
272, 90.
Figure 3. Part of the ZrW2O8 structure where the bonds between W,
O2, and O3 define the 3-fold axis. The O2 atom is on an inversion center.
The O1 atoms are corners of ZrO6 octahedra.
(3) Evans, J. S. O.; Mary, T. A.; Vogt, T.; Subramanian, M. A.; Sleight,
A. W. Chem. Mater. 1996, 8, 2809.
(4) Pryde, A. K. A.; Hammonds, K. D.; Dove, M. T.; Heine, V.; Gale, J.
D.; Warren, M. C. Phase Trans. 1997, 61, 141.
(5) Ernst, G.; Broholm, C.; Kowach, G. R.; Ramirez, A. P. Nature 1998,
396, 147.
(6) Ramirez, A. P.; Kowach, G. R. Phys. ReV Lett. 1998, 80, 4903.
(7) David, W. I. F.; Evans, J. S. O.; Sleight, A. W. Europhys. Lett. 1999,
46, 661.
(8) A ZrW2O8 pellet 0.5 in. in diameter and 1 in. in length was suspended
with Pt wire in a furnace on the BT-1 high-resolution diffractometer at the
NIST reactor. A Cu(311) monochromator with a 90° takeoff angle, θ ) 1.5402
Å, and in-pile collimation of 15 ft were used to collect data from 3 to 168°
2θ with a step size of 0.05°. The sample was heated as rapidly as possible to
1403 K. A neutron diffraction pattern obtained at this temperature indicated
that ZrW2O8 had largely decomposed into ZrO2 and WO3. The temperature
was raised to 1443 K, and another diffraction pattern was obtained which
showed a strong diffraction pattern of cubic ZrW2O8 with a cell edge of 9.102
Å. Refinement of these data by the Rietveld method using GSAS software in
space group Pa3h yielded final agreement factors of xRp ) 6.18% and Rp )
5.11%. Final refined positional parameters with Zr at 0, 0, 0 and O2 at 1/2,
1/2, 1/2 were x(W) ) 0.363(1), x(O1) ) 0.4403(8), y(O1) ) 0.2079(6), z(O1)
) 0.4309(8), and x(O3) ) 0.223(1).
oxygen mobility.3 Thus, this portion on the 3-fold axis is a mixture
of O3-W-O2-W and W-O2-W-O3 configurations, and to
a lesser extent O3-W-O2-W-O3 and W-O2-W configura-
tions. Although the thermal ellipsoids in Figure 3 all have the
shapes expected for normal thermal motion, these ellipsoids reflect
in part the different atomic positions for the different configura-
tions. With increasing temperature, there is a significant expansion
of W-O bond lengths along the 3-fold axis. Distances at 483
and 1443 K are respectively 2.090 and 2.157 Å for W-O2 and
2.148 and 2.215 Å for W-O3. There is, on the other hand, a
considerable shortening for the apparent W-O1 distance of 1.778
to 1.694 Å on going from 483 to 1443 K. This decrease will
directly contribute to the negative thermal expansion of ZrW2O8,
whereas the increasing W-O2 and W-O3 distances have no
impact on the thermal expansion of ZrW2O8. The apparent
10.1021/ja992569+ CCC: $18.00 © 1999 American Chemical Society
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Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 44, 1999 10433
decrease in the W-O distance may be largely due to the
transverse thermal motion of O in the Zr-O-W linkages, as we
and others have previously suggested.2-7 In Y2W3O12 which also
shows strong negative thermal expansion, we found a decrease
for the apparent W-O distance of 1.775 to 1.72 Å from 15 to
1373 K, which was attributed to transverse thermal motion in
Y-O-W linkages.9 The apparent W-O1 distance found for
ZrW2O8 at 1443 K is significantly shorter than that found in
Y2W3O12 at 1373 K. This suggests that an additional factor is
important in the negative thermal expansion of ZrW2O8. The
weakening of the W-O2 and W-O3 bonds with increasing
temperature causes the W-O1 bonds to become stronger and
shorter.
Hydration of ZrW2O8 is not observed at room temperature or
at 100 °C under water. However, we find that exposing ZrW2O8
to water at higher temperature can cause insertion of water into
this open framework structure up to 1 mol of water for 1 mol of
ZrW2O8.10 This water insertion causes a dramatic decrease in the
unit cell size of ZrW2O8 (Figure 4) demonstrating that the ZrW2O8
framework can shrink much more than occurs by thermal
contraction up to 1443 K. Assuming the thermal expansion
coefficient of -3.9 × 10-6 K-1 observed for ZrW2O8 from 483
to 1443 K, a temperature of over 8000 K would be required for
thermal contraction to equal the impact of inserted water. Given
the compressibility of 1.44 × 10-3 kbar-1 measured11 for ZrW2O8,
a pressure of about 700 kbar would be required to equal the
compressive effect of inserted water. Once the water has been
inserted into ZrW2O8, its thermal expansion becomes weakly
positive.12 We conclude that the inserted water pulls in the
structure through hydrogen bonding. The water within the
Figure 4. The 298 K unit cell edge of ZrW2O8 on hydration.
framework disrupts the dynamic rocking motions of the WO4
tetrahedra and ZrO6 octahedra which are primarily responsible
for the thermal contraction of ZrW2O8. The hydration reactions
are readily reversible with anhydrous cubic ZrW2O8 being
reformed on heating the hydrated samples to 423 K in air.
Considering the conditions required for hydration of ZrW2O8, this
hydration reaction will not likely be a problem for applications
of ZrW2O8. However, we have observed that in samples with Mo
substituted for W this hydration reaction occurs slowly in air at
room temperature.
The basic framework formula for ZrW2O8 is ZrW2O6 where
all framework oxygen is shared between one Zr and one W atom.
The additional nonframework oxygen atoms are bound to W only.
Any dimensional change in the framework must be reflected in
framework bond distances or in the Zr-O-W angle. Variation
in the Zr-O-W angle is not a significant factor in the thermal
contraction of ZrW2O8. However, the framework collapse caused
by the water insertion has occurred with a significant decrease in
the Zr-O-W angle and thus also in the Zr-W distance.13
(9) Forster, P. M.; Sleight, A. W. Inter. J. Inorg. Mater. 1999, 1, 123.
(10) A typical experiment was based on heating 1 g of ZrW2O8 in a 40
mL Parr bomb with 15 mL of water for 24 h at 180 °C. Water content of the
product was determined by thermogravimetric analysis.
(11) Evans, J. S. O.; Hu, Z.; Jorgensen, J. D.; Argyriou, D. N.; Short, S.;
Sleight, A. W. Science 1997, 275, 61.
(12) From 15 to 298 K, thermal expansion coefficients of +1.9 and +2.0
× 10-6 K-1 were found for ZrW2O8‚0.55H2O and ZrW2O8‚0.75H2O,
respectively.
(13) Rietveld refinements were conducted on three partially hydrated
samples using data collected at both 15 and 298 K. These refinements confirm
that the framework has remained intact, that the space group is Pa3h, and that
the Zr-O-W angle has decreased by about 3° in a 55% hydrated sample.
Exact placement of the inserted water was precluded by the several types of
disorder present: the partial occupancy of the water and orientational disorder
for both water and the WO4 tetrahedra.
Acknowledgment. This work was supported by NSF grant DMR-
9802488. Assistance from Brian Toby of NIST was essential in the
collection of the neutron diffraction data.
JA992569+