Negative thermal expansion was found for ZrW,08 from 0.3 kelvin to its decomposition temperature of about 1050 kelvin. Both neutron and x-ray diffraction data were used to solve and refine the structure of this compound at various temperatures. Cubic symmetry persists for ZrW,08 over its entire stability range. Thus, the negative thermal expansion behavior is isotropic. Essentially the same behavior was found for isostructural HfW,O,. No other materials are known to exhibit such behavior over such a broad temperature range. These materials are finding applications as components in composites in order to reduce the composites' overall thermal expansion to near zero.Negative thermal expansion 1s known in several oxide systems (1, 2). Usually, the contraction is small and anisotropic and occurs only over a small temperature range. W e have studied negative thermal expansion in cubic phases of the type ZrP2-xV,0, (3, 4). Such materials can show negative thermal expansion at high temperatures, but this behav~or does not persist below 100°C for any member of the series. The negative thermal expansion of these materials has been attributed to the transverse vibration of the central 0 in the 0,P-0-PO, or 0,V-0-VO, groups, coupled with frustration In find~ng a structural transition that would allow an ordered bending of P-0-P or V-0-V bond angles (3). By adjustment of x in ZrP2-,V,Oi, materials wlth near-zero thermal expansion can be obtained (3).The synthesis of ZrW20S was descr~bed many years ago (5, 6). This compound was reported to be cubic, but the atomic structure was not determined. Thermal exaansion data above room temperature suggested that negative thermal expansion ceased near room temperature (7). Our results ( Fig. 1) show negative thermal expansion for ZrW20, from 0.3 to 1050 K. Agreement between d l l a t o~n e t r~ data and neutron diffraction data is very good in the region where both tvoes of data were obtained., LThe neutron diffraction data also indicate that ZrW208 is cubic over the entlre temperature range. Both dilatometry data and neutron diffraction data suggest a phase transition near 430 K. It has been reported that H f W 2 0 , is isomorphous with ZrW208(8), but no thermal expansion data weregiven. Our dilatometry data on H f W 2 0 , show negative thermal expansion behavior essentially identical to that of ZrW208, including the phase transition at about 430 K.W e first solved the structure of ZrW20,

Isostructural ZrW2O8 and HfW2O8 show strong negative thermal expansion from 0.3 K up to their decomposition temperatures of approximately 1050 K. This behavior is especially unusual because these compounds are apparently cubic over their entire existence range. Detailed structural studies of ZrW2O8 were conducted using high-resolution neutron powder diffraction data taken at 14 temperatures from 0.3 to 693 K. Below 428 K, ZrW2O8 adopts the acentric space group P213 and has a well-ordered structure containing corner-sharing ZrO6 octahedra and two crystallographically distinct WO4 tetrahedra. Above the phase transition at 428 K, which appears to be second order, the space group becomes centric Pa3̄. The structure is now disordered with one oxygen site 50% occupied, suggesting the possibility of high oxygen mobility. Oxygen motion above 428 K is also suggested by dielectric and ac impedance measurements. The negative thermal expansion of ZrW2O8 and HfW2O8 is related to transverse thermal vibrations of bridging oxygen atoms. These lead to coupled rotations of the essentially rigid polyhedral building blocks of the structure. A semiquantitative model for both the negative thermal expansion and phase transition of these materials is proposed in light of the diffraction results.

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Raman spectroscopic measurements on negative-thermal-expansion (NTE) material zirconium tungstate Zr(WO4)2 at 20 K over the complete range of phonon frequencies yield 39 out of the predicted 54 optical phonons. The modes are assigned as lattice modes, and translational, librational and internal modes of the WO4 ion. High-pressure measurements in a diamond anvil cell (DAC) have revealed that in addition to the low-frequency rigid-unit modes (RUMs) several other phonons, including the bending modes of the WO4 ion, also exhibit negative Grüneisen parameter in the cubic phase. In the high-pressure orthorhombic phase above 0.3 GPa splitting of phonon modes is found to be consistent with the lowering of symmetry. Pressure-induced amorphization in this system at 2.2±0.3 GPa is argued to arise because a pressure-induced decomposition of the compound into a mixture of ZrO2 and WO3 is kinetically constrained. The temperature dependence of specific heat and thermal expansion coefficient are calculated and compared with reported results. In contrast to earlier models and calculations, which considered only the phonons below 8 meV (64 cm-1) to explain the NTE, it is shown that modes much higher than 8 meV also contribute significantly to NTE in this material.

Bulk thermal expansion properties of 19 members of the A 2 M 3 O 12 family of tungstates and molybdates were determined from room temperature to 800 ± C. The observed behavior ranges from strong negative thermal expansion (a 211 3 10 26 K 21 ) in Sc 2 W 3 O 12 to near zero thermal expansion in Al 1.68 Sc 0.02 In 0.30 W 3 O 12 .

Isotropic negative thermal expansion has been reported in cubic ZrW2O8 over a wide range of temperatures (0-1050 K). Here we report the direct experimental determination of the Grüneisen parameters of phonon modes as a function of their energy, averaged over the whole Brillouin zone, by means of high pressure inelastic neutron scattering measurements. We observe a pronounced softening of the phonon spectrum at P = 1.7 kbar compared to that at ambient pressure by about 0.1-0.2 meV for phonons of energy below 8 meV. This unusual phonon softening on compression, corresponding to large negative Grüneisen parameters, is able to account for the observed large negative thermal expansion.