Negative thermal expansion in ZrW{sub 2}O{sub 8} and HfW{sub 2}O{sub 8}
- Oregon State Univ., Corvallis, OR (United States); and others
Isostructural ZrW{sub 2}O{sub 8} and HfW{sub 2}O{sub 8} 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 ZrW{sub 2}O{sub 8} were conducted using high-resolution neutron powder diffraction data taken at 14 temperatures from 0.3 to 693 K. Below 428 K, ZrW{sub 2}O{sub 8} adopts the acentric space group P2{sub 1}3 and has a well-ordered structure containing corner-sharing ZrO{sub 6} octahedra and two crystallographically distinct WO{sub 4} tetrahedra. Above the phase transition at 428 K, which appears to be second order, the space group becomes centric Pa3. 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 ZrW{sub 2}O{sub 8} and HfW{sub 2}O{sub 8} 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. 71 refs., 15 figs., 3 tabs.
- OSTI ID:
- 518368
- Journal Information:
- Chemistry of Materials, Vol. 8, Issue 12; Other Information: PBD: Dec 1996
- Country of Publication:
- United States
- Language:
- English
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