Negative Thermal Expansion in ZrW2O8 and HfW2O8
Isostructural ZrW 2 O 8 and HfW 2 O 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 2 O 8 were conducted using high-resolution neutron powder diffraction data taken at 14 temperatures from 0.3 to 693 K. Below 428 K, ZrW 2 O 8 adopts the acentric space group P 2 1 3 and has a well-ordered structure containing corner-sharing ZrO 6 octahedra and two crystallographically distinct WO 4 tetrahedra. Above the phase transition at 428 K, which appears to be second order, the space group becomes centric Pa 3̄. 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 2 O 8 and HfW 2 O 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.