Acta Crystallographica Section B

Structural Science

Volume 55, Part 3 (June 1999)

research papers

Acta Cryst. (1999). B55, 333-340 [ doi:10.1107/S0108768198016966 ]

Structural investigation of the negative-thermal-expansion material ZrW₂O₈

J. S. O. Evans, W. I. F. David and A. W. Sleight

Abstract: High-resolution powder diffraction data have been recorded on cubic ZrW₂O₈ [a = 9.18000 (3) Å at 2 K] at 260 temperatures from 2 to 520 K in 2 K steps. These data have confirmed that [alpha] -ZrW₂O₈ has a negative coefficient of thermal expansion, [alpha] = -9.07 × 10^-6 K^-1 (2-350 K). A `parametric' approach to Rietveld refinement is adopted and it is demonstrated that a full anisotropic refinement can be performed at each temperature, despite using a data collection time of only 5 min. Examination of the resulting structural parameters suggests that the origin of the contraction with increasing temperature can be traced straightforwardly to the rigid-body transverse librations of bridging O atoms. [alpha] -ZrW₂O₈ undergoes a phase transition from P2₁3 to Pa3 at 448 K that is associated with the onset of considerable oxygen mobility. The phase transition can be described in terms of a simple cubic three-dimensional Ising model. Unusual kinetics are associated with this phase transition. Hysteresis in the cell parameter through the phase transition is the opposite of that normally observed.