Structural investigation of the negative-thermal-expansion material ZrW2O8

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 α-ZrW₂O₈ has a negative coefficient of thermal expansion, α = −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. α-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.