Structural investigation of the negative-thermal-expansion material ZrW2O8.
High-resolution powder diffraction data have been recorded on cubic ZrW(2)O(8) [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-ZrW(2)O(8) has a negative coefficient of thermal expansion, alpha = -9.07 x 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-ZrW(2)O(8) undergoes a phase transition from P2(1)3 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.
Evans
JS
Department of Chemistry, Science Laboratories, South Road, Durham DH1 3LE, England. john.evans@durham.ac.uk
David
WI
Sleight
AW