Structural investigation of the negative-thermal-expansion material ZrW2O8

High-resolution powder diffraction data have been recorded on cubic ZTW(2)O (8) [a = 9.18000 (3) Angstrom at 2 K] at 260 temperatures from 2 to 520 K i n 2 K steps. These data have confirmed that alpha-ZTW(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 demonstra ted that a full anisotropic refinement can be performed at each temperature , despite using a data collection time of only 5 min. Examination of the re sulting structural parameters suggests that the origin of the contraction w ith increasing temperature can be traced straightforwardly to the rigid-bod y transverse librations of bridging O atoms. alpha-ZrW2O8 undergoes a phase transition from P2(1)3 to Pa (3) over bar at 448 K that is associated with the onset of considerable oxygen mobility. The phase transition can be des cribed in terms of a simple cubic three-dimensional Ising model. Unusual ki netics are associated with this phase transition. Hysteresis in the cell pa rameter through the phase transition is the opposite of that normally obser ved.