COMPRESSIBILITY, PHASE-TRANSITIONS, AND OXYGEN MIGRATION IN ZIRCONIUMTUNGSTATE, ZRW2O8

In situ neutron diffraction experiments show that at pressures above 2 kilobars, cubic zirconium tungstate (ZrW2O8) undergoes a quenchable p hase transition to an orthorhombic phase, the structure of which has b een solved from powder diffraction data. This phase transition can be reversed by heating at 393 kelvin and 1 atmosphere and involves the mi gration of oxygen atoms in the lattice. The high-pressure phase shows negative thermal expansion from 20 to 300 kelvin. The relative thermal expansion and compressibilities of the cubic and orthorhombic forms c an be explained in terms of the ''cross-bracing'' between polyhedra th at occurs as a result of the phase transition.