Neutron powder diffraction study of the orthorhombic to monoclinic transition in Sc2W3O12 on compression

Time-of-flight powder neutron diffraction combined with Rietveld analysis was used to study Sc2W3O12 at pressures of up to 0.6 GPa in a large volume helium gas cell. Orthorhombic (Pnca) Sc2W3O12, bulk modulus 32(3) GPa, transforms to a softer monoclinic (P2(1)/a) structure, 11.8(8) GPa, on compression at between 0.25 and 0.30 GPa. This transition shows significant hysteresis and involves a volume reduction of similar to 1.5%. The volume reduction during compression of the orthorhombic phase is much less anisotropic than that induced by heating this negative thermal expansion phase at ambient pressure. While the volume reduction on heating orthorhombic Sc2W3O12 is associated with pronounced changes ill Sc-O-W angles, there are no major changes in these angles on compression to similar densities. The structural changes seen at the pressure induced orthorhombic to monoclinic transition in Sc2W3O12 are similar, although not identical, to those seen for the equivalent thermally induced transition in Sc2Mo3O12. (c) 2006 Elsevier SAS. All rights reserved.