High pressure synchrotron x-ray powder diffraction study ofSc2Mo3O12 and Al2W3O12

Abstract

Synchrotron x-ray powder diffraction was used to studySc2Mo3O12 and Al2W3O12 at high pressure in a DAC. Both compounds adopt the orthorhombicSc2W3O12 structure under ambient conditions and exhibit anisotropicnegative thermal expansion. A phase transition from orthorhombic(Pnca) to monoclinic(P 21/a) symmetrywas observed at ∼0.25 GPa for Sc2Mo3O12 andat ∼0.1 GPa forAl2W3O12 associated with avolume reduction of ∼1.5–2%. A second crystalline to crystalline phase transition was clearly seen only forSc2Mo3O12 (2.5–3.0 GPa). Peak broadening and almost complete amorphization were observed forSc2Mo3O12 at∼8 GPa, and this was not fully reversible on decompression. At 7 GPa, the amorphization ofAl2W3O12 was not as advanced as for the molybdate and on decompressioncrystalline material was recovered. The compressibility of orthorhombicSc2Mo3O12 is highly anisotropic, but it is almost isotropic for both monoclinicSc2Mo3O12 and Al2W3O12. Both compounds show a reduction in their bulk moduli(K0) atthe orthorhombic to monoclinic transition: 32(2) GPa for orthorhombic and 16(1) GPa for monoclinicSc2Mo3O12, and 48 GPa for orthorhombic and 28(1) GPa for monoclinicAl2W3O12.Sc2Mo3O12 displays very similar high pressure behaviour to the previously studiedSc2W3O12.