High pressure synchrotron x-ray powder diffraction study of Sc2Mo3O12and Al2W3O12

Synchrotron x-ray powder diffraction was used to study Sc2Mo3O12 and Al2W3O12 at high pressure in a DAC. Both compounds adopt the orthorhombic Sc2W3O12 structure under ambient conditions and exhibit anisotropic negative thermal expansion. A phase transition from orthorhombic (Pnca) to monoclinic (P 21/a) symmetry was observed at ~0.25 GPa for Sc2Mo3O12 and at ~0.1 GPa for Al2W3O12 associated with a volume reduction of ~1.5–2%. A second crystalline to crystalline phase transition was clearly seen only for Sc2Mo3O12 (2.5–3.0 GPa). Peak broadening and almost complete amorphization were observed for Sc2Mo3O12 at ~8 GPa, and this was not fully reversible on decompression. At 7 GPa, the amorphization of Al2W3O12 was not as advanced as for the molybdate and on decompression crystalline material was recovered. The compressibility of orthorhombic Sc2Mo3O12 is highly anisotropic, but it is almost isotropic for both monoclinic Sc2Mo3O12 and Al2W3O12. Both compounds show a reduction in their bulk moduli (K0) at the orthorhombic to monoclinic transition: 32(2) GPa for orthorhombic and 16(1) GPa for monoclinic Sc2Mo3O12, and 48 GPa for orthorhombic and 28(1) GPa for monoclinic Al2W3O12. Sc2Mo3O12 displays very similar high pressure behaviour to the previously studied Sc2W3O12.