High-pressure synthesis, crystal and electronic structures of a new scandium tungstate Sc$_{0.67}$WO$_{4}$

Negative thermal expansion (NTE) materials possess a low-density, open structure that can respond to high pressure conditions, with potential for new compounds with unusual physical properties. Here we report that one such NTE material---white, insulating, orthorhombic Sc$_{2}$W$_{3}$O$_{12}$---transforms into a black, electrically conducting compound when treated at 4 GPa and 1400 $^{\circ}$C. The high pressure phase, Sc$_{0.67}$WO$_{4}$, crystallizes in a defect-rich wolframite-type structure, a dense, monoclinic structure (space group $P12/c1)$ containing 1-D chains of edge-sharing WO$_{6}$ octahedra.. The chemical bonding of Sc$_{0.67}$WO$_{4}$ vis-\`{a}-vis the ambient pressure Sc$_{2}$W$_{3}$O$_{12}$ phase can be understood on the basis of the Sc defect structure. Magnetic susceptibility, electrical conductivity, and thermoelectric power measurements reveal that Sc$_{0.67}$WO$_{4}$ is a metallic paramagnet. Conductivity varies linearly with temperature from 3-300 K, which may be understood in terms of weak localization and electron-electron interactions in this poor metal. Oxygen vacancies are suggested as a potential mechanism for generating the carriers in this defective wolframite.