High-pressure synthesis, crystal and electronic structures of a new scandium tungstate, Sc0.67WO4

Negative thermal expansion (NTE) materials possess a low-density, open structure that can respond to high pressure conditions, leading to new compounds and/or different physical properties. Here we report that one such NTE material - white, insulating, orthorhombic Sc₂W₃O₁₂ - transforms into a black compound when treated at 4 GPa and 1400 °C. The high pressure phase, Sc_0.67WO₄, crystallizes in a defect-rich wolframite-type structure, a dense, monoclinic structure (space group P2/c) containing 1-D chains of edge-sharing WO₆ octahedra. The chemical bonding of Sc_0.67WO₄ vis-à-vis the ambient pressure Sc₂W₃O₁₂ phase can be understood on the basis of the Sc defect structure. Magnetic susceptibility, resistivity, thermoelectric power and IR spectroscopic measurements suggest that the Sc_0.67WO₄ sample is a paramagnet whose conductivity is that of a metal in the presence of weak localization and electron-electron interactions. Oxygen vacancies are suggested as a potential mechanism for generating the carriers in this defective wolframite material.