  Negative thermal expansion (NTE) materials possess a low-density, open
structure which 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 oC. The high pressure phase,
Sc0.67WO4, 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 Sc0.67WO4 vis-a-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 reveal that Sc0.67WO4 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.
