Structural Phase Transitions and Semiconductor-Metal Transition in WO₃

Abstract

Semiconducting WO₃ with the donor electrons trapped in oxygen vacancies shows so-called ‘semiconductor-metal transition’ at the triclinic-monoclinic (I) phase transition temperature at about 17°C. Resistivity discontinuities as large as a factor 10² have been observed at the monoclinic: (II)-triclinic phase transition at about −40°C.
The semiconductor-metal transition is interpreted by the consideration that the energy gap between the t_2g orbitals of 5d¹ conduction electrons and the donor level are enlarged by the crystalline distortions due to the condensations of the soft phonon modes. The resistivity discontinuities are interpreted as due to the different energy gaps in the monoclinic (II) and triclinic phases.
The essential difference between WO₃ and ReO₃ having the same crystal structure is discussed.