Electrical and Magnetic Properties of TiO and VO

Abstract

The cubic compounds $\mathrm{Ti}{\mathrm{O}}_{x}$ and $\mathrm{V}{\mathrm{O}}_{x}$ have a broad homogeneity range with $x$ varying from about 0.75 to 1.30 and a total vacancy content varying between 11 and 20%. Golden $\mathrm{Ti}{\mathrm{O}}_{x}$ is a typical metal with a temperature- and composition-independent resistivity of about 3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ \ensuremath{\Omega} cm, a Seebeck coefficient varying from +1 to -10 \ensuremath{\mu}V/\ifmmode^\circ\else\textdegree\fi{}C, a susceptibility of less than ${10}^{\ensuremath{-}4}$ emu/mole and a superconducting transition temperature between 0.4 and 1.0 K for all compositions. $\mathrm{V}{\mathrm{O}}_{x}$ behaves in a qualitatively similar manner for $x<1.0$. However, it exhibits semiconductor behavior for $x>1.0$, where its resistivity is highly temperature and composition dependent with an activation energy ($150<T<300$ K) rising to about 4 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$ eV for $x=1.3$. The Seebeck coefficient curve of $\mathrm{V}{\mathrm{O}}_{x}$ is sigmoid, with $\ensuremath{\alpha}$ increasing from -12 to + 22 \ensuremath{\mu}V/\ifmmode^\circ\else\textdegree\fi{}C as $x$ increases from 0.8 to 1.3. The magnetic susceptibility can be described by using a temperature-independent susceptibility and a Curie-Weiss term. $\mathrm{V}{\mathrm{O}}_{x}$ is not a superconductor above 0.3 K for any composition. The total number of vacancies in these compounds can be reduced as much as 22% by annealing at 1300\ifmmode^\circ\else\textdegree\fi{}C at pressures of about 60 kbar. This decrease in vacancy concentration is accompanied by a decrease in resistivity and Seebeck coefficient. The superconducting transition temperature of $\mathrm{Ti}{\mathrm{O}}_{x}$ is increased to as high as 1.0 K. Although $\mathrm{Ti}{\mathrm{O}}_{x}$ samples as normally prepared by arc melting are cubic with random vacancies at low temperatures, related ordered structures can be produced by annealing certain compositions at atmospheric pressure. Annealing samples with $x=1.0$ below the transition temperature of 900\ifmmode^\circ\else\textdegree\fi{}C produces an ordered monoclinic structure whose properties are discussed.