Structural Aspects of the Metal-Insulator Transitions in Cr-Doped VO2

Abstract

The temperature-composition phase diagram of ${\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{2}$ with $0\ensuremath{\le}x\ensuremath{\le}0.025$ is found to contain four phases: rutile ($R$), normal monoclinic V${\mathrm{O}}_{2}$ (${M}_{1}$), and two monoclinic phases (${M}_{2}$ and ${M}_{3}$). For ${M}_{2}$ with $x=0.024$ at 298 \ifmmode^\circ\else\textdegree\fi{}K, $2{a}_{R}\ensuremath{\approx}{a}_{M}=9.0664$, $2{c}_{R}\ensuremath{\approx}{b}_{M}=5.7970$, ${a}_{R}\ensuremath{\approx}{c}_{M}=4.5255$ \AA{}, and $\ensuremath{\beta}={91.88}^{\ensuremath{\circ}}$. The ${M}_{2}$ and ${M}_{3}$ regions are separated by a volume discontinuity. There are two types of V atoms in these structures. The first type form pairs and the second type form zig-zag chains along the ${b}_{M}$ axis. This contrasts with V${\mathrm{O}}_{2}$ where all the vanadium atoms are paired. The structures $R$, ${M}_{2}$, and ${M}_{3}$ with $x=0.024$ have been refined from single-crystal x-ray data. High-pressure resistivity and x-ray measurements vs temperature give $\frac{dT({M}_{2}\ensuremath{\rightarrow}R)}{\mathrm{dP}}=\ensuremath{-}(0.9\ifmmode\pm\else\textpm\fi{}0.1)$ \ifmmode^\circ\else\textdegree\fi{}K/kbar and ${V}_{R}\ensuremath{-}{V}_{{M}_{2}}=0.11$ ${\mathrm{cm}}^{3}$/mole V${\mathrm{O}}_{2}$. At higher pressures $\frac{dT({M}_{3}\ensuremath{\rightarrow}R)}{\mathrm{dP}}=(0\ifmmode\pm\else\textpm\fi{}0.2)$ \ifmmode^\circ\else\textdegree\fi{}K/kbar. These results cannot be simply interpreted in terms of the homopolar bond model proposed for the ${M}_{1}$ phase of V${\mathrm{O}}_{2}$.