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

The temperature-composition phase diagram of ${\mathrm{V}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_2$ with $0\le x\le 0.025$ 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 $x=0.\mathrm{}024$ at 298 °K, $2a_R\approx a_M=9.\mathrm{}0664$, $2c_R\approx b_M=5.\mathrm{}7970$, $a_R\approx c_M=4.\mathrm{}5255$ Å, and $\beta ={91.88}^{\circ }$. 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 $x=0.\mathrm{}024$ have been refined from single-crystal x-ray data. High-pressure resistivity and x-ray measurements vs temperature give $\frac{dT(M_2\to R)}{\mathrm{dP}}=-(0.\mathrm{}9\pm 0.1)$ °K/kbar and $V_R-V_{M_2}=0.\mathrm{}11$ ${\mathrm{cm}}^3$/mole V${\mathrm{O}}_2$. At higher pressures $\frac{dT(M_3\to R)}{\mathrm{dP}}=(0\mathrm{}\pm 0.2)$ °K/kbar. These results cannot be simply interpreted in terms of the homopolar bond model proposed for the $M_1$ phase of V${\mathrm{O}}_2$.