Structural aspects of the metal-insulator transitions in Cr-doped VO2

The temperature-composition phase diagram of V₁₋ₓCrₓO₂ with 0≤x≤0.025 is found to contain four phases: rutile (R), normal monoclinic VO₂ (M₁), and two monoclinic phases (M₂ and M₃). For M₂ with x=0.024 at 298 °K, $2a_R\approx a_M=9.0664$, $2c_R\approx b_M=5.7970$, $a_R\approx c_M=4.5255$ \AA{}, and β=91.88°. The M₂ and M₃ 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 VO₂ where all the vanadium atoms are paired. The structures R, M₂, and M₃ with x=0.024 have been refined from single-crystal x-ray data. High-pressure resistivity and x-ray measurements vs temperature give $\frac{dT(M_2\to R)}{dP}=-\left(0.9\pm 0.1\right)$ °K/kbar and $V_R-V_{M_2}=0.11$ cm³/mole VO₂. At higher pressures $\frac{dT(M_3\to R)}{dP}=\left(0\pm 0.2\right)$ °K/kbar. These results cannot be simply interpreted in terms of the homopolar bond model proposed for the M₁ phase of VO₂.