Structures and a Two-Band Model for the SystemV1−xCrxO2

The evolution with $x$ of the monoclinic-to-tetragonal transition temperature and of the room-temperature monoclinic structures of the system ${\mathrm{V}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_2$ have been reexamined and extended to cover the entire solid-solution range $0\le x<\mathrm{}0.20\mathrm{}$. The space group of the room-temperature monoclinic phase changes from $\frac{P{2}_1}{c}$ at $x=0\mathrm{}$ to $\frac{C2\mathrm{}}{m}$ for $0.005\le x\le 0.065$ and $\frac{P2\mathrm{}}{m}$ for $0.07\le x<\mathrm{}0.20\mathrm{}$. The monoclinic $\frac{P2\mathrm{}}{m}$ structure is determined for the first time. The distortion from tetragonal symmetry is accomplished by a contraction of the V-O bonds in the tetragonal ${\mathrm{}\left(110\right)\mathrm{}}_r$ planes and an expansion in the ${\left(1\mathrm{}\overline{1}0\right)}_r$ planes. The structures and the conductivity changes are interpreted in terms of two overlapping bands that change their relative stabilities with $x$. The substitutional ${\mathrm{Cr}}^{3+}$ ions, which have a localized-electron $d^3$ configuration, also perturb the electron potential sufficiently to induce, at larger $x$, a $3d$-electron localization on the vanadium ions.