Structure of OrthorhombicV0.95Cr0.05O2

Abstract

A structure determination of the orthorhombic phase of ${\mathrm{V}}_{0.95}$ ${\mathrm{Cr}}_{0.05}$ ${\mathrm{O}}_2$ based on powder data indicates that the probable space group is $F222\mathrm{}$ and the room-temperature lattice parameters are $a_o=13.\mathrm{}015$ Å, $b_o=12.\mathrm{}597$ Å, and $c_o=5.\mathrm{}795$ Å. The structure represents a distortion of the tetragonal rutile structure of high-temperature ($T>\mathrm{}{68}^{\circ }$ C) ${\mathrm{V}}_2$ ${\mathrm{O}}_4$ having cell parameters $a_r\approx \frac{a_0}{2\sqrt{2}}\approx \frac{b_o}{2\sqrt{2}}$ and $c_r\approx \frac{1}{2}{c}_0$. The orthorhombic cell contains 16${\left({\mathrm{V}}_{0.95}{\mathrm{Cr}}_{0.05}\right)}_2$ ${\mathrm{O}}_4$. There are four distinguishable $c$-axis chains of edge-shared octahedra: Cations $V_1$ in $4a$ and $V_2$ in $4b$ occupy alternate positions of one chain, the ${\mathrm{V}}_1$-O separations being nearly uniform ($\approx 1.895$ Å), whereas the ${\mathrm{V}}_2$-O distances along $b_o$ are shorter (1.85 Å) and the four others are longer (2. 04 Å). Cations ${\mathrm{V}}_3$ in $8j$ and ${\mathrm{V}}_4$ in $8i$ exhibit antiferroelectric displacements along half the $c$-axis chains, and cations ${\mathrm{V}}_5$ from ${\mathrm{V}}_5$-${\mathrm{V}}_5$ pairs along the fourth $c$-axis chain. The relationship of this structure to the semiconductor-to-metal transition in V${\mathrm{O}}_2$ is discussed.