Electrical properties of the ${\left({\mathrm{V}}_{1-x}{\mathrm{Cr}}_x\right)}_2{\mathrm{O}}_3$ system

Extensive electrical resistivity and Seebeck-coefficient measurements are reported for ${\left({\mathrm{V}}_{1-x}{\mathrm{Cr}}_x\right)}_2{\mathrm{O}}_3$ single-crystal alloys in the range $0\le x\le 0.1$. For $0\le x\le 0.005$ a metal-insulator transition is encountered at 150-170 K which coincides with the onset of antiferromagnetic ordering. Above room temperature there is a gradual transition to a second metallic phase. The resistivity of the metallic phase at a fixed temperature increases by three orders of magnitude on changing the Cr content from 0 to 1.8 at.%, while the Seebeck coefficient remains virtually unaltered. For fixed alloy composition $0.005<x<\mathrm{}0.018\mathrm{}$, increasing temperature produces three transitions: The alloy is an antiferromagnetic insulator (AFI) at low temperatures, a metal ($M$) at intermediate temperatures, a paramagnetic insulator ($I$) at higher temperature, and then reverts back to a quasimetallic state ($M^{\prime }$) at elevated temperatures. The $M-I$ transformation is accompanied by unusually large hysteresis effects. For $0.018\le x\le 0.1$ only the AFI, $I$, and $M^{\prime }$ phases are encountered. These findings can be accounted for in a semiempirical manner in terms of a Fermi level for the $M$ phase that intersects a local minimum in the density of states, which deepens further and ultimately gives rise to a band gap, either as $x$ is increased past the critical value of 0.0177 at fixed temperature $T$, or as $T$ is changed at constant $x$. The band gap, generated when the $I$ phase appears, is probably due to a Hubbard-type transition that is strongly affected by electron-phonon interactions; it then closes self-consistently with a further increase in $T$. The band gap obtained on cooling is associated with the onset of antiferromagnetic ordering. The generally accepted phase diagram for the system has been amended to include the existence above 450 K of another phase ($M^{\prime }$) which is either a metal or a highly degenerate semiconductor.