Metal-Insulator Transition in(V1−xCrx)2O3

A first-order metal-insulator transition with no change in long-range order occurs in the mixed oxides ${\left({\mathrm{V}}_{1-x}{\mathrm{Cr}}_x\right)}_2{\mathrm{O}}_3$ with increasing $x \left(x\approx 0.01 \mathrm{at} 1 \mathrm{atm}\mathrm{and} 298 {}_{}{}^{\circ }{}_{}{}^{}\mathrm{K}\right)$ or decreasing pressure (at $\approx 10$ kbar for $x=0.04\mathrm{}$ at 298 °K). In a preliminary letter it was shown that the transition had all the qualitative features expected of a Mott transition, i.e., a transition from band to localized behavior. The present paper reports the detailed experimental results. Single crystals of mixed oxides have been made by reaction of ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$, ${\mathrm{V}}_2$ ${\mathrm{O}}_5$, and VN in molten KF. Electrical resistivity and powder x-ray diffraction measurements made as a function of temperature and pressure in oxides with $0\le x\le 0.12$ establish a temperature-pressure-composition phase diagram with three clearly defined phases: metal (M), insulator (I), and antiferromagnetic insulator (AF). The I-M transition is marked by a drop of over two orders of magnitude in the electrical resistivity and a discontinuous decrease in volume of $\approx 1.2$% with no change in crystal structure. For $x\approx 0.01$ the sequence AF→ M→ I is observed with increasing temperature at 1 atm. At 4.2 K an AF→ M transition occurs with increasing pressure ($P=41\mathrm{}\pm 6$ kbar for $x=0.04\mathrm{}$). The phase diagram is compared with other transition-metal oxides, and the differences between the transition in ${\mathrm{V}}_2$ ${\mathrm{O}}_3$ and V${\mathrm{O}}_2$ are discussed.