Critical behavior of the Mott transition in Cr-doped V2O3

The metal-insulator (M-I) transition in Cr-doped V2O3, which has been shown to be a Mott transition, was investigated by following the resistance as a function of pressure at temperatures above 298 degrees K. This study has shown that the M-\I phase boundary terminates at a critical point in the P-T plane, in accordance with the earlier prediction. The discontinuous resistance drop at the transition, which is about two orders of magnitude at 298 degrees K, progressively diminishes with temperature and beyond a certain critical temperature, depending upon the Cr concentration, vanishes altogether. Above the critical temperature only a smooth but somewhat anomalous change in resistance is seen. The critical pressure and temperature for (V1-xCrx)(2)O-3 with x = 0.0375, x = 0.0187, and x = 0.0135 were determined as 12.5 kbar, 390 degrees K; 5.5 kbar, 433 degrees K; and 3.5 kbar, 443 degrees K, respectively. A straight-line extrapolation of these data yields P-c = -1.5 x 0.2 kbar and T-c = (474 +/- 5)degrees K for pure V2O3. The high-temperature resistance anomaly in pure V2O3 near 500-600 degrees K is due to supercritical behavior. X-ray studies at atmospheric pressure on Cr-doped samples with x = 0.0137 to x = 0.004 show that the AV associated with the M-I transition decreases with increasing temperature and finally vanishes near about x = 0.005. The behavior is exactly analogous to the gamma- to alpha-Ce phase boundary which is known to terminate at a critical point. Indeed the M-I transition in Cr-doped V2O3 and the gamma- to alpha-Ce transition have many similarities, and these are discussed.