Critical behavior of the Mott transition in Cr-doped V₂O₃

Abstract

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 °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 resitance drop at the transition, which is about two orders of magnitude at 298 °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 (V_1-xCr_x)₂O₃ with x=0.0375, x=0.0187, and x=0.0135 were determined as 12.5 kbar, 390 °K; 5.5 kbar, 433 °K; and 3.5 kbar, 443 °K, respectively. A straight-line extrapolation of these data yields P_c=-1.5× 0.2 kbar and T_c=(474±5) °K for pure V₂O₃. The high-temperature resistance anomaly in pure V₂O₃ near 500-600 °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 ΔV 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 γ- to α-Ce phase boundary which is known to terminate at a critical point. Indeed the M-I transition in Cr-doped V₂O₃ and the γ- to α-Ce transition have many similarities, and these are discussed.