PRESSURE-DEPENDENCE OF THE METAL-INSULATOR-TRANSITION IN THE CHARGE-TRANSFER OXIDES RNIO3 (R = PR,ND,ND0.7LA0.3)

We have investigated the pressure dependence (up to 20 kbar) of the me tal-insulator (MI) transition displayed by the orthorhombic perovskite s RNiO3 (R = Nd, Pr, and Nd0.7La0.3) by means of electrical resistance measurements. The transition temperature decreases under pressure, wi th a common rate of decrease of dT(MI)/dP = -4.2 K/kbar, in spite of d ifferences in T(MI) (100-200 K). On the basis of the structural effect s associated with the application of pressure in these orthorhombic pe rovskites, we conclude that the metallic phase is stabilized by pressu re because the bandwidth increases and hence the charge-transfer pp is reduced. This decrease with pressure of the charge-transfer gap in th e RNiO3 perovskites is in contrast to the dependence predicted and obs erved in the layered cuprates where the charge-transfer energy dominat es the bandwidth effects. In addition, our experiments show that the f irst-order phase transformation occurring at T(MI) is inhibited by pre ssure, and hence a larger proportion of metallic phase occurs, which l eads to an apparent reentrant metallic behavior at low temperature.