Pressure dependence of the metal-insulator transition in the charge-transfer oxides R${\mathrm{NiO}}_3$ (R=Pr,Nd,${\mathrm{Nd}}_{0.7}$${\mathrm{La}}_{0.3}$)

We have investigated the pressure dependence (up to 20 kbar) of the metal-insulator (MI) transition displayed by the orthorhombic perovskites R${\mathrm{NiO}}_3$ (R=Nd, Pr, and ${\mathrm{Nd}}_{0.7}$ ${\mathrm{La}}_{0.3}$) by means of electrical resistance measurements. The transition temperature decreases under pressure, with a common rate of decrease of ${\mathit{dT}}_{\mathrm{MI}}$/dP=-4.2 K/kbar, in spite of differences in ${\mathit{T}}_{\mathrm{MI}}$ (100–200 K). On the basis of the structural effects associated with the application of pressure in these orthorhombic perovskites, we conclude that the metallic phase is stabilized by pressure because the bandwidth increases and hence the charge-transfer gap is reduced. This decrease with pressure of the charge-transfer gap in the R${\mathrm{NiO}}_3$ perovskites is in contrast to the dependence predicted and observed in the layered cuprates where the charge-transfer energy dominates the bandwidth effects. In addition, our experiments show that the first-order phase transformation occurring at ${\mathit{T}}_{\mathrm{MI}}$ is inhibited by pressure, and hence a larger proportion of metallic phase occurs, which leads to an apparent reentrant metallic behavior at low temperature.