Pressure dependence of metal-insulator transition in perovskitesRNiO3(R=Eu, Y, Lu)

High-pressure experiments on the $R{\text{NiO}}_{3}$ perovskites with smaller rare-earth ${R}^{3+}$ ions complete our study of the crossover from itinerant to localized $e$-electron behavior associated with the low-spin $\text{Ni}(\text{III}):{t}^{6}{e}^{1}$ ion. By measuring second-sintered samples in a multianvil module, we have observed a clear anomaly of $\ensuremath{\rho}(T)$ at ${T}_{\text{IM}}$ and obtained the pressure dependence of ${T}_{\text{IM}}$ for the $R{\text{NiO}}_{3}$ ($R=\text{Eu}$, Y, Lu) that are different from the published data supporting a charge-ordering model. Instead, they are comparable to that for the orbital-ordering insulator ${\text{LaMnO}}_{3}$. Our distinct results show clearly that where the crossover is approached from the localized-electron side by increasing the orbital overlap integral, a cooperative Jahn-Teller distortion prevents charge transfer from molecular $e$ orbitals on strongly bonded ${\text{NiO}}_{6/2}$ clusters to more weakly bonded Ni(III) centers. However, where the crossover is approached from the itinerant-electron side as in the perovskite ${\text{CaFeO}}_{3}$, a disproportionation charge transfer may occur. In a separate experiment in which the powder samples were pressed in a Bridgman anvil device without a pressure medium, we have shown after releasing pressure that the samples were reduced. This irreversible effect should be distinguished from the hydrostatic-pressure effect on the physical properties of the $R{\text{NiO}}_{3}$ perovskites.