Neutron-diffraction study ofRNiO3(R=La,Pr,Nd,Sm): Electronically induced structural changes across the metal-insulator transition

In the R${\mathrm{NiO}}_3$ series (R=La,Pr,Nd,Sm), the metal-insulator (M-I) transition temperature rises systematically as the size of the rare earth decreases and as the subsequent distortion from the ideal cubic perovskite increases. For R=La the system keeps its metallic character down to 1.5 K, while for R=Pr, Nd, and Sm electronic localization occurs at 135, 200, and 400 K, respectively. High-resolution neutron-powder-diffraction experiments have been performed to investigate the structural anomalies across the first-order M-I transition in the orthorhombic ${\mathrm{PrNiO}}_3$ and ${\mathrm{NdNiO}}_3$ compounds. The cell volume undergoes a subtle increase when the compounds become insulating, due to a slight increase of the Ni-O distances. This effect is accompanied by coupled tilts of ${\mathrm{NiO}}_6$ octahedra, which imply changes in the Ni-O-Ni angles (Δ${\mathrm{\Theta }}_{\mathrm{O}\mathrm{-}\mathrm{Ni}\mathrm{-}\mathrm{O}}$≊-0.5°) governing the transfer integral between Ni ${\mathit{e}}_{\mathit{g}}$ and O 2p orbitals. These changes are sterically driven by the observed increase of the nickel-oxygen distances (Δ${\mathit{d}}_{\mathrm{Ni}\mathrm{-}\mathrm{O}}$≊+0.004 Å) in the insulating (low-temperature) phase. The results of valence-bond calculations suggest the existence of ${\mathrm{Ni}}^{3+}$(${\mathit{d}}^7$) and ${\mathit{R}}^{3+}$ states for nickel and rare earth.