Neutron-diffraction study of<i>R</i><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">NiO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>(<i>R</i>=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 (\ensuremath{\Delta}${\mathrm{\ensuremath{\Theta}}}_{\mathrm{O}\mathrm{\ensuremath{-}}\mathrm{Ni}\mathrm{\ensuremath{-}}\mathrm{O}}$\ensuremath{\approxeq}-0.5\ifmmode^\circ\else\textdegree\fi{}) 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 (\ensuremath{\Delta}${\mathit{d}}_{\mathrm{Ni}\mathrm{\ensuremath{-}}\mathrm{O}}$\ensuremath{\approxeq}+0.004 \AA{}) 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.