Neutron-diffraction study of RNiO3 ( R =La,Pr,Nd,Sm): Electronically induced structural changes across the metal-insulator transition

International audience; In the RNiO3 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 PrNiO3 and NdNiO3 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 NiO6 octahedra, which imply changes in the Ni-O-Ni angles (DELTA-THETA-O-Ni-O almost-equal-to -0.5-degrees) governing the transfer integral between Ni e(g) and 0 2p orbitals. These changes are sterically driven by the observed increase of the nickel-oxygen distances (DELTA-d Ni-O almost-equal-to +0.004 Angstrom) in the insulating (low-temperature) phase. The results of valence-bond calculations suggest the existence of Ni3+(d7) and R3+ states for nickel and rare earth.