Metal-insulator transitions, structural and microstructural evolution of RNiO3 (R = Sm, Eu, Gd, Dy, Ho, Y) perovskites:: Evidence for room-temperature charge disproportionation in monoclinic HoNiO3 and YNiO3

RNiO3 nickelates have been-prepared under high oxygen pressure (R = Sm, Eu, Gd) or high hydrostatic pressure (R = Dy, Ho, Y) in the presence of KClO4. The samples have been investigated at room temperature(RT) by synchrotron X-ray powder diffraction to follow the evolution of the crystal structures and microstructures along the series. The distortion of the orthorhombic (space group Pbnm) perovskite progressively increases along the series, leading for the smallest Ho3+ and Y3+ cations to a subtle monoclinic distortion (space group P2(1)/n) which implies the splitting of the Ni positions in the crystal. This symmetry was confirmed by neutron powder diffraction; the crystal structures for R=Ho and Y were refined simultaneously from RT synchrotron and neutron powder diffraction data. In both perovskites the oxygen octahedra around Ni1 and Ni2 positions are significantly distorted; suggesting the manifestation of Jahn-Teller effect, which is almost absent in the nickelates of lighter rare earths. The very distinct mean Ni-O bond distances observed for Nil and Ni2 atoms at RT, in the insulating regime, suggest the presence of a charge disproportionation effect, considered as driving force for the splitting of the Ni positions. The metal-insulator CMI) transitions for RNiO3 (R = Gd, Dy, Ho, Y), above room temperature, have been characterized by DSC. The transition temperatures for Gd, Dy, Ho,and Y oxides in the heating runs are 510.7, 563.9, 572.7, and 581.9 K, respectively. The increasing rate of T-MI for Dy, Ho, and Y materials is lower than that expected from the variation of T-MI for the larger rare earth perovskites. This is probably related to the subtle monoclinic distortion found for Ho and Y nickelates. The high-resolution synchrotron,X-ray powder patterns have revealed changes in the microstructure along the series. Powder patterns for orthorhombic RNiO3 (R = Sm, Eu, Gd, Dy) display asymmetric tails for some reflections which are due to structural mistakes such:as stacking faults or regular intergrowths. These mistakes are not present in monoclinic RNiO3 (R=Ho, Y) nickelates.