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

Abstract

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 P21/n) which implies the splitting of the Ni positions in the crystal. This symmetry was confirmed by neutron powder diffraction; the crystal structures for RHo 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 dist...