Room-temperature monoclinic distortion due to charge disproportionation in RNiO3 perovskites with small rare-earth cations (R = Ho, Y, Er, Tm, Yb, and Lu):: A neutron diffraction study

Polycrystalline samples of the strongly distorted RNiO3 (R=Ho, Y, Er, Tm, Yb, Lu) perovskites have been prepared under high hydrostatic pressure, at 20 kbar. The crystal structure in the insulating phase of these nickelates has been investigated by high-resolution neutron powder diffraction, below the metal-insulator (MI) transition that all of these compounds experience between 573 K (R=Ho) and 599 K (X=Lu). They present a subtle monoclinic distortion (s.g. P2(1)/n) which implies the splitting of the Ni positions in the crystal. NilO(6) and Ni2O(6) octahedra exhibit very distinct mean Ni-O bond distances, interpreted as a charge disproportionation effect (2Ni(3+) -->Ni3+delta + Ni3-delta) which develops at the opening of the gap. In spite of the regular evolution of the beta angle, characterizing the monoclinic distortion, the average (Nil-O) and (Ni2-O) distances do not significantly change along the series; i.e., the disproportionation parameter delta is about 0.3 electrons in the last six members of the RNiO3 series. The observed regular increase of the Nil-O-Ni2 angles, governing the superexchange and the electronic transfer between Ni cations, accounts for the evolution of Neel and MI transition temperatures in these perovskites.