Room-temperature monoclinic distortion due to charge disproportionation inRNiO3perovskites with small rare-earth cations(R=

Polycrystalline samples of the strongly distorted $R{\mathrm{NiO}}_{3}(R=\mathrm{H}\mathrm{o},\mathrm{}\mathrm{Y},\mathrm{}\mathrm{E}\mathrm{r},\mathrm{}\mathrm{T}\mathrm{m},\mathrm{}\mathrm{Y}\mathrm{b},\mathrm{}\mathrm{Lu})$ 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 $(R=\mathrm{Ho})$ and 599 K $(R=\mathrm{Lu}).$ They present a subtle monoclinic distortion (s.g. ${P2}_{1}/n)$ which implies the splitting of the Ni positions in the crystal. ${\mathrm{Ni}1\mathrm{O}}_{6}$ and ${\mathrm{Ni}2\mathrm{O}}_{6}$ octahedra exhibit very distinct mean Ni-O bond distances, interpreted as a charge disproportionation effect $(2{\mathrm{Ni}}^{3+}{\ensuremath{\rightarrow}\mathrm{Ni}}^{3+\ensuremath{\delta}}+{\mathrm{Ni}}^{3\ensuremath{-}\ensuremath{\delta}})$ which develops at the opening of the gap. In spite of the regular evolution of the \ensuremath{\beta} angle, characterizing the monoclinic distortion, the average 〈Ni1-O〉 and 〈Ni2-O〉 distances do not significantly change along the series; i.e., the disproportionation parameter \ensuremath{\delta} is about 0.3 electrons in the last six members of the $R{\mathrm{NiO}}_{3}$ series. The observed regular increase of the Ni1-O-Ni2 angles, governing the superexchange and the electronic transfer between Ni cations, accounts for the evolution of N\'eel and MI transition temperatures in these perovskites.