Room-temperature monoclinic distortion due to charge disproportionation in $R{\mathrm{NiO}}_3$ perovskites with small rare-earth cations $(R=\mathrm{Ho},$ Y, Er, Tm, Yb, and Lu): A neutron diffraction study

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 $(R=\mathrm{Lu}).\mathrm{}$ They present a subtle monoclinic distortion (s.g. ${P2\mathrm{}}_1/n)$ 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 $(2\mathrm{}{\mathrm{Ni}}^{3+}{\to \mathrm{Ni}}^{3+\delta }+{\mathrm{Ni}}^{3-\delta })$ which develops at the opening of the gap. In spite of the regular evolution of the β 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 δ 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 Néel and MI transition temperatures in these perovskites.