High-pressure preparation and characterization of (R,R')NiO3 (R,R'= rare earths) perovskites: Effect of the variance of theR3+ionic sizes

In this paper we describe some novel members of the RNiO3 perovskites, containing Ni3+ and presenting metal-insulator transitions coupled to a charge disproportionation effect. This phenomenon is observed in the insulating state as a subtle monoclinic distortion, due to the presence of two unequivalent Ni ions with different coordination environments, corresponding to expanded and contracted octahedra alternating in the three-dimensional network. Here we examine how the introduction of mixed rare-earth cations at the R sublattice affects the metal-insulator transition and the structural features, from high resolution neutron powder diffraction studies.