RNiO3perovskites (R=Pr,Nd): Nickel valence and the metal-insulator transition investigated by x-ray-absorption spectroscopy

The strongly correlated and nominally trivalent Ni oxides R${\mathrm{NiO}}_3$ (R=Pr, Nd) show a sharp metal to insulator transition at ${\mathit{T}}_{\mathit{M}\mathrm{-}\mathit{I}}$=135 and 200 K, respectively. Rare-earth (3d), Ni (1s,2p), and O (1s) x-ray absorption measurements have been performed above and below the transition temperature in order to investigate (a) the valence of Ni and O atoms and (b) the changes in the electronic structure across the transition. Our results suggest that, in contrast to hole-doped ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Li}}_{\mathit{x}}$O and ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{NiO}}_{4+\mathrm{\delta }}$, the electronic structure is characterized by a strongly hybridized ground state of mainly 3${\mathit{d}}^7$ character. The differences between the absorption edges at low (insulating) and high (metallic) temperatures are discussed in terms of the possible mechanisms responsible for the metal-insulator transition.