RNiO3 Perovskites (R=Pr, Nd): Nickel Valence and the Metal-Insulator Transition Investigated by X-ray Absorption Spectroscopy

The strongly correlated and nominally trivalent Ni oxides RNiO3 (R = Pr, Nd) show a sharp metal to insulator transition at T(M-I) = 135 and 200 K, respectively. Rare-earth (3d), Ni (1s, 2p), and O(Is) 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 Ni1-xLixO and La2-xSrxNiO4+delta, the electronic structure is characterized by a strongly hybridized ground state of mainly 3d7 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.