Structural, magnetic and electronic properties of perovskites (R = rare earth)

RNiO3 perovskites (R = rare earth) provide a remarkable opportunity to study the relationship between structural and physical properties since, by moving along the 4f rare earth series, the evolution of several transport and magnetic properties can be nicely correlated to the steric effects associated with the lanthanide contraction. The most appealing example is probably the metal-insulator transition discovered for the compounds with R ≠ La, whose critical temperature TM-I increases with decreasing size of the rare earth ion. On the other hand, several experimental results show that in RNiO3 perovskites the degree of Ni 3d-O 2p hybridization is extremely high. The purely ionic (Ni3+ O2-) configuration, first suggested by neutron diffraction experiments, has been then substituted by a ground state where the 3d72p6 (Ni3+ O2-) and 3d8L (Ni2+ O-) configurations are strongly mixed. In this paper, a summary of the scientific work performed on the RNiO3 system during the last 6 years is presented. The results of the different experimental techniques are successively reviewed, with special emphasis on the relationship between the structural and the physical properties. The possible mechanisms responsible for the metal-insulator transition, which is the major open question for RNiO3 perovskites, are also discussed.