Neutron diffraction and neutron spectroscopy of rare earth nickelates RNiO3 (R= rare earths)

The series of the rare-earth nickelates RNiO3 (R=rare earths) provide the remarkable opportunity to study the correlation between structural changes and electronic transport properties. In these systems, which belong to the few families of oxides showing metallic conductivity, a very sharp metal-insulator transition occurs as a function of temperature. The metal-insulator transition temperature is strongly correlated with the deviation from the ideal perovskite structure. Over the rare-earth series, an increase of this deviation accompanied an increase of the metal-insulator transition temperature is observed. In this work, the structural properties of the rare-earth nickelates have been studied in detail using elastic neutron scattering. Elastic neutron scattering is the experimental technique of choice in order to obtain accurate values of the oxygen positions in the perovskite structure with the presence of the much heavier rare-earth and transition metal ions. In Pr1-xLaxNiO3 (0≤x≤0.7), the dependence of the structural parameters with internal pressure (substitution of praseodymium by the larger lanthanum) and temperature has been studied. Apart from the metal-insulator transition (x≅0.2), a structural phase transition is observed in this series (x≅0.7). The variation of the structural parameters with external pressure has been investigated in PrNiO3 and NdNiO3. These investigations, together with the structural studies of SmNiO3 and EuNiO3, result in a complete phase diagram of the rare-earth nickelates. The dependence of the metal-insulator transition temperature on the nickel-oxygen bond angle and the nickel-oxygen distance found in these experiments is in accordance with the explanation of the metal-insulator transition induced by a charge transfer from oxygen to nickel. This model also explains the unusual high pressure dependence of the metal-insulator transition temperature. (author) figs., tabs., refs