Charge disproportionation in RNiO3 perovskites:: Simultaneous metal-insulator and structural transition in YNiO3

Neutron and synchrotron diffraction data provide the first observation of changes in the crystal symmetry at the metal-insulator transition in RNiO3 perovskites. At high temperatures, YNiO3 is orthorhombic and metallic but below T-MI = 582 K it changes to a monoclinic insulator due to a charge disproportionation (2Ni(3+) --> Ni3+delta + Ni3-delta) that develops at the opening of the gap. We report the presence of two alternating NiO6 octahedra with expanded (Nil) and contracted (Ni2) Ni-O bonds and a magnetic structure [k = (1/2,0, 1/2)] with unequal moments at Nil and Ni2 octahedra.