Charge disproportionation in RNiO3 perovskites

RNiO3 gerovskites are one of the few families of undoped oxides showing a m etal-insulator transition as a function of temperature. The origin of first -order transition has been one of the long-standing questions associated wi th these compounds. Neutron and synchrotron diffraction data of small rare- earth members of the series provide the first observation of changes in the crystal symmetry at the metal-insulator (MI) transition in RNiO3 perovskit es. At high temperatures, RNiO3 are orthorhombic and metallic and below T-M I they change to a monoclinic insulator. The structural reorganization at T -MI gives rise to two crystallographically independent Ni positions and the insulating phase consist of alternating expanded (NilO(6)) and contracted (Ni2O(6)) octahedra along the three directions of the crystal cell. The mag netic structure shows unequal magnetic moments for the two Ni sites. These findings indicate a Ni charge disproportionation. (C) 2000 Elsevier Science B.V. All rights reserved.