HOLE AND ELECTRON DOPING OF RNIO(3) (R=LA, ND)

The effect of hole and electron doping in the perovskites RNiO(3) (R = rare earths) has been studied. R(1-x)A(x)NiO(3) (R = La, Nd; A = Sr, Th; O less than or equal to x less than or equal to 0.1) were prepared under 200 bars of oxygen and were characterized by X-ray and neutron powder diffraction, thermogravimetric analysis, differential scanning calorimetry, and resistivity measurements. The neutron data for rhombohedral La0.9Sr0.1NiO3 or orthorhombic Nd(0.95)A(0.05)NiO(3) (A = Sr, Th) show that Sr or Th replace at random the rare-earth cations, whereas the oxygen positions remain fully occupied. NiO6 octahedra are either contracted, in the hole-doped Sr-containing samples, or expanded, in the electron-doped Th-containing compounds, as a consequence of the decrease/increase of the Ni-O bond lengths electronically induced by doping. Accordingly, bond-valence calculations give a valence for nickel higher/lower than +3, for the Sr/Th-doped compounds, respectively. The transport properties dramatically change with respect to the undoped compounds. The metal-to-insulator transition present in NdNiO3 (T = 200 K) disappears for Nd1-xSrxNiO3, which remains metallic down to 1.5 K even for the smallest doping rate (5%). This fact can be structurally related to the straightening, by 1.4 degrees for x = 0.05, of the [Ni-O-Ni] angles, which govern the transfer integral between Ni-3d and O-2p orbitals. For Nd0.95Th0.05NiO3 there is a suppression of the metal-insulator transition toward low temperatures (T = 132 K). Electronic factors, related to the injection of holes or electrons into the bands of the solid, are mostly responsible for the structural changes observed in these doped compounds, which cannot be explained by simple steric considerations. (C) 1995 Academic Press, Inc.