Hole and Electron Doping of R₂BaNiO₅ (R = Rare Earths)

Abstract

Black polycrystalline samples of the composition R2-x Cax BANiO5 (Ni-oxidized samples), with R = Y, Nd, Er, Lu, and 0.2 ≤ x ≤ 0.5, have been prepared in air by solid state reaction of R2O3, CaCO3, BaO2, and NiO. They crystallized in the orthorhombic Nd2BaNiO5 structure type, space group Immm, that contains chains of flattened NiO6 octahedra sharing corners along the [100] direction. X-ray and neutron powder diffraction data show that Ca2+ replaces R3+ at random, whereas the oxygen positions remain fully occupied, which induces a proportional increase in the formal oxidation state of Ni with Ca content. The unit-cell volume decreases as a result of the shrinkage of the NiO6 octahedra. The electrical conductivity rises with Ca doping, showing a semiconductor behavior. The linearity of the log σ vs T-1/4 plot suggests a thermally activated electron hopping conduction mechanism, by intervalence transitions between adjacent Ni2+-Ni3+ cations. The samples can be reduced in two steps, giving oxygen-deficient compounds in which the structure is basically unchanged. After the first step all the Ni3+ is reduced to Ni2+. The second reduction process leads to phases, greenish in color, that are thought to contain Ni in both monovalent and divalent oxidation states. The presence of Ca2+ in the structure seems to be essential in the stabilization of Ni+. A neutron diffraction study of the compounds Er2-xCaxBaNiO5-δ, prepared in a H2/N2 flow at 500°C, shows that the O2 axial oxygens are lost during the reduction process. This breaks the continuity of the chains of octahedra, thus hindering the intervalence transitions along the chains. The electrical conductivity is, in fact, several orders of magnitude lower than that of the corresponding Ni-oxidized samples. © 1994 Academic Press, Inc.