HOLE AND ELECTRON DOPING OF R2BANIO5 (R = RARE-EARTHS)

Black polycrystalline samples of the composition R2-xCax BaNiO5 (Ni-ox idized samples), with R = Y, Nd, Er, Lu, and 0.2 less-than-or-equal-to x less-than-or-equal-to 0.5, have been prepared in air by solid state reaction of R2O3, CaCO3, BaO2, and NiO. They crystallized in the orth orhombic Nd2BaNiO5 structure type, space group Immm, that contains cha ins of flattened NiO6 octahedra sharing corners along the [100] direct ion. X-ray and neutron powder diffraction data show that Ca2+ replaces R3+ at random, whereas the oxygen positions remain fully occupied, wh ich induces a proportional increase in the formal oxidation state of N i with Ca content. The unit-cell volume decreases as a result of the s hrinkage of the NiO6 octahedra. The electrical conductivity rises with Ca doping, showing a semiconductor behavior. The linearity of the log sigma vs T-1/4 plot suggests a thermally activated electron hopping c onduction 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. Aft er the first step all the Ni3+ is reduced to Ni2+. The second reductio n process leads to phases, greenish in color, that are thought to cont ain 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-delt a, prepared in a H-2/N2, flow at 500-degrees-C, shows that the O2 axia l oxygens are lost during the reduction process. This breaks the conti nuity of the chains of octahedra, thus hindering the intervalence tran sitions along the chains. The electrical conductivity is, in fact, sev eral orders of magnitude lower than that of the corresponding Ni-oxidi zed samples. (C) 1994 Academic Press, Inc.