Transport properties and stability of Ni-containing mixed conductors with perovskite- and K2NiF4-type structure

The total conductivity and Seebeck coefficient of a series of Ni-containing phases, including La2Ni1-xMxO4+sigma (M = Co, Cu; x = 0.1-0.2) with K2NiF4-type structure and perovskite-like La(0.90)Sr(0.65)Gao(0.65)Mg(0.15)Ni(0.20)O(3-delta) and La0.50Pr0.50Ga0.65Mg0.15 Ni0.20O3-delta, were studied in the oxygen partial pressure range from 10(-18) Pa to 50 kPa at 973-1223 K. Within the phase stability domain, the conductivity of layered nickelates is predominantly p-type electronic and occurs via small-polaron mechanism, indicated by temperature-activated hole mobility and p(O-2) dependencies of electrical properties. In oxidizing conditions similar behavior is characteristic of Ni-containing perovskites, which exhibit, however, significant ionic contribution to the transport processes. The role of ionic conduction increases with decreasing P(O-2) and becomes dominant in reducing atmospheres. All nickelate-based phases decompose at oxygen pressures considerably lower with respect to Ni/NiO boundary. The partial substitution of nickel in La2Ni(M)O4+delta has minor effect on the stability limits, which are similar to that of La0.90Sr0.10 Ga0.65Mg0.15Ni0.20O3-delta. On the contrary, praseodymium doping enhances the stability of La0.50Pr0.50Ga0.65Mg0.15Ni0.20O3-delta down to p(O-2) values as low as 10(-17)-10(-10) Pa at 1023-1223 K. (C) 2003 Elsevier Inc. All rights reserved.