Transport properties and stability of Ni-containing mixed conductors with perovskite- and K 2NiF 4-type structure
The total conductivity and Seebeck coefficient of a series of Ni-containing phases, including La 2Ni 1- xMxO 4+ δ ( M=Co, Cu; x=0.1-0.2) with K 2NiF 4-type structure and perovskite-like La 0.90Sr 0.10Ga 0.65Mg 0.15Ni 0.20O 3- δ and La 0.50Pr 0.50Ga 0.65Mg 0.15Ni 0.20O 3- δ, 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 La 2Ni( M)O 4+ δ has minor effect on the stability limits, which are similar to that of La 0.90Sr 0.10Ga 0.65Mg 0.15Ni 0.20O 3- δ. On the contrary, praseodymium doping enhances the stability of La 0.50Pr 0.50Ga 0.65Mg 0.15Ni 0.20O 3- δ down to p(O 2) values as low as 10 -17-10 -10 Pa at 1023-1223 K.