Effect of calcium and copper/iron co-doping on defect-induced properties of La2NiO4-based materials

Abstract

The La2-xCaxNi1-y(Fe/Cu)yO4+δ solid solutions with х = 0.2, 0.4, 0.5 and y = 0, 0.1, 0.2, 0.3 were synthesized by decomposition of citrate–nitrate precursors. The single-phase La2-xCaxNi1-y(Fe/Cu)yO4+δ materials crystallized in the K2NiF4-type structure, sp. gr. I4/mmm. The crystal structure parameters of La2-xCaxNi1-y(Fe/Cu)yO4+δ were refined by the Rietveld method. The unit cell volume of La2-xCaxNi1-y(Fe/Cu)yO4+δ decreased with x and increased with y. All studied samples exhibited oxygen excess compared to the stoichiometric composition (δ > 0) in the temperature range 30–1100 °C except for La1.6Ca0.4Ni1-yCuyO4+δ (y = 0.2 and 0.3). Calcium and copper doping decreased the total oxygen content while iron doping increased it in La2-xCaxNi1-y(Fe/Cu)yO4+δ. Thermal expansion coefficients (TECs) of La2-xCaxNi1-y(Fe/Cu)yO4+δ slightly increased with dopant concentration and varied in the range (14.5–15.1) × 10−6 K−1. The contribution of chemical expansion was shown to be negligible. The total conductivity of La2-xCaxNi1-y(Fe/Cu)yO4+δ was thermally activated within the whole studied temperature range 30–1000 °C and could be described in terms of small polaron conduction mechanism. The values of total conductivity increased with x and decreased with y in La2-xCaxNi1-y(Fe/Cu)yO4+δ. The analysis of the Seebeck coefficient vs temperature for La2-xCaxNi1-y(Fe/Cu)yO4+δ showed that the main charge carriers are electron holes localized on Ni2+ cations represented by low-spin Ni3+ cations which had a tendency to change the spin configuration gradually to high-spin when the temperature and dopant content increased.