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

Highlights: • Phase boundary limits of La_2-xCa_xNi_1-yFe_yO_4+δ were estimated in air. • Oxygen content, total conductivity and the Seebeck coefficient vs T were measured. • Total conductivity increased with x and decreased with y in La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+δ. • Model fit analysis of the Seebeck coefficient vs T was performed. • Ni³⁺ coexisted in low– (LS) and high–spin (HS) states in La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+δ. The La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+δ solid solutions with = 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 La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+δ materials crystallized in the K₂NiF₄-type structure, sp. gr. I4/mmm. The crystal structure parameters of La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+δ were refined by the Rietveld method. The unit cell volume of La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+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 La_1.6Ca_0.4Ni_1-yCu_yO_4+δ (y = 0.2 and 0.3). Calcium and copper doping decreased the total oxygen content while iron doping increased it in La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+δ. Thermal expansion coefficients (TECs) of La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+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 La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+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 La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+δ. The analysis of the Seebeck coefficient vs temperature for La_2-xCa_xNi_1-y(Fe/Cu)_yO_4+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.