LAUSR

Abstract The La 2- x Ca x Ni 1-y (Fe/Cu) y O 4+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 La 2- x Ca x Ni 1-y (Fe/Cu) y O 4+δ materials crystallized in the K 2 NiF 4 -type structure, sp. gr. I 4/ mmm . The crystal structure parameters of La 2- x Ca x Ni 1-y (Fe/Cu) y O 4+δ were refined by the Rietveld method. The unit cell volume of La 2- x Ca x Ni 1-y (Fe/Cu) y O 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 1.6 Ca 0.4 Ni 1-y Cu y O 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- x Ca x Ni 1-y (Fe/Cu) y O 4+δ . Thermal expansion coefficients (TECs) of La 2- x Ca x Ni 1-y (Fe/Cu) y O 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- x Ca x Ni 1-y (Fe/Cu) y O 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- x Ca x Ni 1-y (Fe/Cu) y O 4+δ . The analysis of the Seebeck coefficient vs temperature for La 2- x Ca x Ni 1-y (Fe/Cu) y O 4+δ showed that the main charge carriers are electron holes localized on Ni 2+ cations represented by low-spin Ni 3+ cations which had a tendency to change the spin configuration gradually to high-spin when the temperature and dopant content increased.