Homogeneity range, oxygen nonstoichiometry, thermal expansion and transport properties of La2−xSrxNi1−yFeyO4+δ

Abstract

A series of La_2−xSr_xNi_1−yFe_yO_4+δ complex oxides adopting the K₂NiF₄-type structure (sp.gr. I4/mmm) was prepared via the decomposition of citrate–nitrate precursors, followed by multiple annealing treatments at 1100 °C in air. Strontium for lanthanum substitution in La_2−xSr_xNi_1−yFe_yO_4+δ leads to a progressive increase of iron solubility (y) which reaches maximum values at x = 1.0–1.2. The crystal structure of single-phase samples was refined by the Reitveld method. The unit cell volume increases with y and decreases with x in La_2−xSr_xNi_1−yFe_yO_4+δ. The solid solutions of La_2−xSr_xNi_1−yFe_yO_4+δ were shown by TGA to be over-stoichiometric (δ > 0) at x = 0.5 and 0.6 and oxygen deficient (δ < 0) at x = 0.8 within the temperature range of 25–1050 °C in air. The thermal expansion coefficient of La_2−xSr_xNi_1−yFe_yO_4+δ increases with x and y, varying within the range of (12–16) × 10⁻⁶ K⁻¹ up to 700 °C in air. In the higher temperature range, the value of the TEC increases up to 20 × 10⁻⁶ K⁻¹ due to a chemical expansion contribution. The total conductivity and Seebeck coefficient were measured in air from RT to 1100 °C. The maximum conductivity value equal to 289 S cm⁻¹ was obtained for La_1.2Sr_0.8Ni_0.9Fe_0.1O_4+δ at 460 °C in air. The conduction is temperature activated for all samples within the composition range under study. The temperature dependencies of the Seebeck coefficient were explained in the approximation of the small-polaron hopping mechanism. The charge carriers were electron holes localized on nickel forming Ni³⁺ cations in low- and high-spin states.