Correlation between crystal and transport properties in LnBa _0.5Sr_0.5Co_1.5Fe_0.5O _{5 + δ} (Ln - Selected lanthanides, Y)

Selected LnBa_0.5Sr_0.5Co_1.5Fe _0.5O_{5 + δ} (Ln - Pr, Nd, Sm, Gd and Y) oxides were studied in terms of their phase composition, formation of cation-ordered crystal structure, oxygen nonstoichiometry and transport properties. Despite partial substitution of bigger Ba^{2 +} by smaller Sr^{2 +} cations, A-site (Ln-Ba,Sr) cation ordering in LnBa_0.5Sr_0.5Co _1.5Fe_0.5O_{5 + δ} is preserved, but only for smaller Ln cations (Y^{3 +}, Gd^{3 +}). In the case of SmBa_0.5Sr_0.5Co_1.5Fe_0.5O _{5 + δ}, the synthesis procedure yielded material partially ordered, while for Pr^{3 +} and Nd^{3 +} cations XRD measurements indicated formation of cation-disordered samples. Oxygen content in the disordered materials is close to stoichiometric one (δ ≈ 1) at room temperature, while for cation-ordered samples δ < 1. Thermogravimetric measurements indicated that above 250 °C a decrease of mass occurs for all of the studied materials. Except for Y-containing sample, larger mass loss at 800 °C occurs for materials with heavier Ln element. The ordered materials show higher activation energy of the electrical conductivity in 25-300 °C temperature range, as well as different temperature dependence of Seebeck coefficient, comparing to the disordered ones. The highest electrical conductivity was measured for PrBa_0.5Sr_0.5Co _1.5Fe_0.5O_{5 + δ}. For this compound the ionic conductivity at 800 °C was determined to be of the order of 0.03 S cm^{- 1}.