Tracking of high-temperature thermal expansion and transport properties vs.oxidation state of cobalt between +2 and +3 in the La2Co1+z(Ti1−xMgx)1−zO6-system

Abstract

The high-temperature thermal expansion and electronic transport properties of the B-site substituted LaCoO₃ with both variable oxidation state of cobalt between +2 and +3 (La₂Co(Ti_1−xMg_x)O₆, 0 ≤ x ≤ 0.5) and variable Co³⁺-content relative to the other B-cations (La₂Co_1+z(Ti_0.5Mg_0.5)_1−zO₆, 0.2 ≤ z ≤ 0.6) have been investigated. Based on the temperature dependence of the thermal expansion, electronic transport properties and Seebeck coefficient, three different groups of compositions according to their symmetries can be allocated. It was found that the thermal expansion coefficients (TECs) of the studied compounds are mainly dependent on the proportion of Co²⁺/Co³⁺. For La₂Co(Ti_1−xMg_x)O₆, the TEC increases from ∼9 (x = 0) to ∼19 ppm K⁻¹ (x = 0.5) with an increase of the oxidation state of cobalt from +2 to +3, respectively. The TECs of La₂Co_1+z(Ti_0.5Mg_0.5)_1−zO₆, z = 0.2–0.6 with Co³⁺-only, remain constant at ∼21 ppm K⁻¹ independent of the cobalt content. Thermoelectric measurements of the system indicate that all samples in the system, except La₂Co_1.6(Ti_0.5Mg_0.5)_0.4O₆, are p-type conductors over the whole temperature range, 300 < T < 1175 K. The conductivities were found to increase with an increase of both Co³⁺ and total cobalt content and are described with a small polaron hopping model. Due to an insignificant number of oxygen vacancies of La₂Co_1+z(Ti_1−xMg_x)_1−zO₆ samples prepared in air at elevated temperatures, the investigated system is proposed as an excellent model system for the investigation of the influence of the Co oxidation state and stoichiometry on different properties in perovskite cobalt oxides.