Effect of sintering temperature and Sr/Ti ratio on the nature of Mn-ion speciation within SrTiO₃-based compounds

Manganese-doped (Mn/Ti = 0.02/0.98) SrTiO₃-based materials with various Sr/Ti mole ratios (1.02, 1.00, 0.97 and 0.92) were obtained by the citrate precursor method. The materials were characterised in terms of microstructure (SEM), structure (XRD, XANES), electrical properties (DC method) and reducibility (TPR). With the introduction of 2 mol.% Mn, the cubic structure of SrTiO₃ is preserved, but significantly limits (from Sr/Ti ≥ 0.8 to Sr/Ti > 0.92) the possibilities of deviations from stoichiometry at 1200°C. The narrowing of the stability limit of the perovskite structure to Sr/Ti > 0.97 with an increase in temperature to 1400°C and segregation of Ti-Mn-enrich species are shown. The location and the oxidation states of Mn in both (Sr and Ti) sublattices depend on the assumed Sr/Ti ratio and sintering temperature. Regardless of the Sr/Ti ratio, Mn-doped SrTiO₃ exhibits very low electrical conductivity with an ionic conductivity mechanism.