Structural Properties and Water Uptake of SrTi_1-xFe_xO_3-x/2-δ

In this work, Fe-doped strontium titanate SrTi_1-xFe_xO_3-x/2-δ, for x = 0-1 (STFx), has been fabricated and studied. The structure and microstructure analysis showed that the Fe amount in SrTi_1-xFe_xO_3-x/2-δ has a great influence on the lattice parameter and microstructure, including the porosity and grain size. Oxygen nonstoichiometry studies performed by thermogravimetry at different atmospheres showed that the Fe-rich compositions (x > 0.3) exhibit higher oxygen vacancies concentration of the order of magnitude 10²²-10²³ cm^-3. The proton uptake investigations have been done using thermogravimetry in wet conditions, and the results showed that the compositions with x < 0.5 exhibit hydrogenation redox reactions. Proton concentration at 400 °C depends on the Fe content and was estimated to be 1.0 × 10^-2 mol/mol for SrTi_0.9Fe_0.1O_2.95 and 1.8 × 10^-5 mol/mol for SrTi_0.5Fe_0.5O_2.75. Above 20 mol% of iron content, a significant drop of proton molar concentrations at 400 °C was observed. This is related to the stronger overlapping of Fe and O orbitals after reaching the percolation level of approximately 30 mol% of the iron in SrTi_1-xFe_xO_3-x/2-δ. The relation between the proton concentration and Fe dopant content has been discussed in relation to the B-site average electronegativity, oxygen nonstoichiometry, and electronic structure.