Electrical conduction behavior and immittance analysis of Gd and Mn substituted strontium titanate

Various compositions in the system Sr1-x GdxTi1-xMnxO3 (x B 0.50) were synthesized by solid-state ceramic route. Solid solution forms in all the compositions investigated. Crystal structure remains cubic up to x = 0.10 similar to SrTiO3, while for compositions with x C 0.30, it becomes orthorhombic, similar to GdMnO3. Average grain size is small in all the compositions investigated, and it decreases with increasing x. For compositions with x B 0.30, Seebeck coefficient is initially positive. It becomes negative at higher temperatures. For x = 0.50, it is negative over the entire temperature range of measurements. Immittance analysis shows the presence of two polarization processes, one due to grains and other due to grain boundaries. Results of DC resistivity, qdc and Seebeck coefficient, a as a function of temperature show that in the samples with x B 0.30, in the lowest temperature range conduction occurs due to hopping of holes among Mn+3 and Mn4+ ions, and for x = 0.50 it occurs due to hopping of electrons among Mn2+ and Mn3+ ions over the entire temperature range. In the intermediate temperature range, for x B 0.30 the conduction occurs by hopping of electrons among Mn2+ and Mn3+ ions. For x = 0.01 and 0.05 conduction occurs due to diffusion of oxygen ions through doubly positively charged oxygen vacancies in the highest temperature range.