Properties of cobalt-doped SrTiO3 derived from two methods: the modified Pechini method and the citrate combustion method

Abstract

Two wet-synthesis methods: citrate-combustion and modified (with PVA addition) Pechini synthesis were used to obtain Co-doped strontium titanate with assumed chemical compositions of SrTi1−xCoxO3 (where x = 0.02; 0.06; 0.10). All samples were calcined at 900 °C and sintered at 1200 °C, with PVA series being also calcined at 700 and 800 °C, since TG measurements indicated for this method materials lower minimal calcination temperature than for the citrate-combustion series materials. In general, the modified Pechini approach lowers calcination temperature of materials (up to 200 °C) and produces more structurally homogeneous materials than in the case of the citrate-combustion methods. However, the HT XRD measurements reveal the presence of extra cobalt oxide phases in pva_STO_xCo materials that were not visible using conventional XRD measurements. The presence of cobalt oxide has been confirmed for both series of materials based on TPR tests. Moreover, these measurements indicate the incorporation of some amount of cobalt into the perovskite structure. The fact of substituting titanium with cobalt in SrTiO3 structure seems to be confirmed by the results of XAS studies. Additionally, XAS results allowed us to state that Co2+/Co3+ ratio fell as cobalt loading increased in the materials. The examination of SrTiO3 lattice parameter changes for sintered samples is consistent with the findings of the XAS tests and indicates that cobalt adopts +2 oxidation state in the case of the material with the lowest Co content, and the share of Co3+ increases with the growth of the Co amount introduced into the system.