Ionic conductivity enhancement in Ti-doped Sr11Mo4O23 defective double perovskites

Abstract

A substantially higher ionic motion can be achieved by partially replacing Mo(VI) by Ti(IV) cations in the novel Sr₁₁Mo_4−xTi_xO_23−δ (with x = 0.0, 0.5 and 1.0) electrolyte oxides, successfully enhancing the oxygen vacancy level. These phases can be rewritten as Sr_1.75□_0.25(Sr)(Mo,Ti)O_5.75−δ highlighting the relationship with conventional double perovskites. This original structure presents a broken corner sharing connectivity of the octahedral framework, hence leading to a complex and highly defective network. These materials have been prepared in polycrystalline form by thermal treatment up to 1300 °C. The structures were refined from X-ray and neutron powder diffraction data collected at room temperature and at 500 and 800 °C for x = 1. At high temperature this perovskite shows a phase transition to cubic symmetry and also evidences a reversible process of removal/uptake of O-atoms as observed in the undoped phase. AC-conductivity measurements from impedance spectroscopy confirm that Ti-doping increases the ionic mobility by 70%, attaining ionic conductivity values as high as 3.2 × 10⁻³ and 1.8 × 10⁻² S cm⁻¹ at 650 and 800 °C, respectively.