Composition and temperature dependent structural investigation of perovskite-type sodium-ion solid electrolyte series Na_1/2-xLa_1/2-xSr_2xZrO₃

Full text: The development of new solid electrolytes is becoming increasingly important, e.g. in rechargeable batteries for electric vehicles, where current organic electrolytes cause major safety concerns. The ABO₃ perovskite metal oxides have shown excellent lithium and sodium ion conductivity owing to their stability and structural flexibility. This has led to the development of several perovskite-type solid electrolytes such as Li₃xLa_2/3-xTiO₃ and Na_1/2-xLa_1/2-xSr_2xZrO₃, which have shown high ionic conductivity. The Na_1/2-xLa_1/2-xSr_2xZrO₃ perovskite-type sodium-ion solid electrolyte system was recently published by Zhao et al. with the x=1/6 member, i.e. Na_1/3La_1/3Sr_1/3ZrO₃, found to have the highest ionic conductivity. The structure was reported to adopt a cubic crystal system with the space group P213. However, this is highly unlikely as both theoretical end members of the series, Na_1/2La_1/2ZrO₃ and SrZrO₃, have orthorhombic symmetry. Given the high ionic conductivity reported for the system, it is important to determine its structure reliable and with the best available data. Using neutron and X-ray powder diffraction data we have been able to confirm that the symmetry across the series is lowered to orthorhombic indeed. Variable temperature neutron powder diffraction data collected for the x=1/6 member of the system from room temperature to 1100 °C helped to identify a structural phase transition from orthorhombic to tetragonal symmetry at 800 °C. (author)