Evidence of High Fluorine Ion Conductivity in SrF₂-Rich SrF₂-TiO₂-Based Compounds

Materials based on Sr-Ti-O have shown remarkable properties and have been used in a wide diversity of applications. However, very little investigation has gone into the Sr-Ti-O-F system, mainly due to the very high stability of its constituents such as SrF₂ and TiO₂. In this work, solid-state reactions under reducing atmospheres on SrF₂ and TiO₂ showed highly interesting properties for the system, including high mixed electronic and ionic conductivity. XPS data further illuminate the results with the Ti 2p peaks shifting to higher binding energy due to fluorine interaction, possibly hinting at the formation of a TiO_xF_y phase. Testing using both ion blocking and electron blocking layers allowed the distinction between the ionic and electronic conductivity of the materials, showing very high ionic conductivity compared to most SrF₂-based compounds. Ionic conductivities of up to 1 mS cm^-1 for 2SrF₂:TiO₂ samples near room temperature were obtained. The ionic transport numbers were found to be 20-60% over the 50-300 °C temperature range. The apparent activation energy for ionic conduction was surprisingly low with E_a (ionic) = 0.05-0.10 eV for the different samples, whereas for electronic conductivity, E_a (electron) = 0.11-0.32 eV.