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

## TL;DR

This study shows that SrF2-rich compounds with TiO2 have high fluorine ion conductivity, making them promising for solid-state applications.

## Contribution

The discovery of high ionic conductivity in SrF2–TiO2-based compounds under reducing conditions.

## Key findings

- Ionic conductivities of up to 1 mS cm–1 were observed near room temperature.
- Ionic transport numbers ranged from 20–60% over 50–300 °C.
- The apparent activation energy for ionic conduction was surprisingly low at 0.05–0.10 eV.

## Abstract

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 TiOxFy 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 Ea (ionic) = 0.05–0.10
eV for the different samples, whereas for electronic conductivity, Ea (electron) = 0.11–0.32 eV.