Publication date: 10th April 2024
Oxyfluoride type materials are very interesting for their potential as mixed conductors as charge carriers for various applications. Some Oxyfluorides can act as O2-, F-, e- and h+ conductors. Materials based on TiO2-x and can have a very wide range on electronic conductivity based on the oxygen-titanium ratio. Materials based on highly doped SrF2 tend to show some ionic conductivity with little to no electronic conductivity. The interaction between these two compounds is highly interesting.
SrF2-TiO2 composite materials were produced by hot-pressing SrF2 and TiO2 powders under a reducing atmosphere at 1100°C. Ratios of 1:1-1:5 were produced including TiO2 and SrF2 only. Up-to three phases were found in all samples. The very SrF2 rich samples (1:3-1:5) showed SrF2 and a minority phase which was not recognized by XRD or SEM. The samples with more TiO2 such as (1:1 and 1:2) showed also a TiO2-x phase in the middle of the unrecognized phase.
The materials were measured in argon atmosphere at 50-300°C using electrochemical impedance spectroscopy. The response was fitted with a modified Debye circuit to separate the electronic and ionic conductivity of the samples showing very high ionic conductivity and comparative electronic conductivity. Proof of the ionic conductivity was received with electron blocking layers of La0.2Sr0.8F2.2 showing that the ionic conductivity is very high for the SrF2 rich samples but less so for the 1:1 ratio compound. XPS analysis showed several Ti oxidation states allowing for electronic conductivity. Activation energies were calculated for electronic and ionic conduction showing ionic conduction with a very low activation energy of under 0.1eV for fluorine conduction and an activation energy of 0.2eV for electronic conduction.
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