Protons in acceptor doped langasite, La3Ga5SiO14

• Water uptake in acceptor doped langasite is confirmed by TG.
• Hydration of oxygen vacancies by formation of protons confirmed by DFT.
• Langasite displays significant proton conduction below 800 °C.

The electrical and defect chemical properties of acceptor doped langasite have been investigated over wide ranges of pH2O, pO2 and temperature. All compositions are pure proton conductors up to 800 °C in wet atmospheres and mixed oxide ion-p-type conductors at higher temperatures. The enthalpy of mobility of protons is 75 ± 3 kJ/mol, while that of oxygen vacancies is 125 ± 7 kJ/mol. The standard enthalpy and entropy of hydration are -100 ± 3 kJ/mol and -157 ± 5 J/mol K, respectively. Langasite based sensors may therefore be affected by dissolution of protons from H2O in the bulk crystal lattice up to temperatures as high as 1000 °C.