Phase relations during water incorporation in the oxygen and proton conductor Sr6Ta2O11

Evolution of the structure and phase relations of the oxygen-deficient cryolite Sr6Ta2O11 and phase Sr5.92Ta2.08O11.12 from homogeneity range have been investigated during water intercalation by gravimetry, dilatometry and X-ray diffraction (XRD) measurements. Intercalation of H2O up to ∼0.50 and 0.30 mol H2O per formula units Sr6Ta2O11 and Sr5.92Ta2.08O11.12, respectively, takes place without change of the crystal structure, which could be described under the cubic space group Fm3̄m with a cell parameters of approximately 8.32 Å. Further intercalation of water leads to structure transformation to the orthorhombic space group Fmmm. The temperature of phase transition increases with increasing pH2O. It is around 475–450 °C at the humidity of surrounding atmosphere (pH2O≈0.02 atm). This phase transition is induced by pH2O (T=constant) and temperature (pH2O=constant) change and can be observed as a jump on gravimetry curves vs. pH2O or T. At 200 °C, the most hydrated phases correspond to the compositions Sr6Ta2O11·0.96H2O (H2O stoichiometry close to the theoretical value of 1.0) and Sr5.92Ta2.08O11.12·0.6H2O (70% of the theoretical value). A fragment of Sr6Ta2O11–H2O phase diagram is proposed.