Phase relations during water incorporation in the oxygen and proton conductor Sr6Ta2O11
Abstract Evolution of the structure and phase relations of the oxygen-deficient cryolite Sr 6 Ta 2 O 11 and phase Sr 5.92 Ta 2.08 O 11.12 from homogeneity range have been investigated during water intercalation by gravimetry, dilatometry and X-ray diffraction (XRD) measurements. Intercalation of H 2 O up to ∼0.50 and 0.30 mol H 2 O per formula units Sr 6 Ta 2 O 11 and Sr 5.92 Ta 2.08 O 11.12 , respectively, takes place without change of the crystal structure, which could be described under the cubic space group Fm 3 m with a cell parameters of approximately 8.32 A. Further intercalation of water leads to structure transformation to the orthorhombic space group Fmmm . The temperature of phase transition increases with increasing p H 2 O. It is around 475–450 °C at the humidity of surrounding atmosphere ( p H 2 O≈0.02 atm). This phase transition is induced by p H 2 O ( T =constant) and temperature ( p H 2 O=constant) change and can be observed as a jump on gravimetry curves vs. p H 2 O or T . At 200 °C, the most hydrated phases correspond to the compositions Sr 6 Ta 2 O 11 ·0.96H 2 O (H 2 O stoichiometry close to the theoretical value of 1.0) and Sr 5.92 Ta 2.08 O 11.12 ·0.6H 2 O (70% of the theoretical value). A fragment of Sr 6 Ta 2 O 11 –H 2 O phase diagram is proposed.