Double perovskites with oxygen structural vacancies: Raman spectra, conductivity and water uptake

The perovskite-related phases Sr6Ta2O11, Sr5.92Ta2.08O11.12(Vos)0.88, Sr5.46Ta2.54O11.81(Vos)0.19, Ba6Nb2O11(Vos)1, Ba4Ca2Nb2O11 , where Vos stands for ‘oxygen structural vacancies’, have been studied by thermogravimetry, AC-conductivity and IR and Raman spectroscopy. A correlation between water uptake and nominal Vos concentration was established, but the nature of the alkaline earth metal atom influences the oxygen sublattice: Ba6Nb2O11(Vos)1 has zero water uptake. The relationship between short-range disorder and water uptake is clear: only compounds having a strong oxygen and cation lattice disorder exhibit maximal water uptake: Sr6Ta2O11 and Ba4Ca2Nb2O11. The protonic species change during cooling–heating.