Chemical diffusion of water in the double perovskites Ba4Ca2Nb2O11 and Sr6Ta2O11

The mechanism and kinetics of water incorporation in the double perovskites Ва4Ca2Nb2O11 and Sr6Ta2O11 has been investigated (T = 300÷500 °C and aH2O = 1 · 10− 3÷2.2 · 10− 2). The formation of hydration products Ba4Ca2Nb2O11·xH2O and Sr6Ta2O11·xH2O (0.2 < x < 0.50) was limited by the diffusion of H2O. It has been found that the concentration dependences of D˜H2O are the same for both samples: small increasing of D˜H2O with increasing x. The temperature dependences of the chemical diffusion coefficients of water for compositions of Ba4Ca2Nb2O11·0.35H2O and Sr6Ta2O11·0.35H2O could be described with close activation energies of Ea = 0.38 ± 0.03 eV and Ea = 0.49 ± 0.03 eV, respectively. The chemical diffusion coefficients of water are nearly one order of magnitude smaller for tantalate Sr6Ta2O11. This result correlates with lower oxygen and proton conductivities in Sr6Ta2O11 as the consequence of lower mobilities.