Proton conductor Ba3Ca1.18Nb1.82O9-delta (BCN18): Effect of reducing environments

The compound Ba3Ca1.18Nb1.82O9-delta (BCN18) has been discussed recently as a promising high-temperature proton conductor. Oxygen loss under very redu cing conditions has been determined quantitatively using thermogravimetry. The thermodynamic parameters Delta H(O)degrees = 2.58 +/- 0.06 eV and Delta S(O)degrees = 1.06 x 10(-3) eV/K provide a suitable thermodynamic descript ion for the equation O-O(x) --> 1/2O(2) + V-O(..) + 2e'. Enthalpy and entro py of the hydrogen absorption reaction have been determined indirectly for BCN18: Delta H(H)degrees = -0.8 eV and Delta S(H)degrees = -6.6 x 10(-4) eV /K, Hydrogen absorption is, therefore, an exothermic process. Valence chang es where Nb5+ + e' --> Nb4+ also have been taken into account under reducin g conditions.