On the Conductivity of KBaPO4and Its Decomposition in Steam and Water

KBaPO4 – recently reported to hydrate in steam at 80°C into a novel low-temperature protonic conductor – was synthesized by a solid state reaction and its electrical properties characterized by impedance spectroscopy. At high temperatures the conductivity of KBaPO4 was low (order of 10−6 S/cm at 600°C) and not significantly different in dry and wet atmospheres, suggesting that KBaPO4 does not take up and conduct protons to any significant extent under these conditions. It was also independent of oxygen partial pressure and increased by acceptor doping (K excess), suggesting that the transport is ionic and by an effectively positive charge carrying defect. Impedance spectroscopy displayed significant grain boundary series resistance, and the bulk and grain boundaries displayed activation energies of approximately 110 and 120 kJ/mol, respectively. At lower temperatures, the conductivity increased, typical of transport in adsorbed water in a porous sample. Exposure of KBaPO4 to steam at 80°C resulted in decomposition into Ba3(PO4)2 and a liquid phase, presumably a molten hydrate of K3PO4. We thus suggest that what literature proposes as protonic conduction in steamed KBaPO4 in fact represents transport of K+, OH− and HPO42− ions in the molten hydrate of the decomposition product K3PO4 held inside porous Ba3(PO4)2.