Structure, Water Uptake, and Electrical Conductivity of TiP_2O_7

摘要

We report here on the structure of TiP_2O_7 and electrical properties of nominally acceptor (Sc, Fe)-doped TiP_2O_7 synthesized by an aqueous phosphoric acid route. Structural characterization, including studies of the high-temperature phase transition in TiP_2O_7, was carried out by powder X-ray and neutron diffraction. Ceramic disks were sintered by the spark plasma technique and their conductivities were characterized as a function of p(O_2) and p(H_2O) in the temperature range of 500°-1000℃ by means of AC constant frequency measurements and impedance spec-troscopy. As reported earlier, the acceptor doping appears not to influence the defect structure of TiP_2O_7 significantly. Effects of H~+/D~+ isotope shift were utilized to identify proton conduction. The conductivity was independent of P(O_2) at 500°-900℃ under oxidizing conditions suggesting predominantly protonic conduction at these temperatures. Under reducing atmosphere it-type conductivity contributed to the total conductivity at the higher temperatures. P(H_2O) dependencies of the conductivities are interpreted in terms of a defect-chemical model involving protons and oxygen interstitials as the dominating defects. The uptake of water was studied by thermogravimetry at high P(H_2O) and the thermodynamics of the hydration reaction was derived. Finally, parameters for the mobility of protons were extracted by combining the conductivity and thermogravimetry data.