High-temperature proton conductivity and defect structure of TiP2O7

Nominally undoped TiP2O2 and TiP2O2 with 2 mol-% substitution of Ti by Al were synthesized from TiO2 (Al2O3) and H3PO4(aq), sintered at 1050 degrees C, and characterized by XRD, TEM and SEM. The electrical conductivity was investigated at 300-1000 degrees C as a function of p(O-2), P(H2O), and p(D2O). The material's phase transition around 700 degrees C is clearly visible in the conductivity curves. Al substitution hardly increased the conductivity. The conductivity was higher in H2O- than in D2O-containing and dry atmospheres, indicating the dominance of proton conduction. The conductivity was accordingly mainly independent of p(O-2). A slight increase in the conductivity with decreasing p(O-2) at the highest temperatures was indicative of a minor contribution of n-type electronic conduction. The p(H2O) and temperature dependencies of the conductivity have been modelled as a sum of proton and electron partial conductivities under a situation with protons charge compensated by oxygen interstitials as dominating defects. (c) 2010 Elsevier B.V. All rights reserved.