Ionic and electronic transport in La2Ti2SiO9-based materials

The total conductivity of monoclinic La2Ti2SiO9 is mixed oxygen-ionic and n-type electronic, and increases on reduction of the oxygen partial pressure down to 10−21 atm at 973–1223 K. The substitution of Ti4+ with Nb5+ decreases both contributions to the conductivity, whilst Pr doping and reducing p(O2) have opposite effects. The oxygen ion transference numbers of La2Ti2SiO9−δ, LaPrTi2SiO9±δ and La2Ti1.8Nb0.2SiO9±δ ceramics, measured by the faradaic efficiency and e.m.f. methods, vary in the range 0.15–0.32, increasing when temperature decreases. In air, the activation energies for the ionic and electronic transport are 1.23–1.40 and 1.59–1.74 eV, respectively. Protonic contribution to the conductivity in wet atmospheres becomes significant at temperatures below 1000 K. The experimental data and the results of atomistic computer simulations suggest that the oxygen-ionic and electronic transport is primarily determined by processes involving TiO6 octahedra. The ionic conduction may occur via both the vacancy and interstitial migration mechanisms, but the former is more favorable energetically and should dominate, at least, in reducing atmospheres. The average thermal expansion coefficients of La2Ti2SiO9-based ceramics, calculated from dilatometric data in air, are (8.7−9.5)×10−6 K−1 at 300–1373 K. The lattice of lanthanum titanate-silicate is almost intolerant with respect to A-site deficiency and to doping with lower-valence cations, such as Sr and Fe.