Transport properties and thermal expansion of La2Mo2O9-based solid electrolytes

The oxygen ion transference numbers of La1.7Bi0.3Mo2O9, La2Mo1.7W0.3O9 and La2Mo1.95V0.05O9 ceramics, determined by modified faradaic efficiency and e.m.f. methods at 973–1173 K, vary in the range 0.995–0.977 in air, decreasing when temperature increases. The activation energies for the ionic and electronic transport are 61–71 kJ/mol and 123–141 kJ/mol, respectively. Reducing oxygen chemical potential leads to increasing n-type electronic contribution to the total conductivity, which remains, however, essentially p(O2)-independent down to oxygen pressures of 10−4–10−3 atm and exhibits reversible drop on further reduction, probably due to phase decomposition. Doping La2Mo2O9 with calcium results in segregation of a CaMoO4-based phase, accompanied with increasing electronic transport. The average thermal expansion coefficients of La2Mo2O9-based materials, calculated from dilatometric data in air, are (14.4–14.8) × 10−6 K−1 at 300–700 K and (16.4–22.5) × 10−6 K−1 at 700–1070 K.