Phase relationships and transport in Ti-, Ce- and Zr-substituted lanthanum silicate systems

The solubility of Ti4+ in the lattice of apatite-type La9.83Si6−xTixO26.75 corresponds to approximately 28% of the Si-site density. The conductivity of La9.83Si6−xTixO26.75 (x = 1–2) is predominantly oxygen-ionic and independent of the oxygen partial pressure in the p(O2) range from 10−20 to 0.3 atm. The electron transference numbers determined by the modified faradaic efficiency technique are lower than 0.006 at 900–950 °C in air. The open-circuit voltage of oxygen concentration cells with Ti-doped silicate electrolytes is close to the theoretical Nernst value both under oxygen/air and air/10%H2–90%N2 gradients at 700–950 °C, suggesting the stabilization of Ti4+ in the apatite structure. Titanium addition in La9.83Si6−xTixO26.75 (x = 1–2) leads to decreasing ionic conductivity and increasing activation energies from 93 to 137 kJ/mol, and enhanced degradation in reducing atmospheres due to SiO volatilization. At p(O2) = 10−20 atm and 1223 K, the conductivity decrease after 100 h was about 5% for x = 1 and 17% for x = 2. The solubility of Zr4+ in the La9.83Si6−xZrxO26.75 system was found to be negligible, while the maximum concentration of Ce4+ in La9.4−xCexSi6O27−δ is approximately 5% with respect to the number of lanthanum sites.