Structure and oxide anion conductivity in Ln2(TO4)O (Ln=La, Nd; T=Ge, Si)

Oxy-silicate and oxy-germanate, Ln2(TO4)O (Ln=La and Nd, T=Ge and Si) compounds have been prepared. Oxy-germanates can be readily obtained as highly crystalline single phases, while, the oxy-silicates are difficult to prepare as pure phases. The crystal structure of Nd2(SiO4)O has been studied from a joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data. The electrochemical characterisation indicates that these compounds display oxide anion conductivity with p-type electronic contribution under oxidising conditions. The apparent activation energies under dry flowing nitrogen, where p-type contribution is minimised, are 0.97(1), 1.05(3) and 1.17(4) eV, for Nd2(SiO4)O, La2(GeO4)O and Nd2(GeO4)O, respectively. The overall conductivities at 1173 K range from 1.2×10−4 S cm−1 for Nd2(SiO4)O to 1.3×10−6 S cm−1 for La2(GeO4)O. Finally, the stability of these compounds under very reducing conditions has been studied and partial degradation is reported.