High temperature protonic conduction in Sr-doped La₂Si ₂O₇

Electrical conduction in the monoclinic lanthanum disilicate La ₂Si₂O₇ was investigated in H ₂O/O₂ (D₂O/O₂) and H ₂O/H₂ (D₂O/D₂) atmospheres at 500-925°C. It was found that La₂Si₂O₇ became to conduct protons under wet conditions by partially substituting Sr for La. From the H/D isotope effect and the partial pressure dependence of the conductivity, it was demonstrated that the protonic conduction in the Sr-doped La₂Si₂O₇ was considerably predominant both in wet oxidizing and in wet reducing atmospheres and that the contribution of electronic conduction to the total conductivity was minor. The conductivities of the 1 mol% Sr-doped La₂Si₂O₇ were 8 × 10^-7-2 × 10^-5 S cm^-1 under moisturized conditions at 550-925°C, and were relatively lower than the conductivities of the acceptor-doped LaPO₄ and LaBO₃. The partial pressure dependencies of the conductivity of the Sr-doped La₂Si ₂O₇, however, seemed to essentially resemble to those of the acceptor-doped LaPO₄ and LaB₃, indicating the similar proton dissolution and conduction mechanism in these oxoacid salts.