Apatite-type La10-xSi6-yAlyO27-3x/2-y/2 (x=0-0.33; y=0.5-1.5) exhibit predominant oxygen ionic conductivity in a wide range of oxygen partial pressures. The conductivity of silicates containing 26.50-26.75 oxygen atoms per formula unit is comparable to that of gadolinia-doped ceria at 770-870 K. The average thermal expansion coefficients are (8.7-10.8) x 10(-6) K-1 at 373-1273 K. At temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics and a moderate degradation of the ionic transport with time are observed under reducing conditions, thus limiting the operation temperature of Si-containing solid electrolytes. (c) 2005 Elsevier B.V. All rights reserved.