Two types of apatite-type lanthanum silicates, La-9.93(Si-5.11 Mg-0.89)O-26 (space group P6(3)/m) and (La-8.48 Mg-1.28)Si6O26 (space group P6(3)), were successfully synthesized. They were, together with La9.33Si6O26 (space group P6(3)/m), examined by optical microscopy, impedance spectroscopy and X-ray powder diffraction. The crystal structures of the former two compounds were refined by single-crystal X-ray diffraction. The oxide-ion conductivity at 773 K was the highest for La-9.93(Si-5.11 Mg-0.89)O-26. followed by La9.33Si6O26 and (La-8.48 Mg-1.28)Si6O26 in this order. The systematic change in conductivity was well accounted for by the atom arrangements of hexagonal channels, consisting of alternately stacking equilateral triangles of La and O along the c-axis. With La-9.93(Si-5.11 Mg-0.89)O-25, the La triangle appreciably expanded as compared with that of La9.33Si6O26, while the size of O triangle was kept almost constant. When the La sites were partially occupied by Mg atoms, the space group changed from P6(3)/m to P6(3), leading to the transformation of equivalent O triangles into the larger and smaller ones. On the assumption that conduction takes place inside the hexagonal channels through the migration of channel oxide-ions, they necessarily pass through the channel triangles. The smaller O triangle would act as a bottleneck hindering the migration of channel oxide ions. The expansion of La triangle would effectively enhance the conductivity. (C) 2010 Elsevier B.V. All rights reserved.