Effect of Mg substitution on crystal structure and oxide-ion conductivity of apatite-type lanthanum silicates

Two types of apatite-type lanthanum silicates, La9.93(Si5.11 Mg0.89)O26 (space group P63/m) and (La8.48 Mg1.28)Si6O26 (space group P63), were successfully synthesized. They were, together with La9.33Si6O26 (space group P63/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 La9.93(Si5.11 Mg0.89)O26, followed by La9.33Si6O26 and (La8.48 Mg1.28)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 La9.93(Si5.11 Mg0.89)O26, 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 P63/m to P63, 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.