CORRELATION BETWEEN STRUCTURES AND IONIC CONDUCTIVITIES OF NA(2)LN(2)TI(3)O(10) (LN=LA, ND, SM, AND GD)

The variations of the structural detail of layered perovskite-type oxi des, Na(2)Ln(2)Ti(3)O(10) (Ln=La, Nd, Sm, Gd), have been refined by Ri etveld analyses of their powder X-ray diffraction data. Although the c -axis strongly decreases from Ln=La to Nd, Sm, or Gd, the length of Na -O bond along the c-axis that is regarded as the sodium layer spacing is not dependent on the unit cell parameter. Such a behavior is explai ned by the fact that Na-O bond is in competition with Ti-O one of the perovskite slab. Increased covalency of this Ti-O bond by the lattice contraction leads to weakening of the attaching strength of Na ion. Th is picture is consistent with the experimental observation that Na ion conductivity of Na(2)Ln(2)Ti(3)O(10) increases from Ln=La to Nd, Sm, or Gd despite strong contraction of the unit cell volume.