Structure and ionic conductivity of NaLnTiO(4); Comparison with those of Na(2)Ln(2)Ti(3)O(10) (Ln=La, Nd, Sm, and Gd)

The crystal structures of NaLnTiO(4) (Ln = La, Nd, Sm, and Gd) were determined by Rietveld refinements on their powder X-ray diffraction patterns. They had K2NiF4-type related superstructure (space group P4/nmm(D-4h(7))) in which NaO and LnO double layers are 1:1 ordered perpendicular to the c axis. Although the c parameter decreases from La = La to Nd, Sm, and Gd, the Na-O distance along the c axis was not proportional to the variation of the unit cell parameter. A strong corrugation of the LnO layer which is the main factor determining the a parameter was observed, which would be induced by the poor charge compensation between NaO and LnO layers. Due to such a corrugation, the a parameter did not show decreasing tendency despite the fact that the constituting lanthanide ion becomes smaller. Ionic conductivity of NaLnTiO(4) lower than that of Na(2)Ln(2)Ti(3)O(10) (Ln = La, Nd, Sm, and Gd) also resulted from the corrugation of the LnO layer with coordination number 9 accompanied by contraction of the NaO layer. (C) 1996 Academic Press, Inc.