few studies on n Ο 1 members that have layers consisting The crystal structures of NaLnTiO 4 (Ln β«Ψβ¬ La, Nd, Sm, and of alkali-metal cations. If we consider the (A, AΠ) 2 (M, Gd) were determined by Rietveld refinements on their powder MΠ)O 4 (A and AΠ Ο alkaline-earth or rare-earth cations) X-ray diffraction patterns. They had K 2 NiF 4 -type related superwith tetragonal K 2 NiF 4 -type structure corresponding to structure (space group P4/nmm(D 7 4h )) in which NaO and LnO n Ο 1 member of the Ruddlesden-Popper series, large A double layers are 1:1 ordered perpendicular to the c axis. Aland AΠ ions are in general randomly distributed among the though the c parameter decreases from Ln β«Ψβ¬ La to Nd, Sm, and larger cation sites. Concerning small cations, a disordered Gd, the Na-O distance along the c axis was not proportional to arrangement of M and MΠ ions in the perovskite slab leads the variation of the unit cell parameter. A strong corrugation to I4/mmm space group while a distribution with ordered of the LnO layer which is the main factor determining the a manner results in P422 space group (10). The evidences parameter was observed, which would be induced by the poor for an 1:1 ordering of M and MΠ cations have been observed charge compensation between NaO and LnO layers. Due to for several kinds of cationic pairs such as (Li, Mn), (Li, such a corrugation, the a parameter did not show decreasing tendency despite the fact that the constituting lanthanide ion Fe), (Li, Co), (Li, Ni), (Mg, Mn), (Mg, Fe), (Mg, Co), (Mg, becomes smaller. Ionic conductivity of NaLnTiO 4 lower than Ni), (Zn, Mn), (Zn, Fe), (Zn, Co), and (Zn, Ni) (11-16). that of Na 2 Ln 2 Ti 3 O 10 (Ln β«Ψβ¬ La, Nd, Sm, and Gd) also resulted The additional energies obtained by cationic ordering are from the corrugation of the LnO layer with coordination numabout 10 Θ 11 kcal/ion (10). Due to the two-dimensional ber 9 accompanied by contraction of the NaO layer.