The crystal structures of these materials were determined by means of Rietveld refinement of their powder X-ray diffraction patterns. They had K2NiF4-type superstructures, P4/nmm(D4h7), in which NaO and LnO double layers were 1:1 ordered perpendicular to the c-axis. Although the c-parameter decreased in going from La to Nd, Sm and Gd, the Na-O distance along the c-axis was not proportional to the variation in the unit cell parameter. A marked corrugation of the LnO layer, which was the main factor which determined the a-parameter, was observed. This was attributed to poor charge compensation between the NaO and LnO layers. Due to such corrugation, the a-parameter did not exhibit a decreasing tendency in spite of the fact that the constituent lanthanide ion became smaller. The fact that the ionic conductivity of NaLnTiO4 was lower than that of Ti3Na2Ln2O10 also resulted from corrugation of the LnO layer, with coordination number 9, together with contraction of the NaO layer.

S.H.Byeon, K.Park, M.Itoh: Journal of Solid State Chemistry, 1996, 121[2], 430-6