Structural characterization of new layered perovskites MLa2Ti2TaO10 (M = Cs, Rb) and NaLa2Ti2TaO10•xH2O (x=2, 0.9, 0)

The crystal structures of n=3 Dion-Jacobson phases MLa2Ti2TaO10 (M=Cs,Rb) and NaLa2Ti2TaO10. xH(2)O (x = 2, 0.9, 0) have been investigated by the Rietveld analysis of their powder XRD patterns. These compounds show the new-type ordering sequence of (Ti1/2Ta1/2) O-6-TiO6-(Ti-1/2 Ta-1/2)O-6 in the tripled octahedra, corresponding to the niobium analogs of MLa2Ti2NbO10. They crystallize in the tetragonal system P4/mmm with lattice constants of a = 3.84733(9) and c = 15.4364(4) Angstrom for CsLa2Ti2TaO10, and a = 3.8342(2) and c = 15.2776(9) Angstrom for RbLa2Ti2TaO10. Sodium-exchanged phase was easily hydrated to NaLa2Ti2TaO10. 2H(2)O, belonging to the space group 14/mmm with a = 3.8399(3) and c = 34.288(3) Angstrom. Upon firing, NaLa2Ti2TaO10. 2H(2)O was dehydrated to NaLa2Ti2TaO10.0.9H(2)O with P4/mmm at around 100 degreesC and then NaLa2Ti2TaO10 with I4/mmm at above 200 degreesC. The Na cations in two hydrates were surrounded with six oxygens, forming face-shared octahedra along (110) direction. These structural models, especially for the coordination environment of Na cations, were proposed for the first time. (C) 2001 Academic Press.