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 ·xH2O (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)O6–TiO6–(Ti1/2Ta1/2)O6 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) ? for CsLa2Ti2TaO10, and a=3.8342(2) and c=15.2776(9) ? for RbLa2Ti2TaO10. Sodium-exchanged phase was easily hydrated to NaLa2Ti2TaO10 ·2H2O, belonging to the space group I4/mmm with a=3.8399(3) and c=34.288(3) ?. Upon firing, NaLa2Ti2TaO10·2H2O was dehydrated to NaLa2Ti2TaO10·0.9H2O with P4/mmm at around 100°C and then NaLa2Ti2TaO10 with I4/mmm at above 200°C. 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.