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 (Ti_1/2 Ta_1/2) O₆-TiO₆-(Ti_1/2Ta_1/2)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) Å for CsLa₂Ti₂TaO₁₀, and a = 3.8342(2) and c = 15.2776(9) Å 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) Å. 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.