Structural Characterization of New Layered Perovskites MLa_2Ti_2TaO_(10) (M = Cs, Rb) and NaLa_2Ti_2TaO_(10) centre dot xH_2O (x = 2, 0.9, 0)

摘要

The crystal structures of n = 3 Dion-Jacobson phases MLa_2Ti_2TaO_(10) (M = Cs, Rb) and NaLa_2Ti_2TaO_(10) centre dot xH_2O (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))O_6-TiO_6-(Ti_(1/2)Ta_(1/2))O_6 in the tripled octahedra, corresponding to the niobium analogs of MLa_2Ti_2NbO_(10). They crystallize in the tetragonal system P4/mmm with lattice constants of a = 3.84733(9) and c = 15.4364(4) A for CsLa_2Ti_2TaO_(10), and a = 3.8342(2) and c = 15.2776(9) A for RbLa_2Ti_2TaO_(10). Sodium-exchanged phase was easily hydrated to NaLa_2Ti_2TaO_(10) centre dot 2H_2O, belonging to the space group I4/mmm with a = 3.8399(3) and c = 34.288(3) A. Upon firing, NaLa_2Ti_2TaO_(10) centre dot 2H_2O was dehydrated to NaLa_2Ti_2TaO_(10) centre dot 0.9H_2O with P4/mmm at around 100 deg 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.