Structural characterization of new layered perovskites MLa

The crystal structures of n=3 Dion-Jacobson phases MLa(sub 2)Ti(sub 2)TaO(sub 10) (M=Cs, Rb) and NaLa(sub 2)Ti(sub 2)TaO(sub 10(centerdot))xH(sub 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 (Ti(sub 1/2)Ta(sub 1/2))O(sub 6)-TiO(sub 6)-(Ti(sub 1/2)Ta(sub 1/2))O(sub 6) in the tripled octahedra, corresponding to the niobium analogs of MLa(sub 2)Ti(sub 2)TaO(sub 10). They crystallize in the tetragonal system P4/mmm with lattice constants of a=3.84733(9) and c=15.4364(4)(angstrom) for CsLa(sub 2)Ti(sub 2)TaO(sub 10), and a=3.8342(2) and c=15.2776(9)(angstrom) for RbLa(sub 2)Ti(sub 2)TaO(sub 10). Sodium-exchanged phase was easily hydrated to NaLa(sub 2)Ti(sub 2)TaO(sub 10)(centerdot)2H(sub 2)O, belonging to the space group I4/mmm with a=3.8399(3) and c=34.288(3)(angstrom). Upon firing, NaLa(sub 2)Ti(sub 2)TaO(sub 10)(centerdot)2H(sub 2)O was dehydrated to NaLa(sub 2)Ti(sub 2)TaO(sub 10)(centerdot)0.9H(sub 2)O with P4/mmm at around 100 C and then NaLa(sub 2)Ti(sub 2)TaO(sub 10) 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