Neutron diffraction and FT-Raman study of ion-exchangeable layered titanates and niobates

On the basis of the structure refinements of neutron powder diffraction data, the Raman bands of A₂La₂Ti₃O₁₀ (A = Na, K, and Rb) were successfully assigned. The existence of a linear A - O - Ti linkage along the c axis in this structure resulted in the strong dependence of their Raman bands on the nature of the A atom. In contrast, the Raman spectra of A₂La₂Ti₃O₁₀·xH₂O, ACa₂Nb₃O₁₀, and ALaNb₂O₇ (A = Na, K, Rb, and Cs) with no linear A-O-Ti or A-O-Nb connection were essentially the same in the 1000-400 cm^-1 region regardless of a change of the A atom. In particular, the band around 930 cm^-1 attributed to the terminal Nb-O bond with double-bond character showed no shift even in the Raman spectra of CsB₂Nb₃O₁₀ (B = Ca, Sr, and Ba) despite a remarkable change of lattice volume depending on the B atom. These results indicate that the Raman shift of ion-exchangeable perovskites depends mainly on the way that the interlayer alkali-metal atoms are connected with the octahedra in perovskite layers.