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 A 2 La 2 Ti 3 O 10 (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 A 2 La 2 Ti 3 O 10 · x H 2 O, ACa 2 Nb 3 O 10, and ALaNb 2 O 7 (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 CsB 2 Nb 3 O 10 (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.