Effects of 1:1 B-cation order on Raman scattering in complex perovskites AB′_(0.5)B″_(0.5)O_3

摘要

Effects of 1:1 B-cation ordering on Raman spectra were analyzed for a double-perovskite CaAl_(0.5)Nb_(0.5)O_3 substituted with 5 mol % CaTiO_3. Raman measurements were conducted on a series of well-equilibrated polycrystalline samples quenched from various annealing temperatures to produce a broad range of a cation order parameter, η. The results demonstrated surprisingly different sensitivities of the ordering-related Raman-active modes, F_(2g), E_g, and A_(1g), to the variations of 77. The differences were attributed to a dissimilar coupling of these vibrations to the mean value (η_e) and to the fluctuations (Δη) of η(η= η_e+Δη). In particular, the integrated intensities of both the F_(2g) and E_g Raman peaks were found to depend primarily on η_e, while that of the A_(1g) peak appeared to be strongly influenced by Δη. These differences were ascribed to the distinct symmetries of the three modes: Only the A_(1g) mode has the same symmetry as η resulting in linear coupling of these oxygen displacements to η.