Effects of 1:1 B-cation order on Raman scattering in complex perovskites AB′B″0.5O30.5

Effects of 1:1 B-cation ordering on Raman spectra were analyzed for a double-perovskite CaAl0.5Nb0.5O3 substituted with 5mol% CaTiO3. 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, F2g, Eg, and A1g, to the variations of η. The differences were attributed to a dissimilar coupling of these vibrations to the mean value (ηe) and to the fluctuations (Δη) of η(η=ηe+Δη). In particular, the integrated intensities of both the F2g and Eg Raman peaks were found to depend primarily on ηe, while that of the A1g peak appeared to be strongly influenced by Δη. These differences were ascribed to the distinct symmetries of the three modes: Only the A1g mode has the same symmetry as η resulting in linear coupling of these oxygen displacements to η.