First-order Raman spectra of AB_½'B_½"O₃ double perovskites

Abstract

First-principles computations of Raman intensities were performed for the perovskite-family compound CaAl_½Nb_½O₃ (CAN). This compound features NaCl-type ordering of Al and Nb superimposed onto the b⁻b⁻c⁺ octahedral tilting. Raman tensor for CAN was computed using the package for first-principles computations ABINIT (URL http://www.abinit.org). Computations performed for both untilted cubic (Fm3⁻ m) and tilted monoclinic (P2₁/n) CAN structures showed that the strongest Raman lines are associated with the ordering of Al and Nb. The computed spectrum agreed qualitatively with the experimental data measured on powder (CAN is available in polycrystalline form only). The effect of cation disorder on the Raman intensities was considered using a phenomenological theory of light scattering in the vicinity of a phase transition. We suggest that, for certain modes, the corresponding Raman intensities depend primarily on the average long-range order whereas, for other modes, the intensities are determined by the fluctuation of the order parameter.