First-order Raman spectra of double perovskites AB$'{1/2}B$''{1/2}O3

TL;DR

This study uses first-principles calculations to analyze the Raman spectra of the double perovskite CaAl$_{1/2}$Nb$_{1/2}$O$_3$, revealing how cation ordering influences vibrational modes and comparing results with experimental data.

Contribution

It provides the first computational Raman spectra for CaAl$_{1/2}$Nb$_{1/2}$O$_3$ and explores the effects of cation disorder on Raman intensities using phenomenological theory.

Findings

Strong Raman lines are linked to Al/Nb ordering.

Computed spectra qualitatively match experimental data.

Raman intensities depend on long-range order and fluctuations.

Abstract

First principles computations of Raman intensities were performed for perovskite-family compound CaAl$_{1/2}$Nb$_{1/2}$O$_3$ (CAN). This compound features 1:1 (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 \underline {http://www.abinit.org}). Computations performed for both untilted cubic ($Fm\bar{3}m$) and tilted monoclinic ($P2_1/n$) 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 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 while, for other modes, the intensities are determined by fluctuations of the order parameter.