Abstract
Raman spectroscopy is an advantageous method for studying the local structure of materials, but the interpretation of measured spectra is complicated by the presence of oblique phonons in polycrystals of polar materials. Whilst group theory considerations and standard ab initio calculations are helpful, they are often valid only for single crystals. In this paper, we introduce a method for computing Raman spectra of polycrystalline materials from first principles. We start from the standard approach based on the (Placzek) rotation invariants of the Raman tensors and extend it to include the effect of the coupling between the lattice vibrations and the induced electric field, and the electro-optic contribution, relevant for polar materials like ferroelectrics. As exemplified by applying the method to rhombohedral BaTiO3, AlN, and LiNbO3, such an extension brings the simulated Raman spectrum to a much better correspondence with the experimental one. Additional advantages of the method are that it is general, permits automation, and thus can be used in high-throughput fashion.
Highlights
Raman spectroscopy is a powerful technique based on the inelastic scattering of light[1], which is being increasingly used as a tool for studying the local structure of materials
We address the aspects of predicting the Raman spectra of fine-grained materials
We highlight the need for an additional averaging procedure in the case of polar materials, which takes into account the directional dependence of the phononfrequencies and Raman tensors caused by the coupling between the lattice vibrations and the induced electric field and electro-optic contribution
Summary
Raman spectroscopy is a powerful technique based on the inelastic scattering of light[1], which is being increasingly used as a tool for studying the local structure of materials. In the case of single crystals, the interpretation is greatly facilitated by group-theoretical considerations[3,8] supported by first principles calculations of lattice dynamics[9,10,11,12,13,14,15,16]. This procedure is possible because single crystals permit well-defined scattering geometries and the theory of Raman scattering in such materials is readily available.
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