Modeling the contribution of point defects to the Raman spectrum of crystalline materials

International audience; Raman spectroscopy is a widely used tool for the characterization ofinsulating or semiconducting materials of various kinds. The Raman shift is relatedto vibrationals modes of the probed sample and, as such, can be related to theatomic scale structure of the materials. However, when Raman spectrometry is usedto probe materials featuring disorder, radiation damage or simply a large enoughconcentration of point defects, the relationship between the spectrum and the atomicstructure cannot be easily unraveled. In this paper we present a method to extendthe scale of the ab initio calculation of first order Raman spectra, based on DensityFunctional Perturbation Theory (DFPT), to cope with larger systems, in order tobe able to describe point defects in the limit of low concentration. The goal is toprovide a quantitative basis for the interpretation of experimental Raman spectra.The procedure consists in embedding force constants matrices, Born effective charges,and Raman tensor, calculated with DFPT for a supercell with a point defect, intoa corresponding perfect bulk matrix to simulate a larger system. After describingin detail the procedure, we present benchmark applications to three quite differentmaterials, containing defects of various kinds: silicon carbide with an intrinsic defect(a carbon antisite), boron carbide with helium impurities |also in combination withvacancies|, and caesium lead iodide with two different alloying impurities. Strengthsand limitations of the approach are discussed in the light of the three examples.