Electronic Raman study ofFe2+in FePX3

The electronic Raman spectrum of ${\mathrm{Fe}}^{2+}$ ions in ${\mathrm{FePS}}_{3}$ and ${\mathrm{FePSe}}_{3}$ layered crystals is studied and interpreted in a quasimolecular approach for the iron and its environment. We apply crystal-field theory, and the transition probability for the electronic Raman process is computed as a function of crystal-field parameters and compared with the experimental results. It is found that trigonal distortion from cubic symmetry and spin-orbit coupling are the main contributions. From the spectrum in polarized conditions, the ${\mathrm{Fe}}^{2+}$ site in ${\mathrm{FePSe}}_{3}$ and ${\mathrm{FePS}}_{3}$ are shown to have trigonal and monoclinic symmetry, respectively. In ${\mathrm{FePS}}_{3}$ the symmetry lowering is explained as a consequence of a magnetostriction effect occurring below the N\'eel temperature. The evolution of the spectra as temperature is raised is also discussed.