Raman scattering in Phonon Harmonic Software ver. 6.15

The Raman scattering can be calculated for any orientation of the monocrystal with respect to the incoming and outgoing light beams, and for any light polarizations. The intensities of the Raman active modes strongly depend on these orientations, and different configurational settings are experimentaly used as a selection rules, allowing to establish the symmetries of the phonon modes. Averaging over polarization of the incomming and outgoing beams the Raman spectra of unpolaried light can be computed. The calculations of the Raman spectra of polycrystals are performed by random averaging of the crystal over many orientations, and averaging of light polarizations.

Raman spectra o f crystals are devided into two groups:

• Crystals with centrosymmetric space groups. In this case only Raman active modes are visible. All modes have a single definite frequency. The infrared modes, in particular these with LO components, are not visible.
• Crystals with non-centrosymmetric space groups. In this case some Raman active modes are also infrared active. Then, infrared active modes may split to TO and LO components, and hence associated Raman spectra may also show separate TO and LO peaks. Phonon is specially compossed to handle these modes. In polycrystals Raman spectra consists of sharp peaks and background bands originating from the LO modes. Ferroelectrics, piezoelectrics, pyroelectrics, photovoltaic materials, proteins usualy belongs to this class of space groups.

The symmetry of each mode or band are determined within Phonon Software. The vibrations of each phonon mode can be animated.

FIGURE.1. Plot of Raman reduced spectra with polarized light for orthorhombic centro-symmetric monocrystal of forsterite Mg2SiO4. Left panels (a-c) show the measurements (black) and calculations (red) published in Ref. D.A.McKeown, M.I.Bell, and R.Caracas, American Mineralogist 95, 980 (2010). Right panels (d-f) show the VASP+Phonon calculations for monocrystal with the respective geometries and the same light polarization, respectively. Crystals on the left (a-c) and right (e-f) were described by the same space groups Pbnm and Pnma, respectively, but in different settings, hence on the plots follow different notations of used light polarization. The inserts show the Raman reduced spectra for low intensity peaks.

FIGURE.2. Plot of Raman spectra obtained with unpolarized lights on polycrystalline sample of non-centrosymmetric tetragonal P-42_1m akermanite Ca2MgSi2O7. (a) Direct spectra from Phonon calculations. (b) The same spectra as (a), but all peaks have been broadened by "experimental" resolution. (c) One of numerous measurements taken from RRUFF Database of Raman Spectra; http://rruff.info.

Last update: July 20, 2014