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Polarization-resolved Raman and infrared phonon modes of ludlamite
Roberto L Moreira1, Ricardo Scholz2, Ariete Righi1
1Departamento de Física, ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil.
None:
The optical vibrational modes of natural monoclinic ludlamite crystal, [Formula: see text] , were investigated by polarized Raman and infrared-reflectivity spectroscopies. High-quality single crystals from a granitic pegmatite at the Boa Vista Mine (Brazil) were selected for this study. Backscattering polarized micro-Raman spectra were acquired on the main {001} cleavage face of the crystal using four different scattering geometries, with parallel and crossed polarizations relative to the crystallographic a and b axes. Near-normal infrared-reflectivity spectra were collected using a Cassegrain microscope on the {001} and {100} cleavage faces, with polarizers aligned along the crystallographic a, b, or c axes. The infrared spectra were analyzed within the Lorentz oscillator model, allowing the determination of the infrared dielectric function projections along specific crystallographic directions. In summary, all 36 Raman-active modes predicted by group theory for the anhydrous framework of ludlamite (18Ag and 18Bg) and 35 infrared-active phonons (18Au + 17Bu) were identified and assigned (2Au and 2Bu modes were missing). Additional bands were identified in Raman and infrared spectra, a two-mode behavior of cation lattice modes, attributed to the partial substitution of Fe2+ by Mg2+ in the natural crystal, as well as some bands associated with hydrogenated phosphate species. The calculated infrared dielectric functions, along with the static and high frequency (background) dielectric constants, were analyzed and found to be consistent with the expected values for ludlamite.
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