Optical phonon features of triclinic montebrasite : dispersion analysis and non-polar Raman modes.
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2015
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Polarized infrared and Raman spectra of triclinic LiAl(PO4)(OH) [montebrasite] single crystal were
recorded for appropriate optical configurations. Dispersion analysis was applied on the infrared
reflectivity spectra taken at low incidence angle (11 ) to determine the oscillator parameters and the
dipole directions of the polar phonons. In particular, all the 27 polar phonons, predicted by group theory
for triclinic P1 structure,were determined. The obtained dielectric tensor parameters have been checked
by comparison between predicted and measured infrared spectra at higher incidence angle (34 ). The
azimuth and co-elevation angles obtained from the dispersion analysis showed that the response of
several polar phonons is close to that of an orthorhombic system. Polarized Raman spectra obtained in
several scattering geometries allowed us to obtain well-defined 24 non-polar modes, also in perfect
agreement with group theory. The selection rule between Raman and infrared phonons was respected,
confirming the centrosymmetric structure and ruling out any relevant influence of defects. The relatively
narrow phonon bands are compatible with a highly ordered structure with fully occupied atomic sites.
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Triclinic crystals
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ALMEIDA, R. M. et al. Optical phonon features of triclinic montebrasite: dispersion analysis and non-polar Raman modes. Vibrational Spectroscopy, v. 77, p. 25-34, 2015. Disponível em: <http://www.sciencedirect.com/science/article/pii/S0924203115000168>. Acesso em: 15 out. 2015.