Optical phonon features of triclinic montebrasite: Dispersion analysis and non-polar Raman modes

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.