Polarised single crystal Raman spectroscopy of sinhalite, MgAlBO4

Polarised single-crystal Raman spectra for sinhalite have been collected at 80 K over the range 1300-10 cm⁻¹. All 36 of the first-order Raman bands predicted by the factor group analysis (FGA) of the olivine structure type have been observed and assigned. Sinhalite has the smallest unit cell volume of any crystal with the olivine structure and is significantly more distorted than are the silicate olivines. This leads to distortion-related consequences in the vibrational spectra, such as greater wavenumber ranges for each type of mode and reduced Raman intensity for symmetric BO₄ modes. Intense Raman bands in the region 796-468 cm⁻¹ are assigned to BO₄ rotations/translations which deform the AlO bond without displacement of the aluminium atoms which lie on the M1 sites. Raman-active modes involving M1 cation displacements are incompatible with the FGA.