High-pressure Raman spectroscopic studies of hydrogarnet, katoite Ca3Al2(O4H4)3

Abstract:

We report in situ Raman spectroscopic studies of katoite in a diamond-anvil cell under hydrostatic conditions up to 10 GPa at room temperature. Two bands near 331 and 539 cm^-1 are observed clearly at 1.1 GPa. In the cubic garnet structure, the lower frequency peak can be assigned to a mode of A_1g + F_2g symmetry and the higher frequency peak can be assigned to a mode with A_1g + E_g + F_2g symmetry. The band at 331 cm^-1 in the A_1g + F_2g mode is assigned to O₄H₄-translational and librational motion and the band at 539 cm^-1 in the A_1g + E_g + F_2g mode is assigned to OH-translational motion. The peak positions and shapes in the Raman spectra agree well with those measured under ambient conditions. With increasing pressure, the frequency of the F_2g mode increases continuously with pressure derivative of 10 cm^-1/GPa. Upon increasing pressure, the A_1g + E_g + F_2g modes split at 5 GPa and the intensity of the this Raman band gradually decrease due to the cubic-tetragonal phase transition. The most striking characteristic of the Raman spectrum of katoite is that compression leads to the extinction of the Raman-active mode derived from A_1g + E_g + F_2g symmetry.