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

Wednesday, 17 December 2014
Masato Kato and Atsushi Kyono, University of Tsukuba, Tsukuba, Japan
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 A1g + F2g symmetry and the higher frequency peak can be assigned to a mode with A1g + Eg + F2g symmetry. The band at 331 cm-1 in the A1g + F2g mode is assigned to O4H4-translational and librational motion and the band at 539 cm-1 in the A1g + Eg + F2g 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 F2g mode increases continuously with pressure derivative of 10 cm-1/GPa. Upon increasing pressure, the A1g + Eg + F2g 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 A1g + Eg + F2g symmetry.