An in situ Raman study on katoite Ca3Al2(O4H4)3 at high pressure

We report on in situ high–pressure Raman spectroscopic study of hydrogrossular, katoite Ca3Al2(O4H4)3, in a diamond–anvil cell under hydrostatic conditions at room temperature. The variations of vibration modes were analyzed theoretically by factor group analysis. Three characteristic Raman peaks observed at 341, 541, and 3649 cm−1 at 1.3 GPa were obtained continuously up to 8.3 GPa. The Raman peak at 3649 cm−1 was assigned to the O–H stretching vibration modes of A1g + Eg symmetry at 3634 cm−1 and F2g symmetry at 3656 cm−1. The pressure dependence of the two O–H stretching vibration modes showed negative pressure shifts, indicating that hydrogen bond became shorter and stronger with compression. The most striking characteristic was that above 5.1 GPa pressure derivative of the O–H stretching vibration mode of F2g symmetry started to decrease significantly. This change suggests that symmetry of the H4O4 tetrahedron changes at 5.1 GPa, implying the phase transition of katoite to its high–pressure phase. Pressure derivatives of the two O–H stretching vibration modes of A1g + Eg and F2g symmetries are −7.2 and −1.1 cm−1/GPa, respectively, which yields negative Grüneisen parameters. In the frequency region of lattice mode, the lower frequency peak observed at 341 cm−1 was assigned to librational R(O4H4) vibration mode of A1g symmetry, whose frequency increased continuously up to 5.1 GPa with pressure derivatives of 6.5 cm−1/GPa. Meanwhile, the higher frequency peak at 541 cm−1 was assigned to T(OH) motions of A1g + Eg symmetry at 523 cm−1 and F2g symmetry at 545 cm−1, whose frequencies increased with pressure derivatives of 4.4 and 4.9 cm−1/GPa, respectively. These pressure coefficients in the lattice mode lead to the isothermal mode Grüneisen parameters varying from 0.49 to 1.11. Values of the full width at half maximum (FWHM) of all observed Raman bands were continuously increased up to 5.1 GPa, but their increasing rates became higher above this pressure. The result is also indicative that katoite transforms to the high–pressure phase above 5.1 GPa.