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Anharmonic OH vibrations in brucite: Small pressure-induced redshift in the range 0–22 GPa

Materials Chemistry, The Angstrom Laboratory, Uppsala University, Box 538, S-75121 Uppsala, Sweden

Correspondence: ^* E-mail: kersti{at}mkem.uu.se

The uncoupled anharmonic OH-stretching vibrational frequency for the layered mineral Mg(OH)₂ (brucite) has been calculated in the pressure range 0–22 GPa. Quantum-mechanical electronic structure (DFT) calculations were performed, followed by quantum-mechanical vibrational energy calculations. The following findings emerged: (1) The calculated d(OH)/dP slope is –4 cm^–1/GPa, in agreement with the experimental literature value [taken as the average between the Raman and IR-measured slopes for Mg(OH)₂]. (2) The calculated (OH) vs. R(O···O) correlation is linear and the slope is much smaller than that of traditional H-bond correlation curves in the literature. (3) The main origin of the small d/dP and d/dR(O···O) slopes is the small electric field variation as the mineral layers are pressed toward each other. (4) At high pressure, the OH^– ions show some tendency to be tilted with respect to the c axis, and a larger tilt angle leads to a larger (OH) downshift. (5) The pressure variation of the D quadrupole coupling constant is approximately –1 kHz/GPa.

Key Words: Brucite • anharmonic OH frequencies • high pressure • electric field • hydrogen bonding • correlation curve • hydrous mineral