Abstract

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 dν(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 ν(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 dν/dP and dν/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 ν(OH) downshift. (5) The pressure variation of the D quadrupole coupling constant is approximately −1 kHz/GPa.

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