Anharmonic OH vibrations in Mg(OH)_2 (brucite): Two-dimensional calculations and crystal-induced blueshift

摘要

A two-dimensional quantum-mechanical vibrational model has been used to calculate the anharmonic OH vibrational frequencies in the layered Mg(OH)_2 (brucite) crystal. The underlying potential energy surface was generated by density functional theory (DFT) calculations. The resulting OH frequencies are upshifted (blueshifted) by about +75 cm~(-1) with respect to the gas-phase OH frequency (+120 cm~(-1) in experiments; the discrepancy is mainly due to inadequacies in the DFT and pseudopotential models). The Raman-IR split is about 50 cm~(-1), both in the calculations and in experiments. We find that the blueshift phenomenon in brucite can qualitatively be explained by a parabolalike “OH frequency versus electric field” correlation curve pertaining to an OH~- ion exposed to an electric field. We also find that it is primarily the neighbors within the Mg(OH)_2 layer that induce the blueshift while the interlayer interaction gives a smaller (and redshifting) contribution.