Pressure-induced OH frequency downshift in brucite: frequency-distance and frequency-field correlations

The ordered Mg(OH)₂ crystal structure has been optimized for different pressures in the pressure range 0-21 GPa, within the trigonal P m1 space group. Anharmonic frequencies for the Raman-active OH vibrational mode were calculated from potential energy curves obtained by B3LYP and PW91 calculations with the CRYSTAL06 program. We find that an external pressure leads to an OH frequency downshift, in agreement with experiment, but there are no hydrogen bonds involved. The frequency downshift depends linearly on the electric field exerted on the OH ions from its neighbours in the 'opposite' layer. In agreement with experiment we also find that the pressure-induced 'v(OH) vs. interlayer R(O - - - O)' curve deviates from the well established correlation curves for hydrogen-bonded systems in the literature; the Δv/ΔR slope for high pressure conditions is much smaller. We also point out the merits of reporting both v(OH) and Δv_gas-to-solid for comparison with experiment.