High-Pressure Behaviors of Hydrogen Bonds in Fluorine-Doped Brucite

Brucite, Mg(OH)₂ (P3̅m1, Z = 1), is a prototype material for studying hydrogen bonds in solid hydroxides. In this study, substitutional effects of fluorine (F) on the hydrogen-bonding geometries of hydrogenated and deuterated brucite were investigated under ambient conditions and at high pressure using combined experimental methods of neutron powder diffraction, Raman spectroscopy, and infrared (IR) spectroscopy. Under ambient conditions, neutron powder diffraction results showed that F substitution decreased the donor-acceptor distance and increased the hydroxyl covalent bond lengths of both hydrogenated and deuterated brucite, strengthening the hydrogen bond. Red shifts of the hydroxyl stretching modes also indicated an elongation of d(O-H) and d(O-D). High-pressure neutron diffraction experiments were performed on Mg(OH)_1.81F_0.19 and Mg(OD)_1.74F_0.26 up to 7.04 and 10.02 GPa, respectively. For both samples, changes in the hydrogen-bonding geometries did not indicate hydrogen-bond strengthening under high pressure. Compared with Mg(OD)₂, the doping of F suppressed the increase of the hydroxyl covalent bond length, the hydrogen-bond angle, and the cone angle, inhibiting pressure-induced hydrogen-bond strengthening. High-pressure Raman and IR absorption spectroscopic measurements on Mg(OD)₂ and Mg(OD)_1.79F_0.21 up to 9.7 and 13.7 GPa confirmed that F substitution restrains pressure-induced hydroxyl elongation.