9:30 AM - 9:45 AM
[SIT14-03] Unconventional high-pressure behaviors of hydrogen bonds in brucite caused by F substitution
Keywords:Brucite, Fluorine, Hydrogen bond, High pressure, Neutron diffraction
Fluorine (F) is supposed to be the most abundant halogen element in the mantle. Fluorine subduction relies on F-bearing hydrous minerals and phases in subducting slabs. Investigations on the high-pressure and high-temperature behaviors of O-H…F hydrogen bonds (H-bonds) in hydrous mineral structures are of significance. Brucite [Mg(OH)2] (P-3m1, Z = 1) is an archetype material for studying H-bonds in hydrous minerals. Previous experimental studies elucidated the hydrogen sublattice structures of Mg(OH)2 and Mg(OD)2, and revealed that the H-bonds get strengthened under high pressure. A recent study on F-doped brucite refined the hydrogen atomic coordinates in both the 2d site (1/3, 2/3, z) and the 6i site (x, 2x, z) [1], because it was hard to precisely determine the hydrogen positions based on the single-crystal XRD data. The authors concluded a promotion effect of F on pressure-induced H-bond strengthening based on the high-pressure IR and Raman spectroscopic data [1], but here we demonstrate that F actually plays a negative role.
In this study, we synthesized polycrystalline hydrogenated and deuterated samples of Mg(OH)2, Mg(OH)1.81F0.19, Mg(OD)2, and Mg(OD)1.74F0.26, together with single-crystal deuterated samples of Mg(OD)2 and Mg(OD)1.80F0.20. Neutron powder diffraction experiments revealed that the H atom of Mg(OH)1.81F0.19 locates in the 2d site, while the D atom of Mg(OD)1.74F0.26 is in the 6i site. Under high pressure, the H-bonding geometries obtained from neutron diffraction experiments revealed no obvious sign of H-bond strengthening in Mg(OH)1.81F0.19 up to 6.91 GPa and in Mg(OD)1.74F0.26 up to 10.06 GPa. The comparison between Mg(OD)2 [2] and Mg(OD)1.74F0.26 showed that F substitution slowed down the pressure-induced H-bond strengthening by restraining the stretching of d(O-D). Pressure-responses of the hydroxyl stretching modes of the single-crystal samples, Mg(OD)2 and Mg(OD)1.80F0.20, were studied using IR absorption and Raman spectroscopic measurements up to ~10 GPa. The results further demonstrated that F substitution suppressed the strengthening of H-bonds by alleviating the O-H and O-D covalent bond elongation under compression. This work has implications for the estimation of proton diffusion in F-doped brucite under high pressure and calls for future researches on proton conduction mechanisms and sound velocities.
References
[1] Miao Y., et al. (2022) Am Mineral, 107:2065-2074.
[2] Parise J. B., et al. (1994) Am Mineral, 79:193-196.
In this study, we synthesized polycrystalline hydrogenated and deuterated samples of Mg(OH)2, Mg(OH)1.81F0.19, Mg(OD)2, and Mg(OD)1.74F0.26, together with single-crystal deuterated samples of Mg(OD)2 and Mg(OD)1.80F0.20. Neutron powder diffraction experiments revealed that the H atom of Mg(OH)1.81F0.19 locates in the 2d site, while the D atom of Mg(OD)1.74F0.26 is in the 6i site. Under high pressure, the H-bonding geometries obtained from neutron diffraction experiments revealed no obvious sign of H-bond strengthening in Mg(OH)1.81F0.19 up to 6.91 GPa and in Mg(OD)1.74F0.26 up to 10.06 GPa. The comparison between Mg(OD)2 [2] and Mg(OD)1.74F0.26 showed that F substitution slowed down the pressure-induced H-bond strengthening by restraining the stretching of d(O-D). Pressure-responses of the hydroxyl stretching modes of the single-crystal samples, Mg(OD)2 and Mg(OD)1.80F0.20, were studied using IR absorption and Raman spectroscopic measurements up to ~10 GPa. The results further demonstrated that F substitution suppressed the strengthening of H-bonds by alleviating the O-H and O-D covalent bond elongation under compression. This work has implications for the estimation of proton diffusion in F-doped brucite under high pressure and calls for future researches on proton conduction mechanisms and sound velocities.
References
[1] Miao Y., et al. (2022) Am Mineral, 107:2065-2074.
[2] Parise J. B., et al. (1994) Am Mineral, 79:193-196.