Hydroxyl group/fluorine disorder in deuterated magnesium hydroxyfluoride and behaviors of hydrogen bonds under high pressure

Metal hydroxyhalides with the laurionite-type structure (Pnma, Z = 4) are representatives of studying hydrogen bonds with halogens as proton acceptors. In this study, high-pressure responses of the O-D center dot center dot center dot F hydrogen bonds in deuterated magnesium hydroxyfluoride [ideally Mg(OD)F] were investigated using neutron powder diffraction and Raman spectroscopy. The Rietveld analysis on the neutron powder diffractogram collected under ambient conditions revealed a chemical formula of Mg(OD)0.920(12)F1.080(12) and hydroxyl group/fluorine disorder (OD/F disorder) in the crystal structure, which gave rise to two hydrogen-bonding configurations. The evolution of the hydrogen-bonding geometries under high pressure were obtained from neutron powder diffraction experiments up to 9.8 GPa, evidencing no clear sign of pressure-induced hydrogen bond strengthening. The Raman spectra collected under ambient conditions showed three hydroxyl stretching bands at 2613, 2694, and 2718 cm-1. The high frequencies of the O-D stretching modes indicated that the hydroxyls in Mg(OD)0.920(12)F1.080(12) should be involved in weak or none hydrogen-bonding interactions, though the presence of hydrogen bonds cannot be ruled out. Up to 20.2 GPa, the mode initially centered at 2694 cm-1 displayed a pressure-induced blue shift, revealing that the hydrogen bonds were not strengthened under compression, confirming the conclusion drawn by the neutron diffraction results. Discussions are made on the existence of hydrogen bonds and the causes of the blue-shifting hydroxyls in Mg(OD)0.920(12)F1.080(12) under ambient conditions and at high pressure.