Compressibility and blue-shifting O–H stretching bands of magnesium hydroxyfluoride Mg(OH)F up to 20GPa

Highlights: • Mg(OH)F has been investigated up to ~20GPa using DACs combined with in-situ synchrotron XRD and FTIR. • The equation of state and axial compressibility have been determined. • Blue-shifting O–H stretching frequencies indicate that the O–H$\cdots$F hydrogen bond does not strengthen under compression. • Compared with O–H$\cdots$O hydrogen bonds, O–H$\cdots$F in the diaspore-type structure would present more complex behaviors under high pressure. In-situ high-pressure synchrotron X-ray diffraction (XRD) and Fourier transform infrared absorption spectroscopy (FTIR) measurements have been conducted up to ~20GPa at room temperature using diamond anvil cells (DAC), to investigate compressibility and spectroscopic properties of magnesium hydroxyfluoride, Mg(OH)F (Pnma, Z=4). Without any phase transition being observed, the pressure-volume correlation is fitted by the second-order Birch-Murnaghan equation of state, yielding K₀=80 (±2) GPa, $K_0^{\prime }$=4.0 (fixed), and V₀=145.2 (±0.3) ų. In accordance with other isostructural compounds crystallizing in the diaspore-type structure, higher contraction occurs along the c axis of Mg(OH)F. Positive pressure dependence of the O–H stretching mode frequencies initially observed at 3537 (1.56cm⁻¹/GPa), 3641 (0.67cm⁻¹/GPa), and 3680cm⁻¹ (2.85cm⁻¹/GPa) at ambient pressure indicates that the O–H$\cdots$F hydrogen bond does not strengthen under compression.