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

In-situ high-pressure synchrotron X-ray diffraction (XRD) and Fourier transform infrared absorption spectroscopy (FTIR) measurements have been conducted up to ~20 ​GPa 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, K0′ ​= ​4.0 (fixed), and V0 ​= ​145.2 (±0.3) Å3. 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 (1.56 ​cm−1/GPa), 3641 (0.67 ​cm−1/GPa), and 3680 ​cm−1 (2.85 ​cm−1/GPa) at ambient pressure indicates that the O–H⋯F hydrogen bond does not strengthen under compression.