THE HYDROTHERMAL SYNTHESIS, CRYSTAL STRUCTURE AND COMPRESSIBILITY OF MAGNESIUM HYDROXYFLUORIDE, MGOHF

The natural forms of many synthetic hydrous phases contain fluorine in significant quantities. This substitution of F for OH may have some bearing on the overall stability, compressibility and hydrogen-bond structure of the phase. In an attempt to investigate these effects and to tailor the effect on hydrogen bonding in intermediate brucite sellaite phases, we have synthesized the MgOHF phase, which is intermediate in composition between brucite, Mg(OH)₂, and sellaite, MgF₂. While the T-X stability of the MgOHF phase is known (with complex buffering), previous studies have not identified its structure. We have succeeded in producing powder samples, under autogenic pressure at subcritical temperatures in from self-buffering reactants. The structure of MgOHF was solved by powder methods from x-ray data collected at ID30, ESRF and is of the diaspore-type. Least-squares cell parameters are a=10.128(5) Å, 4.6789(17) Å and c=3.0759(11) Å with V=145.76(7) ų. It is hoped that with neutron diffraction techniques we might detect whether there is any deviation from Pnma symmetry with ordering on F, OH sites. The analysis of 16 p, V data to 6.8 GPa, collected at ID30 for a sample of MgOHF powder loaded in 4:1 methanol:ethanol, results in equation of state parameters of V₀=145.78(3) ų, K_T=83.0(14) GPa and K'=6.2(5). In terms of density, r_calc=2.748 gcm^-3, this bulk modulus lies somewhat away from the linear trend for the OH-F density systematic, implying that hydrogen bonding may have a significant second-order reaction to pressure. High-pressure neutron diffraction data will be collected for the investigation of OD-O behaviour under compression, with an aim to quantify this effect.