Static compression and H disorder in Mg(OH)2 (brucite) to 11 GPa : a powder neutron diffraction study

Neutron diffraction data suitable for Rietveld refinements were collected on a powder sample of synthetic Mg(OH)(2) by the Polaris time-of-flight spectrometer (ISIS spallation source, U.K.) at 10(-4), 7.8(3) and 10.9(6) GPa. The Paris-Edinburgh high-pressure cell with WC anvils was used. Pressure calibration and equation-of-state results were attained by separate runs with an NaCl internal standard. Interpolation of p(V) data by the fourth-order Birch-Murnaghan e.o.s. yields K-0=41(2) GPa, K'(0)=4(2) and K''(0)=1.1(9) GPa(-1). The bulk modulus obtained is smaller than previously reported results. Rietveld refinements (R(prof)=1.45% and 2.02% at 10(-4) and 10.9 GPa) show that H lies on the threefold axis (1/3, 2/3, z) up to 10.9 GPa, where a model with H disordered in (x, 2x, z) can be refined. In the latter case, a hydrogen bond with O-H=0.902(7), H..O'=2.026(8) Angstrom and < OHO' = 145.9(7)degrees is observed. Differences with previous results for deuterated brucite are discussed. The onset of H disorder, and a jump of the c/a ratio vs. pressure at 6-7 GPa, may be related to a second-order phase transition consistent with recently reported Raman spectroscopic results.