Revision 1 1 Compression and Structure of Brucite to 31 GPa from Synchrotron X - ray 2 Diffraction and Infrared Spectroscopy Studies

23 Synchrotron X-ray powder diffraction and infrared (IR) spectroscopy studies on 24 natural brucite were conducted up to 31 GPa using diamond anvil cell (DAC) techniques 25 at beamlines X17C and U2A of National Synchrotron Light Source (NSLS). The lattice 26 parameters and unit-cell volumes were refined in m1 3 P − space group throughout the 27 experimental pressure range. The anisotropy of lattice compression decreases with 28 pressure due to a more compressible c axis and the compression becomes nearly isotropic 29 in the pressure range of 10-25 GPa. The unit cell volumes are fitted to the third-order 30 Birch-Murnaghan equation of state, yielding K0 =39.4(1.3) GPa, K0 = 8.4(0.4) for the 31 bulk modulus and its pressure-derivative, respectively. No phase transition or 32 amorphization was resolved from the X-ray diffraction data up to 29 GPa, however, 33 starting from ~4 GPa, a new infrared vibration band (~3638 cm) 60 cm below the OH 34 stretching A2u band of brucite was found to co-exist with the A2u band and its intensity 35 continuously increases with pressure. The new OH stretching band has a more 36 pronounced redshift as a function of pressure (-4.7 cm/GPa) than the A2u band (-0.7 37 cm/GPa). Comparison with first principles calculations suggests that a structural change 38 involving disordered H sublattice is capable of reconciling the observations from X-ray 39 diffraction and infrared spectroscopy studies. 40