HR: 0800h
AN: T41B-1173    [Abstracts]
TI: High pressure proton disorder in brucite
AU: * Mookherjee, M
EM: mainak@umich.edu
AF: Department of Geological Sciences,, University of Michigan, C.C. Little Building,, Ann Arbor, MI 48109 United States
AU: Stixrude, L
EM: stixrude@umich.edu
AF: Department of Geological Sciences,, University of Michigan, C.C. Little Building,, Ann Arbor, MI 48109 United States
AB: Brucite [Mg(OH)$_{2}$] is one of the simplest hydrous phases and serves as a model system for complex hydrogen bearing silicates of Earth's mantle. Brucite belongs to a class of M(OH)$_{2}$ compounds that appear to show unusual behavior at high pressure: the protons disorder while the MO sub-lattice remains crystalline. The nature and extent of proton disorder is uncertain. One can envision two types of proton disorder: 1) dynamic; the hydroxyl bond is tilted away from the c-axis and the proton hops between three symmetrically related potential wells or 2) static; each proton occupies a single well, and long-range order is frustrated by the hexagonal symmetry of the lattice. We explore the structure and physical properties of brucite over a wide range of pressure with density functional theory using the variable cell shape plane wave pseudopotential method in the local density (LDA) and generalized gradient (GGA) approximations. In this present study, we probe the energetics underlying the structure and dynamics of the proton sub-lattice by performing a series of constrained and unconstrained static calculations based on an energetically stable supercell wherein protons occupy 6i Wyckoff positions as opposed to 2d positions. We find that the equation of state and variation of lattice parameters of the superstructure with compression agree well with experiment. The displacement of the hydrogen from the three-fold axis (2d position) increases smoothly with increasing pressure. This means that even in the absence of dynamic disorder (i.e. at 0 K), the protons are frustrated and would be expected to exhibit long range disorder akin to a spin glass. In order to shed light on the dynamic nature of the proton hopping between the three energetically equivalent 6i sites, we have determined the activation barrier for such jumps. We find that the energy barrier increases with compression, possibly indicating a transition from dynamic proton disorder at lower pressures to static disorder at higher pressure. We have also investigated the possibility of proton jumps across the interlayer, by determining the potential energy well along the O\cdots\)O vector. We infer that proton jumps across the interlayer are either strongly limited, or highly cooperative since we do not find any evidence of a double well along the O\cdots\)O vector. This picture supports the view that brucite does not exhibit hydrogen bonding.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting