First-principles Study Of Dense Hydrous Magnesium Silicate Phase H

Phase H is one of the Dense Hydrous Magnesium Silicate(DHMS)minerals,which might play an important role in depositing and transporting water in the deep mantle.The structural,elastic,and vibrational properties of phase H under high pressures were studied by the method of first-principles simulation based on density functional theory,and the effect of symmetrization of hydrogen bonds were analyzed in order to figure out the influence of phase H on lower wave velocities in the mantle.The main achievements are listed as follows:1.Two likely structures:Pm symmetry and P2/m symmetry are propounded to be the structure of phase H based on former theoretically simulated and experimental results.The main difference between these two structures are the position of hydrogen atoms.As for P2/m symmetry the hydrogen atoms are located on the right side in the middle of two oxygen atoms.On the other hand,hydrogen atoms for Pm symmetry are not located in the middle of two oxygen atoms.There are significant differences between lattice parameters of the two structures under low pressures.As pressure rises to about 30GPa,Pm symmetry transforms into P2/m symmetry along with the symmetrization of hydrogen bonds.Third order Birch-Murnaghan equation of state fitting are conducted to find out the bulk moduli,the first derivatives of the bulk moduli versus pressure,and cell volumes at 0 K are 156.2 and 177.9 GPa,4.40 and 4.18,59.3 and 58.4 A3,respectively.2.The elastic constants,bulk moduli,shear moduli,shear wave velocities and longitudinal wave velocities of phase H from 0 to 100 GPa are calculated based on the relationship of stress and strain.Phase transformations between phase D,phase H and ?-AIOOH are discussed,and the variation laws between pressure and seismic anisotropies of phase H are analyzed too.Under pressures range from 0 to 30 GPa,the bulk modulus,shear modulus,shear wave velocities and longitudinal wave velocities of Pm symmetry are smaller than P2/m symmetry;these two structures show similar elastic properties under higher pressures.The anisotropiesof both two structures increase with pressure,while Pm symmetry shows higher intensity of anisotropies.The trends of densities and seismic wave velocities of phase D,phase H and 6-AIOOH reflect the relationship of phase transformation between these three minerals:phase D transforms into phase H under pressures from 40 to 50 GPa,then phase H and 6-AIOOH turn into solid solution under higher pressures.Anisotropy of phase H indicates that it might be responsible for ultralow-velocity regions and large low-shear-velocity provinces in the lower mantle.3.Vibrational and Raman properties of phase H are studied using density-functional perturbation theory and factor group analysis methods.The Raman active vibrational modes for Pm and P2/m symmetry can be represented as ?Raman =14A'+7A" and ?Raman =4Ag+ 2Bg,respectively.The number of Raman active modes for P2/m and Pm symmetry are 6 and 21,respectively.Raman active modes for P2/m are listed as:A549?A560?A725?A834?A941 and A1040.Visual analysis shows that symmetrization of hydrogen bonds restricts the movement of hydrogen atoms,which results in the disappearance of Raman active modes related to hydrogen atoms in Pm symmetry.Deviations of vibrational frequencies are linearly dependent to pressure for P2/m symmetry under 0 to 100 GPa.The deviation ratios of vibrational frequencies for Pm symmetry under 0 to 20 GPa are significantly larger than those under 25 to 100 GPa.