[S-IT14] Deep Earth Sciences

[SIT14-P14] Ab initio calculation of the polarized IR spectra of hydrous Bridgmanite

*Jun Tsuchiya¹, Kikuyo Inagaki¹, Yanyao Zhang³, Jung-Fu Lin², Shun-ichiro Karato⁴, Jennifer Kung⁵, ChingChien Li⁵ (1.Geodynamics Research Center, Ehime University, 2.The University of Texas at Austin, 3.Stanford University, 4.Yale University, 5.National cheng kung university)

Nominally anhydrous minerals (NAMs), which contain hydrogen as an impurity, are the major form of water in the Earth's interior, and elucidation of their structure and physical properties is important for understanding the evolution and dynamics of the Earth. In recent years, much attention has been paid to the states of water in bridgmanite, the most abundant mineral in the Earth's interior. The purpose of this study is to establish a structural model that can explain the recently reported results of Fourier-transform infrared spectroscopy (FTIR) of hydrous bridgmanite (Fu et al. 2019) by first-principles calculations. The structure of NAMs (hydrous bridgmanite and hydrous stishovite) were calculated using ab initio calculations based on density functional theory. The generalized gradient approximation in the Perdew-Burke-Ernzerhof form (GGA-PBE, Perdew et al. 1996) was used for the exchange correlation functionals and norm-conserving pseudopotentials were used for Si, Al, O, and H (Troullier and Martins 1991). The pseudopotential for Mg was generated by von Barth-Car’s method (Karki et al.2000). These pseudopotentials have been extensively tested in the calculations of hydrous minerals and NAMs (e.g. Tsuchiya and Tsuchiya 2009). The electronic wave function was expanded in plane waves using a kinetic energy cutoff of 80 Ry. The structures used in the calculations were supercells with 2×2×1 of the unit cell of MgSiO₃ bridgmanite, with two hydrogen atoms in the Mg atom vacancies (81 atoms), Si atoms replaced with four hydrogen atoms (83 atoms), and Si replaced with Al + H (81 and 83 atoms). Brillouin zone sampling is carried out on 2×2×4 Monkhorst-Pack mesh for the supercells (Monkhorst and Pack 1976). All structural parameters are fully relaxed at static 0 K conditions using damped variable cell shape molecular dynamics (Wentzcovitch 1991) implemented in the Quantum-espresso code (Giannozzi et al. 2009) until residual forces became less than 1.0×10^-5 Ry/au. After relaxation of the structure, we calculated phonon frequencies using density functional perturbation theory (DFPT) (Baroni et al., 2001). In this presentation, the location of hydrogen in the structure and its vibrational properties will be discussed based on the theoretically obtained polarized infrared spectra of hydrous bridgmanite from the above calculations.