First-Principles Calculation on Crystal Structure and Elastic Properties of Py-FeO2, Py-FeOOH and ε-FeOOH under High Pressures.

Py-FeO₂, Py-FeOOH and ε-FeOOH are important components of mantle and core-mantle boundary. Their physical evolution characteristics at high temperature and high pressure are important for understanding the composition, structure and dynamic process of mantle. The crystal structures and elastic properties of Py-FeO₂ and Py-FeOOH at 0-350 GPa and ε-FeOOH at 0-170 GPa were calculated by first principles in this study. The Py-FeO₂ and Py-FeOOH belong to the cubic crystal system, their lattice constants decrease gradually with increasing pressure. The ε-FeOOH belongs to orthorhombic crystal system, and its lattice constants decrease with increasing pressure, however, a and b axes bump up at 33 GPa, while c axis swoop at 33 GPa. The cell densities of Py-FeO₂, Py-FeOOH and ε-FeOOH increase with pressure, and the relative cell density among these three phases is Py-FeO₂ > Py-FeOOH > ε-FeOOH. The bulk moduli of Py-FeO₂, Py-FeOOH and ε-FeOOH increase linearly with pressure, and the shear moduli of Py-FeO₂ and Py-FeOOH increase linearly with pressure, but the shear modulus of ε-FeOOH mutates at 33 GPa. Py-FeO₂ has the highest bulk modulus, and Py-FeOOH and ε-FeOOH have almost the same bulk modulus at high pressures. In addition, Py-FeO₂ has the largest shear modulus, and ε-FeOOH has the smallest shear modulus. The compression wave velocities of Py-FeO₂, Py-FeOOH and ε-FeOOH decrease gradually with the increasing pressure, while the shear wave velocities of Py-FeO₂ increase gradually with the increasing pressure. The shear wave velocity of Py-FeOOH decreases with the increasing depth in the range of 0-2 000 km, and the variation is small in the range of 2 000-6 000 km (5.8 km/s < vs < 6.0 km/s). The shear wave velocities of ε-FeOOH mutate at 33 GPa (about 900 km depth). The wave velocity of ε-FeOOH is the lowest, while that of Py-FeO₂ is the highest. Based on comprehensive theoretical calculations, it is found that Py-FeO₂ and Py-FeOOH have the high density and low wave velocity characteristics, which are consistent with the properties of the mantle ultra-low velocity zone (ULVZs). Py-FeO₂ and Py-FeOOH may enrich and sink to the core-mantle boundary after formation, becoming the source of the ULVZs. The hydrogen bond symmetry of ε-FeOOH under 33 GPa may affect the crystal structure of ε-FeOOH, the atomic interactions of ε-FeOOH, and then the elastic properties and seismic wave velocity of ε-FeOOH. [ABSTRACT FROM AUTHOR]