Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p11c1291f&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P11C-1291
Other
5410 Composition (1060, 3672)
Scientific paper
We have performed density functional electronic structure calculations for oxygen at 350 GPa, using the quantum espresso package with ultrasoft pseudopotentials and PBE exchange and correlation functional. The random search method was used to determine the enthalpies and lattice parameters of monatomic oxygen at 350 GPa. Other chalcogen elements (Po, Te, Se, S) all take a rhombohedral β-Po structure when compressed. Polonium forms its structure at zero pressure, Te at 11 GPa, Se at 60 GPa, and Sulfur at 153 GPa. As you move up the table, higher pressures are needed to make the element take a β-Po form. My hypothesis was that at pressures near 350 GPa, oxygen would have a β-Po structure as well. Within random search, several starting configurations are randomly chosen and relaxed, until the final structures, with the lowest enthalpy, are found several times. First, the crystal lattice is generated by randomly selecting cell-vector lengths between 0.5 and 1.5 (in arbitrary units) and three cell angles between 40° and 140°. The cell vectors are then scaled to match a new volume, which is also chosen randomly between 0.5 and 1.5 of some physically sensible volume. Then, atomic positions are obtained by generating three random numbers between 0 and 1 for each atom, which represents the positions of the atoms in terms of the crystal vectors. First-principles methods are then used to relax the cell towards the closest minimum in enthalpy. We performed random search using one oxygen atom per unit cell. Therefore, only the six cell degrees of freedom were necessary. 150 random starting systems were selected and then converged towards equilibrium at 350 GPa. 1000 iterations or more were necessary for each structure to reach hydrostaticity of the stress tensor. The systems' enthalpies were then calculated and the structures with the lowest enthalpies were analyzed. Eight different values for enthalpy local minima were obtained. The global minimum was obtained with 14 out of 150 samples. It had an enthalpy that was 50 meV lower than that of the closest local minima and had a C2/m space group. The results show that oxygen forms a monoclinic arrangement of 1-D chains at 350 GPa and zero temperature. For the global minima, the spacings between oxygens along the chains is 1.44 Å, while the distance from one chain to the closest other is 1.98 Å. Keywords: DFT, high pressure, oxygen, structural search
Farnsworth C. V.
Montoya Javier A.
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