First-principles calculations of the epsilon phase of solid oxygen

Abstract

The crystal, electronic and magnetic structures of solid oxygen in the epsilon phase have been investigated using the strongly constrained appropriately normed (SCAN) + rVV10 method and the generalized gradient approximation (GGA) + vdW-D + U method. The spin-polarized SCAN + rVV10 method with an 8-atom primitive unit cell provides lattice parameters consistent with the experimental results over the entire pressure range, including the epsilon-zeta structural phase transition at high pressure, but does not provide accurate values of the intermolecular distances d 1 and d 2 at low pressure. The agreement between the intermolecular distances and the experimental values is greatly improved when a 16-atom conventional unit cell is used. Therefore, the SCAN + rVV10 method with a 16-atom unit cell can be considered the most suitable model for the epsilon phase of solid oxygen. The spin-polarized SCAN + rVV10 model predicts a magnetic phase at low pressure. Since the lattice parameters of the predicted magnetic structure are consistent with the experimental lattice parameters measured at room temperature, our results may suggest that the epsilon phase is magnetic even at room temperature. The GGA + vdW-D + U (with an ad hoc value of U eff = 2 eV at low pressure instead of the first-principles value of U lr eff ~ 9 eV) and hybrid functional methods provide similar results to the SCAN + rVV10 method; however, they do not provide reasonable values for the intermolecular distances.

Electronic structure of dense solid oxygen from insulator to metal investigated with X-ray Raman scattering

“…An X-ray diffraction study (6,8) indicates that the zeta phase is stiffer than the epsilon phase. This is also consistent with the recent calculation (16).…”

“…The first interpretation implies that the change in compressibility at the metallization pressure is similar to the behavior of the continuum band, which is described later in this paper. The second interpretation derives that the electronic structure changes not only in the a-b plane, as reported in the theoretical studies (15,16), but also parallel to the c axis (i.e., O 2 molecular axis) in the metallization.…”

“…The negative shift in Pi* observed in the present measurements likely reflects the metallization in the a-b plane. Theoretical calculations pointed out that metallic conduction is realized when the distance between O 8 clusters becomes shorter than the intermolecular distance within the cluster (15,16).…”

“…Calculations were performed at the non-spin-polarized condition as dense solid oxygen (the epsilon phase) is considered nonmagnetic or does not have any long-range magnetic order according to neutron studies (23,25). Details of the geometrical optimizations can be found elsewhere (16). The geometrical optimization indicated that the metallic zeta phase appears at 80 GPa and higher-pressure conditions.…”

The Cs2AgRhCl6 Halide Double Perovskite: A Dynamically Stable Lead-Free Transition-Metal Driven Semiconducting Material for Optoelectronics

“…We note that the newly-proposed SCAN-rVV10 functional is one of the strongly constrained and appropriately normed metageneralized gradient approximation (meta-GGA) functionals that is considered to model well metallic, insulating and semiconducting materials (Sun et al, 2015;Sun J. et al, 2016;Buda et al, 2017). The rVV10 part of the functional accounts for the non-local correlation part required to appropriately describe van der Waals (vdW) interaction (Peng et al, 2016;Chakraborty et al, 2018;Zhang et al, 2018;Anh et al, 2019). Bokdam et al have demonstrated that the SCAN functional accounts for short range dispersion effects-which conventional hybrid functionals do not account for-and is the most suitable functional to study the atomic structure of hybrid perovskite materials (Bokdam et al, 2017).…”