Long oxygen molecules beyond diatomic oxygen are physically possible under extreme conditions.
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Scientific studies and structure prediction methods demonstrate that oxygen can form larger polymeric structures and molecules beyond diatomic oxygen, such as spiral chain O4 and cyclic O6 formations, under extreme high-pressure conditions.
report here the prediction of the dissociation of molecular oxygen into a polymeric spiral chain O 4 structure (space group I 4 1 / acd , θ -O 4 ) above 1.92-TPa pressure using the particle-swarm search method. The θ -O 4 phase has a similar structure as the high-pressure phase III of sulfur. The molecular bonding in the insulating ε -O 8 phase or the isostructural superconducting ζ -O 8 phase remains remarkably stable over a large pressure range of 0.008–1.92 TPa. The pressure-induced softening of a transverse acoustic phonon mode at the zone boundary V point of O 8 phase might be the ultimate driving force for the formation of θ -O 4 . Stabilization of θ -O 4 turns oxygen from a superconductor into an insulator by opening a wide band gap (approximately 5.9 eV) that originates from the sp 3 -like hybridized orbitals of oxygen and the localization of valence electrons.
Keywords: solid oxygen, spiral chain structure
As a long-standing problem in physics and chemistry, as well as earth and planetary sciences, high-pressure dissociation of diatomic molecules, such as H 2 , N 2 , O 2 , F 2 , Cl 2 , Br 2 , and I 2 , has attracted a lot of attention. Among these molecular systems, solid oxygen is a system of particular interest and exhibits many unusual physical properties by virtue of its molecular spin and the resultant spin-spin interactions, which make the system a critical test case for condensed-matter theory ( 1 , 2 ). Oxygen is also the third most abundant element in the Solar System, and its behavior under extreme pressures provides important insight into the oxygen-related systems for a better understanding of the physics and chemistry of planetary interiors.
Oxygen exhibits a rich polymorphism with seven unambiguously established crystalline phases. Upon cooling at ambient pressure, oxygen is in turn solidified to the paramagnetic γ-phase, the magnetically disordered (short-range ordered) β-phase ( 3 , 4 ), and ultimately the antiferromagnetic α-phase ( 5 ). Up
Spiral chain O4 form of dense oxygen | PNAS
Contents
## Abstract
Oxygen is in many ways a unique element: It is the only known diatomic molecular magnet, and it exhibits an unusual O8 cluster in its high-pressure solid phase. Pressure-induced molecular dissociation as one of the fundamental problems in physical sciences has been reported from theoretical or experimental studies of diatomic solids H2, N2, F2, Cl2, Br2, and I2 but remains elusive for molecular oxygen. We report here the prediction of the dissociation of molecular oxygen into a polymeric spiral chain O4 structure (space group I41/acd, θ-O4) above 1.92-TPa pressure using the particle-swarm search method. The θ-O4 phase has a similar structure as the high-pressure phase III of sulfur. The molecular bonding in the insulating ε-O8 phase or the isostructural superconducting ζ-O8 phase remains remarkably stable over a large pressure range of 0.008–1.92 TPa. The pressure-induced softening of a transverse acoustic phonon mode at the zone boundary V point of O8 phase might be the ultimate driving force for the formation of θ-O4. Stabilization of θ-O4 turns oxygen from a superconductor into an insulator by opening a wide ban
Article https://doi.org/10.1038/s41467-025-61390-0
Machine Learning simulations reveal
oxygen’s phase diagram and thermal
properties at conditions relevant to
white dwarfs
Yunlong Wang1,5, Jiuyang Shi1,5, Zhixin Liang1, Tianheng Huang1, Junjie Wang1,
Chi Ding1 , Chris J. Pickard 2,3, Hui-Tian Wang 1
, Dingyu Xing1,
Dongdong Ni4 & Jian Sun 1
Current studies show that oxygen does not aggregate into a polymeric phase
even under pressures up to 10 TPa. To address the critical knowledge gap in
understanding dense oxygen, here we show the complete polymerization
process of oxygen, by using structure prediction methods. We determine the
crystal structures of oxygen up to 1 PPa (1000 TPa), identifying a novel twodimensionally bonded body-centered tetragonal (bct) phase and a fully polymerized hexagonal close-packed (hcp) phase. Electronic structure analysis
reveals significant bond softening in the bct phase with increasing pressure,
which may affect the dynamic behavior under finite temperatures. So, we
employ the machine learning potential molecular dynamics and the two-phase
method to construct the melting curve of oxygen up to 200 TPa (200 TPa,
23,740 K) and identify abnormal melting
Everything we examined (4) — 3 independent sources
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