An Initio Study Of The Magnetism And Electrical-structure Of Solid Oxygen Under High Pressure

In many aspects, solid oxygen is an unusual crystal. O₂ is the only elementary molecule that carries a magnetic moment. Among the simple molecular systems (O₂, H₂, N₂ and I₂) studied, solid oxygen has the unique property of retaining molecular magnetism up to relatively high pressure. There is a particular interest in the high-pressure behavior of solid oxygen. Superconductivity of solid oxygen had been found in numerous experimental studies about 100 GPa.Experimentally, solid oxygen has been studied up to a pressure of 130 GPa. At ambient pressure, solid oxygen is crystallizes inα,β, andγphase in the low temperature ranges. Theγphase is cubic and paramagnetic, theβphase is rhombohedral and short-range magnetically ordered, whereas theαphase is monoclinic and exhibits a long-range antiferromagnetic order, space group C2/m. At about 3 GPa, theα–O₂ phase transforms to orthorhombicδ-O₂ (space group Fmmm). A new type of magnetic order with ferromagnetic stacking of the antiferromagnetic O₂ planes was discovered inδphase by neutron diffraction, at P = 6.2 GPa. The stability range ofδ-O₂ extends up to nearly 10 Gpa, Where it transforms into a third insulatingε-O₂ phase, X-ray diffraction has suggested thatε-O₂ has a monoclinic space group (C₂/m) with eight molecules per cell, a structure that consisted of an O8 cluster with 4 molecules. Finally, at a pressure of 96 GPa,ε-O₂ transforms to the metallicζ-O₂ phase. It is known that high-pressure crystal structures of theδandεoxygen solid occur at low temperature, but the evolution of the magnetic properties and the electronic structure ofδ-O₂ andε-O₂ with increasing pressure are not fully known. Afterδ→εphase transition, the long-range antiferromagnetic order disappears. If the natural magnetism of oxygen molecular is disappeared with the phase transition, this needs the experiment to be test.Pseudopotential plane-wave ab initio calculations are performed within the framework of Density Functional Theory (DFT) through the CASTEP code, use the generalized gradient approximation (GGA) in the Perdew-Burke-Eruzerhof (PBE) scheme to describe the exchange and correlation effect.Firstly, we perform the geometry optimization for theδandεphase. The results are follows:1. The structures with antiferromagnetic order become energetically more favorable then the structure without magnetic order in theδandεphase.2. The structures with antiferromagnetic order have the bigger volume than the structure without magnetic order in theδandεphase. So, they have the lower energy density.The results shows that theεphase with the antiferromagnetic order have the lower energy. But the experiment observe that theεphase exhibit the magnetic collapse. Secondly, the electronic band structure for theδandεphase are calculated along high-symmetry direction in high pressure. The calculations show that theδphase has a direct energy gap and theεphase has the indirect band gap. It is interesting that the band gapes for the two phases are crossed at 10 GPa. It is means that the phase transition may occur at 10 GPa. The changed energy band gap may as a proof for the phase transition from theδtoεphase. Our results depend on the experiment to be confirmation.