| Solid oxygen has attracted much attention due to its unique properties and the peculiar magnetic interactions between oxygen molecules. At very high pressures, solid oxygen changes from an insulating to a metallic state; at very low temperatures, it even transforms to a superconducting state. At present, six solid-state phases were found, which include three low-temperature phases: theα,β, andγphase stable at ambient pressure, and three high pressure phases at room temperature: the orange-coloredδphase stable between 9.6 and 10 GPa, the dark-redεphase between 10 and 96 GPa, and the metallicζphase above 96 GPa(Our predicted the metallicζphase occurs at 42 GPa). The molecular character is still present in the metallicζphase until 220 GPa.In the paper, the phase transitions from theδ-εand theε-ζhave been investigated under high pressure, using pseudopotential plane-wave and full-potential linearized augmented plane-wave (FLAPW) methods, respectively. The phase transition pressures of theδ-εand theε-ζwere predicted, respectively. At the section of the phase transition from theδphase to theεphase, we discuss our results on the change of magnetic, the electronic structure and the optical properties. The structural information in theεphase and theζphase were investigated, particularly. The mechanism of the phase transition between theε-ζphases was obtained. We found that the change of the k-edge of the oxygen atom can be used as a means to identify the phase transition between the two phases experimentally.Firstly, the phase transition from theδ-εhas been discussed. We found that theδ-εphase transition is a first order transition with a discontinuous volumetric change and from the antiferromagneticδ-O2 phase to the nonmagnieticε-O8 phase, consistent with the experimental results. The calculated energy band in theδ-O2 phase shows that the direct band gap of 1.22eV smaller than the experimental value of 2.2 eV, which is significantly underestimate by the LDA. It is found thatε-O8 at 17.6 GPa is an insulator with an indirect band gap of 0.64 eV (Г-L). The optical properties can be used for identifying the phase transition fromδ-O2 toε-O8. In theε-O8 phase, the highest peak happens at about 2 eV (the visible portion) and the intensity is obviously stronger than the highest peak in theδ-O2 phase. It is comes from the transition between the O atom 2s to O atom 2p.Secondly, we predicted the structure of metallicζ-O2 using density functional theory and the plane-wave pseudopotential method. Our geometry optimizations show that the a and b lattice constants are abruptly changed at 50 GPa, where a becomes larger and b smaller than the experimental results. The c lattice parameter is in excellent agreement with the experimental data in the whole pressure range. However the slopes of a and b lattice parameters with pressure are consietent with the experimental measurement. Consequently, an isostructuralε-ζbegins at 42 GPa and completes at 50 GPa, that is, the two phases coexist in the pressure range from 42 to 50 GPa.The charge density contours of at theε-O8 at 17.6 GPa show that the (O2)4 clusters are clearly observed. However, there are electrons appear in the space between O8 cluster along b axis in the charge density contours of the metallicζ-O2 in 50 GPa, that is, the clusters connect each other by covalent bonds along b-axis.The pressure dependences of the distances between nearest neighbors: the intra-cluster distances d1 and the inter-cluster distances d2 in theε-O8 andζ-O2 phase, are calculated. It is interesting to note that d1 is slightly larger than d2 above 50 GPa. Judging from the calculated data, theε-O8 clusters collapse in the metallicζ-O2 phase. The L (O1– O1) shows a pressure-induced step change at the transition. After the phase transition, the bond lengths L (O1– O1) and L (O2– O3) have similar rates of change with pressure, hence, there is nearly the same compressibilities in the metallicζ-O2 phase.The near-edge x-ray absorption fine structure (NEXAFS) spectra and crystal structures of theε-O8 phase and the metallicζ-O2 phase have been investigated. Both theσ* andπ* peaks in the NEXAFS spectra of the O1 atom k-edge have changed, but for the O2 atom k-edge, the change of theσ* peak is relatively smaller with itsπ* peaks practically unchanged.These changes of the NEXAFS spectra with pressure can be used for identifying the phase transition fromε-O8 toζ-O2 phase experimentally. There is no phonon softening at the phase transition pressure. Pressure induced softening of phonon frequencies is not found during the phase transitionAt last, we also investigated the phase transition of an important AX2 compounds of CaCl2 under high pressure. We present the studies in CaCl2 (Pnnm, Z = 2) andα-PbO2 (Pbcn, Z = 4) phases in the appendix, respectively. We estimate the transition pressure between the two phases by the crossing point of their enthalpies, which are equivalent to the Gibbs free energy at zero temperature, after optimizing the structures under high pressure. Our results show that the transition happens at about 2.9 GPa, which is in agreement with the experimental data 3 GPa. We calculated the structural parameters, milliken population analyses, density of states and optical proanaperties. The calculated results show that there are no charge transfers in CaCl2 when phase transition happens. It is found that the transitions from the Cl 3p to Ca 4s and Cl 3s to Ca 3p orbitals contribute mainly to the dielectric function. The optical properties of CaCl2 (Pnnm, Z = 2) do not vary much under pressure. However, some of the dispersion curves of optical constants in theα-PbO2 (Pbcn, Z = 4) phase are changed significantly induced pressure. |
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