Solid oxygen

First-principles equation of state and electronic properties of warm dense oxygen

“…X-ray diffraction and optical experiments reveal that oxygen condenses to a molecular solid with a rich phase diagram made up of at least ten different structural phases [1][2][3][4][5][6] . Static compression experiments on the solid have been performed up to 1.3 Mbar and 650 K 1 .…”

“…In addition, the solid phases exhibit a complex magnetic structure with various degrees of ordering due to a strong exchange interaction between O 2 molecules that becomes suppressed under pressure and acts in tandem with weak van der Waals forces holding the lattice together 1,12,13 .…”

Spin dynamics in liquid and rotationally disordered solid oxygen

“…Within the ␤ phase there exist magnetically short-range-ordered ͑SRO͒ regions that coexist with a background of paramagnetic excitations. Such shortrange-ordered regions 8,32 are characterized by an average correlation length ␤ ϰ a ͱ exp͑ J nn / ͱ 3T͒ ͑ 16͒ which is given in terms of the lattice constant a and the nearest-neighbor exchange constant varies from about 3a Ϸ 10 Å at 24 K ͑just above the ␣ → ␤ transition͒ down to some 2a Ϸ 6 Å at 43 K ͑just below the transition into the ␥ phase͒, while the corresponding variation in the lattice volume can only account for about 3% of such a reduction. Also, the number of such regions n is expected to increase as the temperature is decreased.…”

“…Such an arrangement of molecules is also a dominant structural motif in the rotatorphase ͑plastic͒ crystal which forms by cooling the liquid below 54.4 K at ambient pressure. 8 Freezing the liquid into the rotationally disordered crystal known as ␥-O 2 ͑GOX͒ is mediated by an anomalously small drop in molar volume, together with a meager 3.5% increase in susceptibility, and yields a cubic unit-cell structure of symmetry Pm3n, an A15 structure with eight molecules per cell. One fourth of these molecules sit at the corners of the cubic cell ͑2a sites͒ and exhibit complete ͑isotropic͒ molecular reorientations 9 as well as a spherically symmetric electron-density distribution, whereas the rest sit at the cube faces ͑6d sites͒, forming one-dimensional ͑1D͒ chains characterized by an oblateshaped electron distribution.…”

“…Molecules in these chains rotate within disk-shaped volumes at right angles to the ͗100͘ crystal axis. Explicit calculations show that such a structure is mostly stabilized by the isotropic part of the intermolecular potential 8 while the action of anisotropic interactions is almost entirely felt by the disks. The magnetic structure of ␥-O 2 also resembles that of the liquid, showing a broad diffuse peak in the single-differential magnetic cross section for polarized neutrons 5,9 centered at about 1.0 Å −1 .…”