| Research Abstract |
The phase transition of solid oxygen has been investigated at low-temperatures of 19 K, 180 K and 240 K and at high-pressures of up to 12 Gpa using monochromatic synchrotron x-ray diffraction techniques, and the phase relation among α-,β-δ-, and ε-0_2 has been revealed, based on the structural data. The monoclinic α-0_2 phase at 19K, with increasing pressure, directly transformed to theε-0_2 phase at 7.6 Gpa. The α-δ phase boundary was located on a higher temperature region than 240 K and the α-0_2 phase was stable over a wide pressure and temperature range. The b/a ratio of the lattice constants at 19 K exhibited a monotonic increase with pressure and reached 0.708 at 7.6 GPa, while it abruptly decreased to 0.690 by the α-ε transition. From the anisotropy of compressibility, the existence of a collinear antiferromagnetic order in the α-0_2 phase was confirmed at pressure up to 7.2 Gpa. The equation of state of the α-0_2 phase at 19 K has also been determined.
The structure and lattice
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compression of β-0_2 phase have been studied at 277 K and 300 K. The pressure dependence of lattice constants, a and c, obtained from a powder indicated an anisotropic compressibility, a is almost seven times as compressible as c.This large linear compressibility of a may be attributed to the antiferromagnetic interaction in the basal plane. An analogy of the lattice deformation between the α-δ and α-β phase transitions was also suggestive of the magnetic order in theδ phase.
The structure of the metallic ζpnase above 100 Gpa was determined to be a monoclinic lattice (SG : P2/m) with Z=2. This lattice consisted of 0_2 molecules. Therefore, this phase was concluded to be molecular semi metallic.
The single crystal growth of β, δ, and ε-0_2 phases and the structure analyses of these crystals were carried out at room temperature. The sample quality was very good and the full width at half maximum was about 0.005^o. The data collection was carried out by several methods. It is found that the thermal parameter in basal planeβ_<11>, was considerably large. Less
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