Theoretical calculations for solid oxygen under high pressure

TL;DR

This paper uses theoretical calculations to study the crystal structure of solid oxygen under high pressure, explaining structural stability, distortions, and phase transitions up to 7 GPa.

Contribution

It provides a detailed theoretical analysis of solid oxygen's structure under pressure, including stability, distortions, and phase transition insights, aligning well with experimental data.

Findings

Monoclinic alpha structure stable up to 6 GPa

Lattice parameters agree with experiments

Energy difference between alpha and delta structures becomes very small near 7 GPa

Abstract

The crystal structure of solid oxygen at low temperatures and at pressures up to 7 GPa is studied by theoretical calculations. In the calculations, the adiabatic potential of the crystal is approximated by a superposition of pair-potentials between oxygen molecules calculated by an ab-initio method. The monoclinic alpha structure is stable up to 6 GPa and calculated lattice parameters agree well with experiments. The origin of a distortion and that of an anisotropic lattice compressibility of the basal plane of alpha-O2 are clearly demonstrated. In the pressure range from 6 to 7 GPa, two kinds of structures are proposed by X-ray diffraction experiments: the alpha and orthorhombic delta structures. It is found that the energy difference between these structures becomes very small in this pressure range. The relation between this trend and the incompatible results of X-ray diffraction experiments is discussed.