Some Aspects of Phase Diagrams of Homonuclear Diatomic Assemblies, Including Melting and Critical Point Properties

Abstract

Some aspects of the phase diagrams are considered for the homonuclear diatomics H2, N2, O2 and the halogens I2, Br2 and Cl2. It is first pointed out that, at the critical point, N2 and O2 resemble argon, while H2 has common elements with He. Attention is focussed on similarities between H2 and I2, and the halogens Br2 and Cl2 under pressure, though it is stressed that totally different ranges of pressure are involved when comparing these materials. However, zig-zag chains appear to play a role in the solid-state structures of all these materials, and these lead to two-dimensional structures, and correspondingly low melting points, with increasing pressure. After a survey of some further facts on crystal transitions under pressure, model predictions plus computer experiments are invoked. Some possible directions for future progress arc proposed.