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.

Key Words:

• High pressures
• insulator-metal transitions
• orientational correlations