Mechanical lattice instability and thermodynamical properties in classical solids

TL;DR

This paper investigates the mechanisms behind the instability of classical solids using self-consistent phonon theory, identifying two distinct processes that lead to melting and their thermodynamic signatures.

Contribution

It provides a detailed analysis of solid instability mechanisms across the phase diagram, linking them to thermodynamic properties and identifying their signatures.

Findings

Two mechanisms of solid instability identified: one at high temperatures, one near melting.

The second mechanism correlates with melting and shows divergence in compressibility and lattice expansion.

The first mechanism occurs at high temperatures without thermodynamic singularities.

Abstract

In this paper we revisit the onset of the instability of the solid state in classical systems within self-consistent phonon theory (SCPT). Spanning the whole phase diagram versus volume and versus pressure, we identify two different kinds of mechanism: one mainly relevant at constant volume, associated with the vanishing of the SCPT solution; and one related to the disappearing at a spinodal temperature of the solid phase as a metastable energy minimum. We show how the first mechanism occurs at extremely high temperatures and it is not reflected in any singular behavior of the thermodynamical properties. In contrast, the second one appears at physical temperatures which correlate well with the melting temperature and it is signalized by the divergence of the thermal compressibility as well as of the the lattice expansion coefficient.