Supercritical anomalies and the Widom line for the isostructural phase transition in solids

The representation of the Widom line as a line of maximums of the correlation length and a series of thermodynamic response functions above the critical point were introduced to describe anomalies observed in water above the hypothetical critical point of the liquid–liquid transition. The supercritical region for the gas–liquid transition was also described later in terms of the Widom line. It is natural to assume that an analogue of the Widom line also exists in the supercritical region for the first-order isostructural transition in crystals, which ends at a critical point. We use a simple semiphenomenological model, close in spirit the van der Waals theory, to study the properties of the new Widom line. We calculate the thermodynamic response functions above the critical point of the isostructural transition and find their maximums determining the Widom line positions.