Distinguishing Vibrational and Structural Equilibration Contributions to Thermal Expansion

Abstract

Geometric attributes of a many-body-system potential energy function suggest a natural description of that system in terms of inherent structures (local potential minima), and intra-basin vibrational displacements away from those discrete structures. This description is applied herein to the isobaric thermal expansion property (α). Starting with the virial form of the pressure equation of state, two distinct contributions to α can be unambiguously identified. The first (αvib) arises from intra-basin anharmonic thermal vibrations; the second (αstr) stems from thermally induced structural shifts in basin occupancy. Only αvib appears for nondefective crystals, and for glasses below their Tg's. An approximate analysis for liquid water suggests that αstr alone manifests a liquid-phase density maximum, thought shifted to slightly higher temperature than the observed density maximum at 4 °C.