A mean-field model for structural phase transitions based on ab initio band calculations

Abstract

Abstract We study a microscopic model of the structural phase transition in ordered systems. Our model is based on the idea that, in anharmonic potentials, smali harmonie displacements can trigger first-order phase transitions. This model is in contrast with the older conjectures of phonon softening, since a complete softening of a phonon branch is no longer necessary. In order to model the smali harmonie displacements we introduce thermally induced fluctuations of the geometrical order parameters describing the respective transition. It is shown that such a fluctuation model justifies the original ad hoc assumption of a weakly temperature-dependent phonon frequency. To demonstrate the applicability of our model, we use total energy data derived from quantum-mechanical band-structure calculations for the bcc ↔ hcp transition in sodium. It is shown that our model successfully leads to a first-order phase transition without complete phonon softening.