Atomic motion in solids with dimpled potentials

TL;DR

This paper introduces a model for atomic motion in polymorphic solids with low energy barriers, predicting phase transitions and entropy changes, supported by phonon calculation methods and experimental comparisons.

Contribution

It proposes a new model for dynamically polymorphic solids with multiple local potential minima and analyzes phase transitions due to ergodic atomic motion.

Findings

Prediction of symmetry-breaking phase transition upon cooling

Finite change in lattice entropy during transition

Comparison of phonon calculation methods with experimental data

Abstract

Polymorphic solids of the same chemical composition can have different atomic structures; in each polymorph atoms vibrate around a local potential energy minimum (LPEM). If transformations to other structures have sufficiently high enthalpy barriers, then each polymorph is either stable or metastable; it is stationary and does not spontaneously change with time. But what happens, if those barriers are low? As examples, we consider NiTi shape memory alloy exhibiting a large elastocaloric effect, and selected elemental solids. We suggest a model for dynamically polymorphic solids, where multiple LPEMs are visited by ergodic motion of a single atom. We predict that upon cooling a dynamically polymorphic phase should undergo a symmetry-breaking first-order phase transition, accompanied by a finite change of the lattice entropy. We discuss 3 methods used to calculate phonons in solids with non-harmonic dimpled atomic potentials, and compare theoretical predictions to experiment.