Classical model of negative thermal expansion in solids with expanding bonds

TL;DR

This paper investigates the mechanisms of negative thermal expansion (NTE) in solids using a classical model with anharmonic bonds, showing that NTE can occur without rigid units and depends on bond flexibility and local atomic coordination.

Contribution

It demonstrates that NTE arises from flexible bonds and low-energy bond angle changes, challenging the notion that rigid units are necessary for NTE in solids.

Findings

NTE occurs in models with flexible bonds and low-energy bond angle changes.

Rigid octahedral units suppress NTE, leading to positive thermal expansion.

Similar behavior observed in diamond lattice models with varying second-neighbor interactions.

Abstract

We study negative thermal expansion (NTE) in model lattices with multiple atoms per cell and first- and second-nearest neighbor interactions using the (anharmonic) Morse potential. By exploring the phase space of neighbor distances and thermal expansion rates of the bonds, we determine the conditions under which NTE emerges. By permitting all bond lengths to expand at different rates, we find that NTE is possible without appealing to fully rigid units. Nearly constant, large-amplitude, isotropic NTE is observed up to the melting temperature in a classical molecular dynamics model of a $\mathrm{ReO}_3$-like structure when the rigidity of octahedral units is almost completely eliminated. Only weak NTE, changing over to positive expansion is observed when the corner-linked octahedra are rigid, with flexible second-neighbor bonds between neighboring octahedra permitting easy rotation. We observe similar changes to thermal expansion behavior for the diamond lattice: NTE when second-neighbor interactions are weak to positive thermal expansion when second-neighbor interactions are strong. From these observations, we suggest that the only essential local conditions for NTE are atoms with low coordination numbers along with very low energies for changing bond angles relative to bond-stretching energies.