Anomalous dynamics of interstitial dopants in soft crystals

TL;DR

This paper reveals that classical models fail to accurately describe interstitial dopant dynamics in thermally excited soft crystals, especially near melting, due to disordered potential landscapes and dopant interactions.

Contribution

It demonstrates through simulations that classical assumptions break down in entropically-stabilized BCC crystals, highlighting the importance of disordered landscapes and dopant interactions.

Findings

Classical theories fail near melting due to thermal fluctuations.

Dopant dynamics become localized and heterogeneous.

Disordered potential landscapes and dopant interactions are key factors.

Abstract

The dynamics of interstitial dopants governs the properties of a wide variety of doped crystalline materials. To describe the hopping dynamics of such interstitial impurities, classical approaches often assume that dopant particles do not interact and travel through a static potential energy landscape. Here we show, using computer simulations, how these assumptions and the resulting predictions from classical Eyring-type theories break down in entropically-stabilised BCC crystals due to the thermal excitations of the crystalline matrix. Deviations are particularly severe close to melting where the lattice becomes weak and dopant dynamics exhibit strongly localised and heterogeneous dynamics. We attribute these anomalies to the failure of both assumptions underlying the classical description: i) the instantaneous potential field experienced by dopants becomes largely disordered due to thermal fluctuations and ii) elastic interactions cause strong dopant-dopant interactions even at low doping fractions. These results illustrate how describing non-classical dopant dynamics requires taking the effective disordered potential energy landscape of strongly excited crystals and dopant-dopant interactions into account.