  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.
