  An analysis of thermal transients from non-equilibrium ab initio
molecular-dynamics simulations can be used to calculate the thermal
conductivity of materials with a short phonon mean-free path. We adapt the
approach-to-equilibrium methodology to the three-dimensional case of a
simulation that consists of a cubic core region at higher temperature
approaching thermal equilibrium with a thermostatted boundary. This leads to
estimates of the lattice thermal conductivity for the glassy state of the
phase-change memory material, Ge$_2$Sb$_2$Te$_5$, which are close to previously
reported experimental measurements. Self-atom irradiation of the material,
modelled using thermal spikes and stochastic-boundary conditions, results in
glassy models with a significant reduction of the lattice thermal conductivity
compared to the pristine glassy structure. This approach may prove to be useful
in technological applications, e.g. for the suppression of thermal cross-talk
in phase-change memory and data-storage devices.
