Tuning thermal energy transport via solid-state structure manipulation is a challenge and of vital technological importance in energy-related devices. Herein, we provide microscopic insights into the correlations among atomic motions, anharmonic lattice dynamics, and thermal switch across the ultrafast amorphous-to-crystalline transition in a promising subnanosecond phase-change material (PCM) ${\mathrm{Sc}}_{\text{0.2}}{\mathrm{Sb}}_{\text{2}}{\mathrm{Te}}_{\text{3}}$. We show a reversible octahedron-heptagon reconfiguration related to the Sc-centered atomic motifs with a slight distortion during the phase transition, which is responsible for the exceptional recrystallization kinetics. Our combined density fluctuations analysis with classical hydrodynamic theory demonstrates a jump of thermal conductivity $\left(\kappa \right)$ with the highest predicted switch ratio of 2.4 accompanied by the order-disorder transition, corresponding to the octahedral alignment of Te atoms with an increased long-range translational symmetry. Our unified lattice dynamical approach rationalizes the $\kappa$ evolution by linking Te-dominated anharmonic lattice dynamics and thermal switching, which is further benchmarked by the heat-flux-independent, nonequilibrium simulations. These results provide physical insights into PCM's complex atomic dynamics and thermal switch mechanism.

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• Thermal conductivity
• Transport phenomena
• Ultrafast phenomena
• Density functional calculations
• Machine learning