Spatial heterogeneity is an intrinsic structural characteristic of amorphous materials and has been shown to be closely linked to numerous physical properties. However, its correlation with crystallization behaviors, particularly in phase-change materials, remains poorly understood. Here, we systematically investigate the spatial heterogeneity and medium-range order (MRO) in amorphous $\mathrm{G}{\mathrm{e}}_2\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_5$ (GST) materials using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and atomic electron tomography (AET), complemented by differential scanning calorimetry (DSC) to analyze the crystallization kinetics. We reveal that the correlation length of spatial heterogeneity increases progressively with subglass transition-temperature (sub-$T_{\mathrm{g}}$) annealing time, accompanied by the growth and interconnection of crystal-like MRO networks. Concurrently, the crystallization activation energy decreases markedly. Our findings reveal a relationship between MRO-associated enhancement of spatial heterogeneity and the reduced kinetic barrier to primary crystallization, offering a promising route to optimize phase-change memory performance via precisely controlled thermal annealing protocols.

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• Phase-change materials
• Disordered systems
• Glasses
• Annealing
• Crystallization
• Dark-field microscopy
• Electron microscopy