Here, we present experimental measurements probing the influence of crystallization and surface oxidation on the Casimir force from ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST225) thin films, highlighting the effects of material state and environmental exposure on fluctuation-induced interactions. Using an Au-coated microsphere attached to a cantilever of an atomic force microscope, the Casimir force was measured above amorphous and crystalline GST225 thin films in a nitrogen-purged chamber following vacuum evacuation to minimize surface contamination and moisture that lead to strong capillary forces. Crystallization of the amorphous GST225 films led to a consistent increase in the measured Casimir force by approximately 40% at surface separations less than 100 nm, which is attributed to higher optical reflectivity and dielectric contrast spanning the infrared to visible-UV spectral range. In contrast, prolonged exposure to ambient air reduced the force by 20–30%, because of oxidation-induced changes in surface composition and dielectric function, as verified by transmission electron microscopy and spectroscopic ellipsometry. Therefore, both the phase state and surface chemistry significantly influence Casimir interactions from GST225 thin films, offering insights into material-dependent fluctuation forces relevant to nanoscale device environments.

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• Casimir effect
• Crystallization
• Dielectric properties
• Phase-change materials
• Amorphous materials
• Thin films
• Atomic force microscopy
• Ellipsometry