Charge-density-wave (CDW) order in ${\mathit{R}\mathrm{Pt}}_2{\mathrm{Si}}_2$ $(\mathit{R}=\mathrm{Pr}, \mathrm{Nd})$ develops through a complex sequence of structural and electronic transitions. Using atomic pair distribution function analysis and large-scale structural modeling, we show that both compounds exhibit local lattice distortions dominated by locally static Pt displacements already present at room temperature, prior to the onset of long-range CDW order. Upon cooling, these distortions become progressively correlated and evolve into superstructures characterized by alternating short and long Pt-Pt distances, forming lattice modulations along different crystallographic directions. Our results suggest a predominantly order-disorder character of the higher-temperature second-order transitions, whereas the lower-temperature first-order transitions involve a significant additional displacive component, superimposed on the preexisting Pt displacements, and further reorganization of the CDW state. These findings establish a structural connection between intrinsic lattice instabilities and the development of long-range CDW order in ${\mathrm{PrPt}}_2{\mathrm{Si}}_2$ and ${\mathrm{NdPt}}_2{\mathrm{Si}}_2$.

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• Charge density waves
• Rare-earth alloys
• Monte Carlo methods
• X-ray scattering