We employ polarization-resolved Raman spectroscopy combined with first-principles calculations to study the sodium-ion lattice dynamics in a sodium zigzag ordered cobaltate compound ${\mathrm{Na}}_{0.5}{\mathrm{CoO}}_2$. We detect two sodium phonon modes at 129 and $157{\mathrm{cm}}^{-1}$ for the first time, and their mode frequencies are consistent with first-principles phonon calculations based on an orthorhombic unit cell. We find that they appear below around $T^{*}\sim 300\pm 50\mathrm{K}$ with large linewidth broadening, much lower than the sodium zigzag ordering temperature $T_{\text{S}}\sim 460\mathrm{K}$, and then narrow at lower temperatures. We interpret the sodium-phonon anomalies occurring at $T^{*}$ as a dynamical-to-static crossover involving mainly the motion of sodium ions. Our results suggest that the gradual freezing of the sodium ions and the well-defined static sodium-zigzag order below $T^{*}$ set the stage for the emergent electronic and magnetic orders in the ${\mathrm{CoO}}_2$ layer of ${\mathrm{Na}}_{0.5}{\mathrm{CoO}}_2$.

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• Ionic conductivity
• Lattice dynamics
• Phonons
• Batteries
• Density functional calculations
• Raman spectroscopy