${\mathrm{TbMnO}}_3$ is a model type-II multiferroic where ferroelectricity appears with spiral magnetic order, but its microscopic origin remains unresolved. Utilizing novel approach using polarized Raman spectroscopy, we isolate individual Raman tensor and directly provide evidence of anionic displacements across the ferroelectric transition $\left({\mathrm{T}}_{\mathrm{FE}}\right)$, which is not possible by structural probes. The tensor element related to $c$ axis polarizability showed a sharp anomaly below ${\mathrm{T}}_{\mathrm{FE}}$. Quantitative analysis resulted in atomic displacement of ∼121 fm, in agreement with first-principles calculations, demonstrating that ferroelectricity in ${\mathrm{TbMnO}}_3$ is driven by atomic displacements.

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• Lattice dynamics
• Magnetic coupling
• Spin-phonon coupling
• Multiferroics
• Single crystal materials
• Density functional theory
• Raman spectroscopy