A thorough understanding of lattice dynamics, particularly how they couple with spin through spin-phonon interactions, is crucial for pioneering new spin-caloritronic applications. Despite extensive studies on ${\mathrm{Cr}}_2{\mathrm{O}}_3$, the origin of nonlinear thermal expansions and the unusual stiffening of optical phonon modes is still elusive. Through inelastic neutron scattering and atomistic calculations, we found that these behaviors can be attributed to the renormalization of electron states owing to the magnetic transition, and we ruled out the effects from thermal expansion, phonon anharmonicity, magnetostriction, or electron-phonon interactions. Importantly, our quantitative modeling suggests that the common belief that dynamic spin-phonon interactions are the origin of anomalous phonon energy stiffening in magnetic compounds does not apply in ${\mathrm{Cr}}_2{\mathrm{O}}_3$.

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• Electron-phonon coupling
• Phonons
• Spin-phonon coupling
• DFT+U
• Inelastic neutron scattering