Chiral phonons, lattice vibrations carrying finite angular momentum, are at the forefront of a fast-developing field for exploring and controlling quantum materials in captivating ways. Phonon chirality originating from topological phonon bands is physically interesting but generally small and limited to a few material classes. Here, we report the observation of significant phonon chirality in the triangular-lattice rare-earth compound ${\mathrm{KNdSe}}_2$. We find that the chirality is activated by a distinct local mechanism—crystalline electric field (CEF) excitations—rather than by a global topology. Using helicity-resolved magneto-Raman spectroscopy, we observe a clear splitting ($\sim 3{\mathrm{cm}}^{-1}$ under 9 T, or equivalently $\sim 0.4{\mu }_B$) of the degenerate $E_g$ phonon mode that exhibits perfect circular polarization selection rules—the unambiguous fingerprint of chirality. The magnitude, field dependence, and thermal evolution of the splitting are quantitatively reproduced by a microscopic theory of CEF-phonon coupling, which we solve by employing the Dyson equation formalism and incorporating a careful analysis of symmetry and angular momentum conservation. Our findings demonstrate a deterministic, CEF-activated mechanism for realizing phononic chirality and identify the rare-earth chalcogenides as a highly tunable playground for investigating the rich physics of coupled electronic and vibrational quasiparticles.

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• Chirality
• Crystal-field excitations
• Phonons
• Rare-earth magnetic materials
• Triangular lattice
• Density functional calculations
• Raman spectroscopy