Exciton-phonon interactions play a central role in defining the optical response of hexagonal boron nitride (hBN), yet their quantitative determination has remained incomplete. Here, we reveal the Fröhlich-type exciton-phonon coupling in boron-10-enriched hBN using low-temperature cathodoluminescence. We resolve the indirect exciton $5.95\pm 0.02\mathrm{eV}$ together with its longitudinal optical phonon replica detuned by $184\pm 56\mathrm{meV}$, enabling the extraction of a Fröhlich-coupling constant $\alpha =0.159$, and a larger exciton binding energy of 161 meV, larger than previously reported values for natural abundance hBN, which is attributed to isotope enrichment. The inferred polaron radius exceeds the lattice constant, indicating large-polaron behavior. We deduced an exciton scattering time $\sim 97\mathrm{fs}$ (homogeneous linewidth $\sim 6.76\mathrm{meV}$), a polaron binding energy of magnitude $\sim 48\mathrm{meV}$, and an effective mass of $1.045m_0$. These results provide a direct quantitative characterization of exciton-phonon coupling in isotopically engineered hBN and establish a foundation for tailoring its phonon-polaritonic and quantum-optical properties.

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• Luminescence
• Phonons
• Polaritons
• Polarons