We have measured second-order Raman scattering in high-quality monoisotopic $h$-BN crystals for all possible boron and nitrogen isotope combinations. The second-order spectra are dominated by phonon combinations and are well aligned with density-functional perturbation-theory calculations of the two-phonon density of states. The isotopic shifts are well described by second-order perturbation of the dynamical matrix. The analysis of the Raman spectra of the monoisotopic samples and their isotopic shifts allows us to unambiguously assign the second-order peaks and to determine phonon frequencies at $M$ and $K$ zone-edge points, as well as on the $T$ line. This study demonstrates the usefulness of Raman spectroscopy as a tool for probing the frequencies of zone-edge phonons. These play a prominent role in the phonon-assisted emission in $h$-BN. The study also serves as a stringent test of the density-functional theory calculations commonly used to estimate these frequencies.

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• Optical phonons
• Boron nitride
• Density functional calculations
• Raman spectroscopy