Spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design
of flexible two-dimensional quantum sensing platforms. Here we rely on hBN crystals isotopically
enriched with either 10B or 11B to investigate the isotope-dependent properties of a spin defect
featuring a broadband photoluminescence signal in the near infrared. By analyzing the hyperfine
structure of the spin defect while changing the boron isotope, we first unambiguously confirm that
it corresponds to the negatively-charged boron-vacancy center (V−
B ). We then show that its spin
coherence properties are slightly improved in 10B-enriched samples. This is supported by numerical
simulations employing cluster correlation expansion methods, which reveal the importance of the
hyperfine Fermi contact term for calculating the coherence time of point defects in hBN. Using crossrelaxation spectroscopy, we finally identify dark electron spin impurities as an additional source of
decoherence. This work provides new insights into the properties of V−
B spin defects, which are
valuable for the future development of hBN-based quantum sensing foils.