Two-dimensional (2D) materials hosting color centers and spin defects are emerging as key platforms for quantum technologies. However, the impact of reduced dimensionality on the spin-lattice relaxation time ($T_1$) of embedded defect spins—critical for quantum applications—has remained largely unexplored. In this study, we present a systematic first-principles investigation of the negatively charged boron-vacancy (${\mathrm{V}}_{\text{B}}^{-}$) defect in monolayer boron nitride (BN), as well as in ${\mathrm{AA}}^{\prime }$-stacked hexagonal BN (hBN) and ABC-stacked rhombohedral BN (rBN). Our results reveal that the $T_1$ times of ${\mathrm{V}}_{\text{B}}^{-}$ in monolayer BN and hBN are nearly identical at room temperature. Surprisingly, despite the symmetry reduction in rBN that opens additional spin relaxation channels, ${\mathrm{V}}_{\text{B}}^{-}$ exhibits a longer $T_1$ compared to hBN. We attribute this effect to the stiffer out-of-plane phonon modes in rBN, which activate spin-phonon relaxation at reduced strength. These findings suggest that ${\mathrm{V}}_{\text{B}}^{-}$ in rBN offers enhanced spin coherence properties, making it a promising candidate for quantum technology applications.

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• Quantum information with solid state qubits
• Spin-phonon coupling
• Boron nitride