Experimental and computational 11B NMR comparative study of MOVPE-grown rhombohedral and bulk hexagonal boron nitride

Layered boron nitride (BN) is a two-dimensional (2D) insulator that exhibits several polytypes, which differ in the stacking sequence of adjacent atomic planes. The relative shift and rotation of subsequent planes modify the crystal symmetry and thereby determine the crystal's fundamental properties. While most previous NMR-oriented studies have focused on AA'-stacked hexagonal BN (hBN), ABC-stacked rhombohedral BN (rBN) has not yet been investigated. In this study, we present the magic-angle spinning NMR spectra of rBN and compare them with those of hBN and with DFT calculations. We also measure spin-lattice relaxation of both polytypes. Our findings suggest that both phases exhibit nearly identical NMR parameters, including chemical shifts and quadrupole couplings. However, our rBN and hBN samples differ significantly in spin-lattice relaxation, which may be attributed to different growth methods, resulting in different concentrations of paramagnetic centers. Interestingly, we also observe extremely long spin-lattice relaxation times for terminal groups in hBN, which have rendered them unobservable in many previous studies.