13C spin–lattice relaxation in natural type-IIb semiconducting diamond

13C spin–lattice relaxation times in the laboratory frame have been measured at 295 K in a field of 4.7 T for two natural type-IIb diamonds using nuclear magnetic resonance spectroscopy (NMR). 13C spin–lattice relaxation is mainly due to mobile hole carriers. The contribution of fixed paramagnetic impurities is at least an order of magnitude smaller. At 295 K, the carrier concentration is estimated to be 1.2×1014 cm−3, resulting in an η, the electron density at the nucleus, of about 320 for natural type-IIb diamonds.