10B/11B 1D/2D solid-state high-resolution NMR studies on boron-doped diamond

In this work, three boron-doped diamond samples prepared from a high-pressure high-temperature synthesis method with the same starting materials but with different initial ratios for boron are studied. Magnetic susceptibility measurement shows that the increment of the initial amount of boron does not straightforwardly bring lower superconducting transition temperature. In accordance with our previous 11B high-resolution magic-angle spinning (MAS) NMR results, we show that there are at least four boron signal components and the one at 28.5 ppm is ascribed to the substitutional boron in the diamond structure providing the carriers responsible for conductivity. From observed relative intensities of the four signal components, it is suggested that the excess boron, which does not contribute to the conductivity appears as a broad signal at around 65.5 ppm. We apply two-dimensional (2D) NMR to examine 1H dipolar broadening and 11B–11B boron spin diffusion, and candidates purported so far for the excess boron, that is, a boron + hydrogen complex and –B–B– and/or –B–C–B– clusters are negated. From its chemical-shift value, it is suggested that the excess boron exists as trigonally coordinated boron. We further apply 2D nutation NMR to examine the size of its quadrupolar coupling and show that it is not very large. As for a structure that compromises the trigonal coordination invoked from the chemical-shift value and the small quadrupolar coupling, we postulate boron locally in a graphite-like structure but the symmetry of the electric field gradient around it is high. Furthermore, we show that 10B MAS NMR is useful to selectively observe the substitutional boron in the diamond structure appearing at 28.5 ppm, whose quadrupolar coupling is much smaller than that of the excess boron at 65.5 ppm.