10B and 11B NMR Study of Elemental Boron

Abstract

Elemental boron typically exists in either of two states: crystalline or amorphous. In the synthesis of boron-based superhard materials, such as WB4, elemental boron is in some instances a side product that is difficult to separate from the desired superhard material. In the present study, both crystalline and amorphous boron are characterized by 10B and 11B nuclear magnetic resonance spectroscopy as a prelude for the study of boron-based superhard materials. The 11B spectrum of a static sample reflects both bulk magnetic susceptibility and second-order quadrupolar line shapes of quadrupolar frequencies ranging from 0 to 680 kHz. The 10B spectrum of a static sample shows quadrupolar frequencies ranging from 0 to 142 kHz. In contrast to the previous literature indicating relaxation of quadrupolar origin, the variable temperature spin–lattice relaxation data indicate that the 11B relaxation at 248 K and below is dominated by spin diffusion from paramagnetic centers. Above 248 K, relaxation is dominated by a thermally activated interaction with the conduction charge carriers originating from the boron vacancies. Relaxation in amorphous elemental boron shows an additional insulating component with a comparatively long time constant of 44 s.