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 WB 4 , 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 10 B and 11 B nuclear magnetic resonance spectroscopy as a prelude for the study of boronbased superhard materials. The 11 B spectrum of a static sample reflects both bulk magnetic susceptibility and second-order quadrupolar lineshapes of quadrupolar frequencies ranging from zero to 680 kHz. The 10 B spectrum of a static sample shows quadrupolar frequencies ranging from zero to 142 kHz. In contrast to the previous literature indicating relaxation of quadrupolar origin, the variable temperature spin-lattice relaxation data indicate that the 11 B relaxation at 248 K and below is dominated by spindiffusion 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 44s.

Boron‐Doped Polygonal Carbon Nano‐Onions: Synthesis and Applications in Electrochemical Energy Storage

“…B NMR spectroscopy is frequently used to monitor the chemical environment of B in various materials, including elemental B, B compounds, oxides, glasses, and zeolites . There are two B isotopes with nuclear spins: 11 B and 10 B, with natural abundances of approximately 80 and 20 %, respectively.…”

Synthesis and characterization of aluminum diboride products using 27Al, 11B NMR and ab initio studies

“…In elemental boron, partial occupancy of unit cell sites results in defects yielding paramagnetic centers that can be observed by electron paramagnetic resonance (EPR) spectroscopy [7]. EPR spectra (not shown) were also acquired on these boron-rich phases and are nearly identical to those of elemental boron [7].…”

“…The ''second-order quadrupole effect'' [33] and ''two distinct boron sites'' [34] had been proposed in the literature, this spectral effect in elemental boron was demonstrated to arise from isotropic bulk magnetic susceptibility [7]. In elemental boron, partial occupancy of unit cell sites results in defects yielding paramagnetic centers that can be observed by electron paramagnetic resonance (EPR) spectroscopy [7]. EPR spectra (not shown) were also acquired on these boron-rich phases and are nearly identical to those of elemental boron [7].…”

“…If such a ''doublet'' arises from isotropic bulk magnetic susceptibility, as in the cases of elemental boron and the boron-rich aluminum boride phases, then the splitting will be dependent upon the particle size [7]. In addition, the sample will yield a measurable EPR spectrum.…”

“…We began this series of investigations with a 10 B and 11 B NMR study of elemental boron [7], which has interesting properties of its own and also provides insight into boron-rich metal boride phases. This was followed by 11 B studies of ReB 2 [8] and WB 2 [9].…”