10B and 11B NMR Study of Elemental Boron - American Chemical

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Article 10

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B and B NMR Study of Elemental Boron

Christopher L Turner, Robert E. Taylor, and Richard B. Kaner J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04705 • Publication Date (Web): 20 May 2015 Downloaded from http://pubs.acs.org on May 26, 2015

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The Journal of Physical Chemistry

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B and 11B NMR Study of Elemental Boron

Chris Turner1, R. E. Taylor*,1 and Richard B. Kaner1 1

Department of Chemistry and Biochemistry University of California, Los Angeles Los Angeles, CA 90095-1569 USA

*Corresponding author: R. E. Taylor Email address: [email protected]

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 10 B and 11B nuclear magnetic resonance spectroscopy as a prelude for the study of boronbased superhard materials. The 11B 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 10B 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 11B 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.

Key Words 10

B NMR; 11B NMR; boron; spin-lattice relaxation; half-integer quadrupolar nuclei.

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Introduction Transition metal borides exhibit an interesting range of physical properties, including being superhard1. These superhard materials are characterized by high valenceelectron density and bond covalency and are typically metallic. In the synthesis of boronbased superhard materials, such as WB4 2, elemental boron is, in some instances, a side product that is difficult to separate from the desired superhard material. For characterization of such boron-based superhard materials by nuclear magnetic resonance (NMR) spectroscopy, it is desirable to have a good characterization of elemental boron in order to investigate the NMR responses arising from the transition metal borides. Discerning the spectroscopic behavior of boron alone will thus aid in a thorough comprehension of its behavior within the metal-boride lattice. The interaction between boron and metal atoms within the structure contains many different bonding environments. Elucidation of the boron-metal interactions will further allow the “prediction” of plausible structures. Historically, the borides have been characterized through X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and a host of physical measurements, such as micro- and nano-indentation and thermo-gravimetric analysis. Clearly defining boron’s interactions allows corroboration with previously used techniques to further realize metal-boride structures. Comprehensive recognition of boron’s bonding characteristics allows more precise tailoring of metal boride lattices. Bulk crystallographic properties are both theoretically predicted3 and experimentally determined2, but boron’s bonding environments currently leave room for further understanding. Elemental boron also displays interesting properties of its own. Characterization of these properties, arising from the electron-deficient bonding, still remains a research challenge. In an overview of the history of the element given by Oganov and Solozhenko4, they describe boron as “arguably the most complex element in the Periodic Table” and note that “boron still remains a poorly understood element”. In thermodynamically stable β-rhombohedral boron, the idealized unit cell has 105 atoms5. However, many of these crystallographic sites are only partially occupied6. These intrinsic defects arising from the numerous partially occupied sites give rise to both semiconductive behavior7 and “frustration” arising from antiferromagnetic correlations8,9. In the present study, both crystalline and amorphous boron are characterized by 10 B and 11B nuclear magnetic resonance (NMR) spectroscopy as a prelude to the study of boron-based superhard materials. Both resonant lineshape and spin-lattice relaxation data are presented and compared and contrasted with previous literature results. As noted above that boron “remains a poorly understood element”, several of the NMR results reported here are simply at odds with previous explanations in the literature.

Experimental Samples prepared for NMR spectroscopy were used as received -- amorphous boron (99+%, Strem Chemicals) and crystalline boron (99%, Materion). Both samples contain the naturally occurring abundance ratio of 10B to 11B. Using a 325 mesh (44 µm)

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screen (Humboldt Mfg.), the boron powders were independently screened to ensure a uniform, maximum particle size for the NMR experiments. For characterization by X-ray diffraction, the samples were deposited onto silicon (511) “zero-background” plates, with excess material removed by razor blade to guarantee flatness. Diffraction patterns were collected from 10° to 76° 2θ using an X’Pert Pro Bragg-Bentano geometry laboratory Xray diffractometer (PANalytical) with nickel-filtered CuKα radiation ((λKα1 = 1.540593 Å, λKα2 = 1.5444274 Å), flat sample state, 0.04 Soller slits, and X’Celerator position sensitive detector. The NMR data were acquired with a Bruker DSX-300 spectrometer operating at frequencies of 96.29 MHz for 11B and 32.24 MHz for 10B. The NMR properties of the two boron isotopes are given in Table I. Magic-angle spinning (MAS) spectra were acquired with a standard Bruker MAS probe using a 4-mm outside diameter zirconia rotor with a sample spinning rate of 12 kHz. The 11B π/2 pulse widths for the MAS experiments were 3 µs as measured on an aqueous solution of boric acid. The 11B background from the boron nitride stator in the MAS probe was minimized by the use of the Elimination of Artifacts in NMR SpectroscopY (EASY) pulse sequence10. Static powdered samples of either crystalline or amorphous boron were placed in a standard Bruker X-nucleus wideline probe with a 5-mm solenoid coil. The 11B background from sodium borosilcate glass was avoided by the use of a polyimide coil support and a quartz sample tube. As a result, the wideline 11B spectral data could be directly acquired without the use of background suppression techniques such as EASY. Each sample was ground to 325 mesh to avoid radiofrequency (RF) skin-depth effects at these NMR frequencies. Samples were confined to the length of the RF coil. The 11B π/2 pulse width was 5 µs as measured with an aqueous boric acid standard. However, the “solid-state” ninety degree pulse width was reduced by a factor of (I + ½), where I is the nuclear spin, in comparison with the ninety-degree pulse width for the same nucleus measured in solution11,12. Data for determining the spin-lattice relaxation times (T1) were acquired with a saturation-recovery technique13. The 11B and 10B chemical shift scales were calibrated using the unified Ξ scale14, relating the nuclear shift to the 1H resonance of dilute tetramethylsilane in CDCl3 at a frequency of 300.13 MHz. The reference compound for defining zero ppm on each chemical shift scale is BF3 etherate14. The chemical shift referencing was experimentally verified with the 11B and 10B resonances of an aqueous solution of boric acid15 at pH = 4.4. Spectral simulations were performed with the solids simulation package (‘‘solaguide’’) in the TopSpin (Version 3.1) NMR software program from Bruker BioSpin. The electron paramagnetic resonance (EPR) spectrum was acquired with a Bruker EMX spectrometer operating at 9.77 GHz.

Results and Discussion

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The idealized crystal structure of β-rhombohedral elemental boron, typically written as ((B12)4(B28)2B or B84(B10)2B with 105 atoms per unit cell, is based on icosahedral subunits8,9,16-18. The X-ray diffraction spectra collected (independently) validated the elemental purity of each boron sample (Fig. 1). The crystalline boron spectrum contains the sharp, distinct peaks expected for a crystalline compound. The broader, less defined peaks obtained from the amorphous boron illustrate the less ordered structure expected for an amorphous material. The consistencies between peak locations, as shown in Fig. 1, provide agreement on the elemental purity (and identity) of each sample analyzed. There were no detectable impurities present in either sample. The 11B spectrum of a static sample of 325-mesh crystalline β-rhombohedral elemental boron is shown in Fig. 2B. Rather than being the broad featureless spectrum typically observed with wideline NMR of polycrystalline samples, this spectrum has distinct features and is also quite similar to those of previous NMR studies16,19,20. Two different explanations of this spectral feature of “two split lines”19 observed with natural abundance 11B in the static sample have been offered in the literature. Such differing explanations in the literature are an indication of the statement by Oganov and Solozhenko4 that “boron still remains a poorly understood element”. An explanation for the observed 11B linehsape of a static sample of polycrystalline boron needs to be given. One explanation of the lineshape given in the literature is the suggestion that this feature arises from the “second order quadrupole effect”19. Under the assumption that the lineshape of the static sample is dominated by the “second order quadrupolar effect”, then the parameters extracted from simulations of the second-order quadrupolar interaction for the central transitions of both the MAS and static-sample spectra are given in Table II. Two issues are immediately apparent. The extracted parameters, i.e., the shifts and quadrupolar coupling constants, from the simulations of a single second-order quadrupolar interaction in the two spectra of the same compound are not in reasonable agreement. Also, while the simulation does describe the general features of the static lineshape, the simulation is not a particularly good fit. These issues suggest that the observed lineshape does not arise from the second-order quadrupolar interaction. The combination of both wideline and MAS experiments shows that this explanation in the literature is incorrect. An alternative suggestion in the literature raises the possibility of “two distinct boron sites”20 (although a multiplicity of sites would be expected from the 105 atoms in the unit cell5,16). The MAS spectrum, showing a single resonance in Fig. 2A, suggests that this two-site hypothesis is not correct. However, Lee and co-workers20 did note that these two apparent peaks in the spectrum of the static sample shifted as a function of particle size. In Fig. 6 of Reference 20, the two resonances observed for the “coarse….boron powder” appear at 114.4 and -150.2 ppm, while they appear at 66.8 and -109.3 ppm for the “fine…boron powder”. With the 325 mesh sample used for this study, the resonances appeared at 45.5 and -56.9 ppm. It is worth noting that the spectra acquired by Lee and co-workers20 and the spectra reported here were obtained using the same magnetic field strength of 7.05 T. The dependence of the “two peaks” upon particle size (and presumably shape) suggests that bulk magnetic susceptibility21 contributes to the observed lineshape. Spectral broadening from isotropic bulk magnetic susceptibility will not be observed in the MAS experiment22 (although anisotropic bulk susceptibility may still contribute to the

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broadening). The removal of spectral broadening from bulk magnetic susceptibility of the static sample under MAS (with averaging of the second-order quadrupolar interaction) explains the observation of a single peak in the MAS spectrum. Noting how the static, one-dimensional 11B lineshape changes as a function of particle size in one single magnetic field also allows other interactions to be ruled out. This change in the lineshape at one single magnetic field strength is not characteristic of the chemical shift anisotropy interaction, allowing it to be ruled it out as the dominant interaction. This lineshape behavior also rules out both homo- and heteronuclear dipolar broadening as the dominant mechanism, as they would not be altered by particle size. In particular, with the shortest internuclear boron distance of 0.162 nm16, the homonuclear 11 B dipolar coupling is calculated to be only 2.9 kHz while the heteronuclear dipolar coupling to 10B would be 0.97 kHz. In contrast to the two explanations for the observed lineshape currently in the literature, spectral broadening from isotropic bulk magnetic susceptibility provides an explanation for the observed lineshape. In particular, the magnetic susceptibility arises from the intrinsic defects in β-rhombohedral boron that result from partial occupancy of unit cell sites7-9. The resulting paramagnetic centers are observed in the EPR spectrum shown in Fig. 3. Effects upon the NMR spectral features arising from paramagnetic centers are to be expected. The spectral features of the 11B spectrum of a static sample shown in Fig. 2B result from a combination bulk magnetic susceptibility with the lineshapes arising from the second-order quadrupolar interactions of varying magnitude. The evidence of different quadrupolar coupling constants arises from the skewed resonance shown in the multiple quantum MAS (MQMAS)23,24 spectrum in Fig. 4. The 10B spectrum of a static sample of 325-mesh crystalline β-rhombohedral elemental boron is shown in Fig. 5. The major features observed in the spectrum can be achieved with a simple model. A simulation of first-order quadrupolar interactions from only three major sites, all with a chemical shift of -26 ppm but with differing quadrupolar coupling constants, reproduces the major spectral features. While using an asymmetry parameter, η, of zero for all three sites, the quadrupolar frequencies, νQ, used in the simulation are zero Hz, 84 kHz, and 142 kHz. With this simple model, the values of the asymmetry parameter, η, are difficult to determine. Varying the value of η between zero and 0.1 still reproduce the major features of the experimental spectrum. The difficulty in narrowing down the asymmetry values arises because the spectral features associated with the asymmetry parameter in the simulations can also arise simply from small variations in the quadrupolar frequencies. There is much interest in establishing the number of unique sites within elemental boron. However, this is a very challenging task that still has not yet been accomplished. The earlier single crystal and powder NMR study16 of β-rhombohedral elemental boron specifically discussed the issues that limited the sites that they were able to characterize. These issues include the fact that there are 105 boron sites in the unit cell and that any boron sites situated next to a boron vacancy will differ yet still. As a result, they limited reporting their results as three groups of boron sites based on the 11B quadrupolar frequencies, νQ, centered around values of zero, 130 ± 30 kHz, and 680 ± 70 kHz. The 11 B MQMAS and wideline spectra reported here are consistent with quadrupolar frequencies over these ranges. The 10B NMR spectrum reported here also indicates a similar grouping of three, with νQ centered around values zero Hz, 84 kHz, and 142 kHz.

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(These quadrupolar frequencies, which depend upon the nuclear spin that differs between 10 B and 11B, may be converted to the quadrupolar coupling constants by use of the equations given in the footnotes of Table II. The use of quadrupolar frequencies here is for comparison with the previous literature.) All of these quadrupolar frdequencies are still smaller than those obtained from the single second-order quadrupolar simulations of the 11B spectra as presented in Table II. This provides further evidence that the spectrum of the static sample does not arise solely from the second order quadrupolar interaction. NMR relaxation studies provide information regarding both the structure and dynamics of chemical compounds. Usually the spin-lattice relaxation time constant is measured as a function of some parameter, e.g., temperature, pressure, or magnetic field strength. For this study of elemental boron, 11B spin-lattice relaxation rates have been acquired as a function of temperature. The results of a 11B saturation-recovery experiment with a static sample of polycrystalline boron at 296 K are shown in Figure 6. The plot shows the integrated area of the resonance of the central transition as a function of the time after application of a single ninety-degree saturation pulse. The spin-lattice relaxation of a half-integer quadrupolar nucleus in a non-cubic singe crystal is typically characterized by a multi-exponential function regardless of whether the relaxation mechanism is quadrupolar or magnetic in origin25. Of course this assumes that the crystal is oriented in the static magnetic field such that there are no accidental degeneracies of the satellite energy levels with that of the central transition. The multi-exponential recovery results as the quadrupolar interaction creates unequal energy spacings in the Zeeman interaction. However, for spin-lattice relaxation of the same half-integer quadrupolar nucleus in polycrystalline samples, an exponential recovery is typically observed26. The suggestion27 has been made that the mathematical process of orientational averaging28 of quadrupolar relaxation gives rise to a spin-lattice relaxation well characterized by a single exponential function. On the other hand, there are other mechanisms which produce non-exponential spin-lattice relaxation for these half-integer quadrupolar nuclei in solids, such as relaxation through spin diffusion from dilute paramagnetic centers29. The EPR spectrum in Fig. 3 suggests that the paramagnetic centers may play a role in the spin-lattice relaxation. The 11B saturation-recovery data from a static sample of polycrystalline boron at 296 K shown in Figure 6, which is non-exponential, have been fit with three single-exponential functions. The time constants extracted from fitting three singleexponential functions to the 11B saturation-recovery data obtained as a function of temperature are shown in Fig. 7. The shortest time constants, T1, are on the order of microseconds. There is scatter in these extracted time constants (± 200%), but no discernable trend with temperature was noticed. However, the mid-range and longest time constants remain relatively constant at 248 K and below. Relaxation independent of temperature is a hallmark of relaxation by paramagnetic centers30 for a sample in the rigid-lattice limit. Above 248 K, both the mid-range and longest time constants indicate a thermally activated relaxation mechanism. For numerical comparison with the shortest time constant noted above, the mid-range T1 is 97 ms (± 20%) and the longest is 526 ms (± 10%), both at 296 K. As analyses of both the mid-range and longest time constants yield the same activation energy of 15.8 kJ/mol (0.16 eV), there is a single relaxation mechanism responsible. This activation energy is in reasonable agreement

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with the 0.19 eV determined by photoconductivity measurements7 for “localized states” in the band gap attributed to boron vacancies (e.g., see Fig. 12 in Ref. 7). In short, the relaxation mechanism above 248 K is dominated by interaction with the conduction charge carriers originating from the boron vacancies, i.e., defect states in the band gap. The difference in the T1s of these two components represents the difference in the hyperfine interaction between the conduction charge carriers with these two components. The earlier single crystal and powder NMR study16 of β-rhombohedral elemental boron reported a single spin-lattice relaxation measurement at ambient temperature and suggested that the relaxation mechanism was quadrupolar in origin. A “back of the envelope” calculation was used to rule out spin diffusion as a relaxation mechanism at ambient temperature. However, the variable temperature spin-lattice relaxation results reported here clearly show that the relaxation below 248 K is dominated by spindiffusion from paramagnetic centers (as indicated by the independence of the spin-lattice relaxation rate with temperature), while relaxation above 248 is dominated by a thermally-activated interaction with the conduction charge carriers. The shortest T1 arises from boron nuclei in the immediate vicinity of the paramagnetic centers. It should be noted that the observed variable temperature spin-lattice relaxation results are fully explained by spin-diffusion from paramagnetic centers at low temperature and a thermally activated mechanism from charge carriers being excited from defect states in the band gap. No other thermally activated mechanism, for example, such as “plastic crystalline” behavior associated with the icosahedral geometry of the bonding that might facilitate such motions, was observed in this temperature range. There is no indication of motion to which the spin-lattice relaxation behavior is sensitive. In addition, there is no evidence of slow molecular motions that would lead to a change or narrowing of the static lineshape over this temperature range. The onedimensional 11B spectra acquired at 173 K, 295 K, and 423 K can all be overlaid with no apparent differences visible. There is also interest in the properties of amorphous elemental boron31-33 and comparison of its properties to those of crystalline elemental boron. The NMR results obtained for the amorphous elemental boron are quite similar to those obtained for the crystalline sample. The 11B spectrum of a static sample of 325-mesh amorphous elemental boron is virtually identical to that of the crystalline β-rhombohedral elemental boron shown in Fig. 2B. However, the 11B spin-lattice relaxation between the two samples does slightly differ. There are three time constants for amorphous boron that are quite similar to the three found for the crystalline β-rhombohedral elemental boron. The difference is that there is an additional component with a very long time constant of 44 s at ambient temperature. This additional component observed in the spin-lattice relaxation experiments was the only difference between the two samples. This very long time constant suggests the presence of isolated domains without boron vacancies, i.e., an insulating material.

Conclusions

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Both amorphous and β-rhombohedral elemental boron were investigated with 10B B NMR. In contrast to the previous literature suggesting that the 11B lineshape

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from a static boron sample arises from either the second-order quadrupolar lineshape or from two distinct boron sites, the 11B 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 10B spectrum of a static sample shows quadrupolar frequencies ranging from zero to 142 kHz. In contrast to the previous literature suggesting spin-lattice 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 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, i.e., defect states in the band gap. Relaxation in amorphous elemental boron shows an additional insulating component with a comparatively long time constant of 44s. The loss of translational symmetry in amorphous boron, in comparison with crystalline boron, results in insulating domains free of defects, i.e., no boron vacancies.

Acknowledgment This material is based upon work supported by the National Science Foundation under Grant DMR 1106364 and Equipment Grant #DMR-9975975.

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(33) Berezin, A. A.; Golikova, O. A.; Kazanin, M. M.; Khomidov, T.; Mirlin, D. N.; Petrov, A. V.; Umarov, A. S.; Zaitsev, V. K. Electrical and Optical Properties of Amorphous Bonron and Amorphous Concept for β-Rhombohedral Boron. J. NonCrystal. Solids 1974, 16, 237-246.

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Tables Table 1. NMR Properties of Boron Nucleia Natural Abundance

Magnetogyric Ratio

Quadrupolar Moment

(%)

(107 rad s-1 T-1)

(fm2)

3/2

80.1

8.5847044

4.059

3

19.9

2.8746786

8.459

Isotope

Spin

11

B

10

B

a

From Ref. 14

Table II. The “Second Order Quadrupole Effect” Modela for the 11B NMR Lineshape: Parameters Extracted from Spectral Simulations of Crystalline Boronb CQd υQe Ηf Compound δisoc Linebroadening (ppm) (MHz) (MHz) (Hz) (ppm) Crystalline 33.4 3.864 1.932 0 8100 Boron (static) Crystalline Boron (MAS)

14.2

2.339

1.170

a

Lineshape Model from Ref.19. Spectral parameters obtained from simulations in Figure 2. c Chemical shifts referenced to the unified Ξ scale (Ref.11). d Quadrupolar coupling constant { ݁ ଶ ‫ܳݍ‬/ħ}. e Quadrupolar frequency {(3݁ ଶ ‫)ܳݍ‬/[2‫(ܫ‬2‫ ܫ‬− 1)ħ]}. f η = asymmetry {{0 ≤ η ≤ 1} b

12


0

2498

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Figures and Captions

Figure 1: X-ray diffraction spectra of crystalline (blue) and amorphous (red) boron. The presence of fine peaks (blue line) indicates high crystallinity and structure. Conversely, the amorphous boron (red line) shows less apparent ordering (color online).

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Figure 2: 11B MAS spectrum (top) of polycrystalline boron at 296 K with the 11B wideline spectrum (bottom) of static polycrystalline boron at 296 K. The simulated lineshapes are the smooth lines in red (color online). The difference in axes should be noted. Parameters extracted from the simulations are given in Table II.

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Figure 3: EPR spectrum of polycrystalline boron at 296 K.

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Figure 4:

11

B MQMAS spectrum of polycrystalline boron at 296 K.

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Figure 5: 10B wideline spectrum (in blue, color online) of static polycrystalline boron at 296 K. The simulated spectrum (in red) is the summation of the simulations of three sites, presented above and described in the text.

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Figure 6: 11B NMR saturation-recovery data for static polycrystalline boron at 296 K with the variable time delay on a logarithmic scale so that the data points are evenly spaced. The smooth line (in red online) is the fit of three exponential functions to the data.

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Figure 7: 11B NMR saturation-recovery time constants T1 for static polycrystalline boron as a function of temperature from 173 K to 423 K.

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Table of Contents Graphic

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10B and 11B NMR Study of Elemental Boron - American Chemical

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