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

Abstract

Understanding different bonding environments in various metal borides provides insight into their structures and physical properties. Polycrystalline aluminum diboride (AlB 2 ) samples have been synthesized and compared both with a commercial sample and with the literature. One issue that arose is the relative ease with which boron-rich and aluminum deficient phases of aluminum borides can be presented in AlB 2 . Here, we report 27 Al, 11 B nuclear magnetic resonance (NMR) spectroscopy and first-principles calculations on AlB 2 in order to shed light on these different bonding environments at the atomic level and compare the structural and electronic properties of the products of different preparations. Along with the aforementioned, the present study also takes an indepth look at the nature of the 11 B and 27 Al nuclear spin-lattice relaxation recovery data for the AlB 2 and other superhard materials. The nuclear spinlattice relaxation has been measured for a static sample and with magic-angle spinning. The combination of NMR and band structure calculations highlights the synthetic challenges with superhard materials.

DFT-GIPAW 27Al NMR Simulations for Intermetallics: Accuracy Issues and Magnetic Screening Mechanisms

“…In the referred spectrum, two additional peaks at 1640 and 0 ppm were assigned to elemental Al and aluminum oxide, Al 2 O 3 , respectively, impurities claimed to be inevitably present in the samples. At this point, it is important to recall that the experimental shifts of these two latter compounds have already been successfully predicted with the GIPAW method and with the same PBE PAW data sets used in this work, , which indicates a fair possibility of revisiting the peak assignments made in the literature for AlB 2 spectra, proposing that the actual experimental shift of pure AlB 2 contributes to the sharp and intense peak around 1680 ppm in the spectrum reported in ref and not to the broad peak at 880 ppm. Naturally, this also further explains the huge disagreement between the experimental 27 Al chemical shift of AlB 2 and its theoretical counterpart, as depicted in Figure .…”

“…In a recent study, Turner et al 63 brought up challenges in synthesizing pure AlB 2 and hence in obtaining an ssNMR spectrum with the only broad peak at 880 ppm expected for this compound. In the referred spectrum, two additional peaks at 1640 and 0 ppm were assigned to elemental Al and aluminum oxide, Al 2 O 3 , respectively, impurities claimed to be inevitably present in the samples.…”

GIAO versus GIPAW: Comparison of Methods To Calculate 11B NMR Shifts of Icosahedral Closo-Heteroboranes toward Boron-Rich Borides

“…However, the experimental reference does not distinguish between both 11 B species (−10.3 ppm) leaving it open whether this is caused by (solvation) effects which are not included here or from different accuracies of GIAO and GIPAW. Similarly, B (4,6,8,11) and B(9,10) in (15) are indistinguishable by experimental observation. In both cases, GIPAW predicts those accidental degeneracies more accurately than GIAO.…”

“…In this regard, ab initio calculations of the electronic structure indicated that a deficiency of aluminum in the measured probe of AlB 2 caused a reduction of the measured 27 Al shift. 6 These are just a few of numerous examples that the literature draws a close connection between bonding situations and measurable magnetic parameters of NMR in general.…”

“…Recent studies using solid-state nuclear magnetic resonance (SSNMR) spectroscopy on crystalline (β-) and amorphous boron as well as on the metal borides ReB 2 , AlB 2 , and WB 4 show that this analytical technique has the capability to study structural and electronic properties, for example, bonding situations and (spin–lattice) dynamics, on an atomic scale for solids containing boron. − Furthermore, it may help to clarify ambiguities and overcome limitations of the conventional X-ray diffraction (XRD) investigations. Combined with theoretical methods, SSNMR experiments may even provide an understanding of (unexpected) observed effects.…”