NMR Crystallography of Aluminum Carbide: Impurities in the Reagent and Improved ²⁷Al NMR Tensors

Abstract

The structure of aluminum carbide (from two different manufacturers) has been interrogated by 27 Al solid-state NMR in an effort to accurately characterize the material. Quadrupolar line shapes for 27 Al sites such as these are important to catalogue, as efforts are underway to employ NMR crystallography tools to understand and predict both solid-state structures and perturbations to these that are relevant to NMR analyses. Impurities present in both of the samples are revealed to be aluminum oxycarbides, as well as an aluminum metal impurity in one and aluminum nitride in anotherpotentially important information for groups employing aluminum carbide in syntheses or in other applications. Triple-quantum magic-angle spinning (3Q-MAS) 27 Al NMR was employed to help determine the quadrupolar parameters of the two crystallographically inequivalent aluminum sites in Al 4 C 3 . Revised values for the quadrupolar tensors of this material were measured and simulated through use of CASTEP-NMR.

Bulk and Surface Chemistry of the Niobium MAX and MXene Phases from Multinuclear Solid-State NMR Spectroscopy

“…MHz. 49,50 Hayes et al identified aluminum oxycarbide impurities with smaller CQs in their recent study of commercial Al4C3 so a related aluminum species is plausible here. 50 Another possibility is the tetrahedral aluminum nitride environment that appears at around 110-115 ppm in AlN and in aluminum oxynitrides with a CQ of a few MHz, consistent with the resonance observed here.…”

“…49,50 Hayes et al identified aluminum oxycarbide impurities with smaller CQs in their recent study of commercial Al4C3 so a related aluminum species is plausible here. 50 Another possibility is the tetrahedral aluminum nitride environment that appears at around 110-115 ppm in AlN and in aluminum oxynitrides with a CQ of a few MHz, consistent with the resonance observed here. [51][52][53][54] Note that the 27 Al signal from Nb2AlC covers a range of ~90-170 ppm, so the impurity at 115 ppm is less pronounced but it can be clearly seen as a shoulder that appears as the interval between scans increases ( Figure S2 depending on the preparation conditions, with NbCx (0.7 ≤ x < 1) yielding quadrupolar 93 Nb signals.…”

Bulk and Surface Chemistry of the Niobium MAX and MXene Phases from Multinuclear Solid-State NMR Spectroscopy

“…49,50 Hayes et al identified aluminum oxycarbide impurities with smaller CQs in their recent study of commercial Al4C3 so a related aluminum species is plausible here. 50 Another possibility is the tetrahedral aluminum nitride environment that appears at around 110-115 ppm in AlN and in aluminum oxynitrides with a CQ of a few MHz, consistent with the resonance observed here. [51][52][53][54] Note that the 27 Al signal from Nb2AlC covers a range of ~90-170 ppm, so the impurity at 115 ppm is less pronounced but it can be clearly seen as a shoulder that appears as the interval between scans increases (Figure S2).…”

Bulk and Surface Chemistry of the Niobium MAX and MXene Phases from Multinuclear Solid-State NMR Spectroscopy

“…4.5 MHz while Al4C3 has two Al sites with CQ magnitudes of 14-17 MHz. 49,50 Hayes et al identified aluminum oxycarbide impurities with smaller CQs in their recent study of commercial Al4C3 so a related aluminum species is plausible here. 50 Another possibility is the tetrahedral aluminum nitride environment that appears at around 110-115 ppm in AlN and in aluminum oxynitrides with a CQ of a few MHz, consistent with the resonance observed here.…”

“…49,50 Hayes et al identified aluminum oxycarbide impurities with smaller CQs in their recent study of commercial Al4C3 so a related aluminum species is plausible here. 50 Another possibility is the tetrahedral aluminum nitride environment that appears at around 110-115 ppm in AlN and in aluminum oxynitrides with a CQ of a few MHz, consistent with the resonance observed here. [51][52][53][54] Note that the 27 Al signal from Nb2AlC covers a range of ~90-170 ppm, so the impurity at 115 ppm is less pronounced but it can be clearly seen as a shoulder that appears as the interval between scans increases (Figure S2).…”