consistent with octahedral AlO6, as found in e.g. boehmite, gibbsite, or g-alumina. The oxide environments could arise from secondary oxide phases, or from the oxide layers previously observed by XPS to form on the surface of MAX particles. The resonance at 115 ppm appears at a higher frequency than for typical aluminum oxides or fluorides ; it does coincide with the shift range of aluminum carbide Al4C3 (111-120 ppm) but the maximum CQ value for the observed resonance is ca. 4.5 MHz while Al4C3 has two Al sites with CQ magnitudes of 14-17 MHz. 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. 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. Note that the 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 ). Quantitative spectra recorded with recycle delays of 90-100 s (Figure ) revealed the relative ratios of the MAX and impurity (or surface) species in the samples from the perspective of aluminum content. For the Nb2AlC sample, a ratio of 89:9:2 was found for the MAX signal at 169 ppm and the impurity resonances at 16 and 115 ppm, respectively. For the Nb4AlC3 sample, a ratio of 56:34:7:3 was found for the MAX signal at 319 ppm and the impurity resonances at 17, 5, and 115 ppm, respectively. b The sign of the quadrupolar coupling can be calculated but is not determined experimentally from the spectrum of a polycrystalline sample.