Multiple magnetic field27Al solid-state NMR study of the calcium aluminates CaAl4O7 and CaAl12O19

Calcium and Strontium Hexaluminates: NMR Evidence that “Pentacoordinate” Cation Sites Are Four-Coordinated

“…Experimental and simulated 27 Al MAS for CA6 and SA6 are shown in Figures 1, 2, and 3. Those at 14.1 T are quite similar to previously published data, 13,14,16,17 and are similar to each other, as expected for isostructural compounds. The higher frequency peak (about 65 ppm) can be fitted with a single quadrupolar doublet (Table 1), consistent with the single "normal" tetrahedral site in the structures (Al(3), following previous nomenclature 4,5 ).…”

“…The region between about 20 and 0 ppm contains two narrow peaks, which, again as previous authors have found, are consistent with the relatively symmetrical octahedral Al(1) and Al(4) sites. 16,17 This region of the spectrum also contains a broader peak with no well-defined quadrupolar line shape; previous authors have fitted this with two components representing the less symmetrical Al(5) octahedron, plus the "pentahedral" Al(2) site. 16,17 In the 18.8 T spectra, however, careful examination of the baseline, and adjustment of the sample spinning rate to optimize sideband positions, revealed a previously unreported component for both SA6 and CA6, which can be accurately fitted with C Q values of about 21 MHz, η ) 0, and δ iso of about 57 ppm, with relative areas of close to the 8.3% expected for the Al(2) site (Figure 3, Table 1).…”

“…16,17 This region of the spectrum also contains a broader peak with no well-defined quadrupolar line shape; previous authors have fitted this with two components representing the less symmetrical Al(5) octahedron, plus the "pentahedral" Al(2) site. 16,17 In the 18.8 T spectra, however, careful examination of the baseline, and adjustment of the sample spinning rate to optimize sideband positions, revealed a previously unreported component for both SA6 and CA6, which can be accurately fitted with C Q values of about 21 MHz, η ) 0, and δ iso of about 57 ppm, with relative areas of close to the 8.3% expected for the Al(2) site (Figure 3, Table 1). This new component was observed in at least two samples of each phase; it is most apparent at 18.8 T, but its lower frequency singularity can readily be identified at 14.1 T as well.…”

“…As part of a larger study of 27 Al and 17 O spectra of Sr aluminates, an 17 O-enriched sample of SA6 was also synthesized, necessitating somewhat different methods to ensure retention of the isotopic label. Carefully dried SrO was ground together with 0.2 wt % Co 3 O 4 (to speed 17 O spin-lattice relaxation) and the stoichiometric amount of ≈20% 17 O-enriched Al 2 O 3 , the latter obtained by hydrothermal exchange of Al(OH) 3 with labeled H 2 O followed by dehydration under Ar. The starting mixture was packed into a Pt tube, which was then welded shut and heated for 8 h at 1420 °C.…”

“…Most recently, a study at fields to 14.1 T of CA6 reached similar conclusions: that the signal from the AlO 5 site was poorly resolved from those of the AlO 6 sites, but could be modeled with a surprisingly low δ iso value of 20 ppm. 17 A value of η ) 0 was retained in the model, consistent with the 3-fold symmetry axis that passes through the site in all crystal structure refinements.…”