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

Abstract

We report new, high-resolution solid state 27Al NMR data for two members of the magnetoplumbite group of structures, strontium hexaluminate (SrAl12O19, “SA6”) and calcium hexaluminate (CaAl12O19, “CA6,” synthetic hibonite), acquired at 14.1 and 18.8 T fields. The high-field data allow, for the first time, the accurate observation of the NMR signal from the Al(2) sites, which have been previously described as “pentacoordinate” but which appear instead to be very distorted tetrahedra, which give rise to 27Al isotropic chemical shifts of about 57 ppm and nuclear quadrupolar coupling constants of about 21 MHz. This finding is fully consistent with “split-atom” refinements of these crystal structures, in which the most probable position of the Al atom is displaced away from the average trigonal pyramidal site center, along the crystallographic 3-fold axis. The NMR data also demonstrate, however, that the “splitting” is the result of static, not dynamical, disorder among these sites.