27 Al NMR Chemical Shifts in Oxide Crystals: A First-Principles Study

In the framework of density functional theory using periodic boundary conditions, 27 Al nuclear magnetic resonance (NMR) parameters of 20 aluminum sites in 12 aluminum-containing crystalline oxides, i.e., Al 2 O 3, LiAlO 2, SiAl 2 O 5, MgAl 2 O 4, YAlO 3, AlVO 4, and their polymorphs, are investigated. The present method excellently reproduces the 27 Al NMR parameters and the well-known empirical correlation of the isotropic chemical shifts, δ iso, of 27 Al nuclei with its number of coordinating oxygen atoms, i.e., coordination number. Through systematic calculations, we demonstrate that the mean Al−O bond order, Q Al−O, can be a better parameter to correlate with 27 Al δ iso than merely averaged bond length or the coordination number. The relationship between δ iso and Q Al−O is also found to be valid for α-Al 2 O 3 under hydrostatic pressures in which the coordination number is unchanged from six.