Local versus Average Structure in LaSrAl3O7: A NMR and DFT Investigation

LaSrAl3O7 belongs to the family of (M2T(1)T(2)O7)-T-1-O-2 compounds with tetragonal melilite-like layered structure. In such compounds the local structure and properties may be very different from the average ones, mainly due to cationic disorder on the M site. In this work, solid-state Al-27 NMR spectroscopy and periodic density functional theory calculations were used to highlight the differences between local and average order in the LaSrAl3O7 crystal. The Al-27 isotropic chemical shifts and quadrupolar coupling constants were computed by employing the gauge including projector augmented wave (GIPAW) and PAW formalisms, respectively. An impressive linear relationship between the computed quadrupolar coupling constant C-Q and the Al tetrahedra (T-1, T-2) distortion was observed. In particular, our calculations showed that the distortion of the T-2 environment is determined by the La/Sr speciation around the apical nonbridging oxygen.