Structural Study of MgyAl(8+x–2y)/3O4–xNx (0 < x < 0.5, 0 < y < 1) Spinel Probed by X-ray Diffraction, 27Al MAS NMR, and First-Principles Calculations

The deep understanding of the crystal structure and composition-structure relationship is important for modifying and designing solids to obtain functional materials with customized properties. However, because of multiple compositions and complex structures in some spinel solid solutions, the composition-structure relationship is unknown, or becomes very complicated and difficult to be controlled. In this work, the solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR) technique and powder X-ray diffraction (XRD) Rietveld refinement were combined to characterize the crystal structure of quaternary disordered MgyAl(8+x-2y)/3O4-xNx solid solutions in detail, which was supported by the first-principles density functional theory calculations. Diffraction data indicate that Mg ions preferentially enter the tetrahedron structure in MgyAl(8+x-2y)/3O4-xNx solid solutions because the sum of the bond valence in tetrahedron is closer to the atomic valence of Mg. With the compositional change, the coordination polyhedra in the crystal structure will also adjust the volume according to the changes in lattice parameter, anion parameter, and inversion parameter. In addition, the populations, chemical shifts, and quadrupole coupling constants of Al located in different coordinated environments of the solid solutions were detected through the simulation and integration of 27Al MAS NMR spectra, which were related to the structural parameters by the bond valence method. It turns out that 27Al MAS NMR parameters are highly sensitive to the subtle changes in the local environment of Al caused by the preferential occupation of Mg in the tetrahedron. These results provide deep insights into the crystal structural details of novel spinel materials with multiple disorders.