Silicon/aluminum and oxygen/nitrogen ordering in lanthanum U−phase (La 3Si 3Al 3O 12N 2)

Abstract

High-resolution 29Si, 27Al, 15N, and 17O MAS NMR spectra have been obtained for both glass and crystalline samples of lanthanum U-phase, La 3Si 3Al 3O 12N 2. Previous X-ray single-crystal studies have shown that this phase is iso-structural with rare-earth gallogermanates of the type Ln 3Ga 5GeO 14, the atomic arrangement consisting of layers of [(Si,Al)(O,N) 4] tetrahedra in the x, y plane of the hexagonal unit cell, linked together in the z-direction by [AlO 6] octahedra, with rare-earth cations occupying the large holes in this network. However, previous studies obtained only a limited amount of information about cation and anion ordering, mainly deduced from bond-length data. NMR spectra have not only enabled the change in structure from glass to crystalline ceramic to be monitored, but have also given detailed ordering information about the latter, indicating partial disorder of both (Si,Al) and (O,N) on their respective sites in the tetrahedra.