"114"-Type Nitrides LnAl(Si_4-x Al_x )N₇ O_δ with Unusual [AlN₆ ] Octahedral Coordination

Aluminum-nitrogen six-fold octahedral coordination, [AlN₆ ], is unusual and has only been seen in the high-pressure rocksalt-type aluminum nitride or some complex compounds. Herein we report novel nitrides LnAl(Si_4-x Al_x )N₇ O_δ (Ln=La, Sm), the first inorganic compounds with [AlN₆ ] coordination prepared via non-high-pressure synthesis. Structure refinements of neutron powder diffraction and single-crystal X-ray diffraction data show that these compounds crystallize in the hexagonal Swedenborgite structure type with P6₃ mc symmetry where Ln and Al atoms locate in anticuboctahedral and octahedral interstitials, respectively, between the triangular and Kagomé layers of [SiN₄ ] tetrahedra. Solid-state NMR data of high-purity La-114 powders confirm the unusual [AlN₆ ] coordination. These compounds are the first examples of the "33-114" sub-type in the "114" family. The additional site for over-stoichiometric oxygen in the structure of 114-type compounds was also identified.