Crystal Structure and Bond-Valence Investigation of Nitrogen-Stabilized LiAl₅O₈ Spinels

Abstract

An in-depth insight into the effect of nitrogen substitution on structural stabilization is important for the design of new spinel-type oxynitride materials with tailored properties. In this work, the crystal structures of ordered and disordered LiAl 5 O 8 obtained by slow cooling and rapid quenching, respectively, were analyzed by a X-ray diffraction (XRD) Rietveld refinement and OccQP program. The variation in the bonding state of atoms in the two compounds was explored by the bond valence model, which revealed that the instability of spinel-type LiAl 5 O 8 crystal structure at room temperature is mainly due to the severe underbonding of the tetrahedrally coordinated Al cations. With the partial substitution of oxygen with nitrogen in LiAl 5 O 8 , a series of the nitrogenstabilized spinel Li y Al (16+x−y)/3 O 8−x N x (0 < x < 0.5, 0 < y < 1) was successfully prepared. The crystal structures were systematically investigated by the powder XRD structural refinement combined with 7 Li and 27 Al magic-angle spinning nuclear magnetic resonance. All the Li + ions entered the octahedra, while the Al resonances may be composed of multiple non-equivalent Al sites. The structural stability of spinel Li y Al (16+x−y)/3 O 8−x N x at ambient temperature was attributed to the cationic vacancies and high valence generated by the N ions, which alleviated the under-bonding state of the tetrahedral Al−O bond. This work provides a new perspective for understanding the composition−structure relationship in spinel compounds with multiple disorders.

Crystal structure investigation of spinel‐type LiAlON with multiple atomic disorders

“…Chen et al. proposed that the Li + at 16d site appears overbonded with valence bond excesses of ∼0.8 vu (valence units) in spinel‐type LiAl 5 O 8 13 . The formation of Li–N bonds contributes to exacerbate the overbonded state, which may lead to an unstable crystal structure.…”

“…Chen et al proposed that the Li + at 16d site appears overbonded with valence bond excesses of ∼0.8 vu (valence units) in spinel-type LiAl 5 O 8 . 13 The formation of Li-N bonds contributes to exacerbate the overbonded state, which may lead to an unstable crystal structure. As a result, the formation enthalpy of the system increases significantly as the number of Li-N bonds in the unit cell increases.…”

“…Based on the Rietveld refinement, Chen et al obtained the lattice constant of LiAlON and proposed that all the Li + enter into 16d sites, and a small number of cation vacancies are distributed at 8a positions. 13,16 However, the multi-disordered structure from experimental determinations still makes it difficult to construct the reasonable supercell model for theoretical investigations. 18,19 In this paper, the multi-disordered crystal structure of LiAlON was systematically studied by combining the firstprinciples simulations, solid-state NMR, and the Rietveld structure refinement.…”

“…Clay et al. developed a series of research studies on the synthesis and preparation of LiAlON transparent ceramics using LiAl 5 O 8 , Al 2 O 3 , and AlN as raw materials 13–15 . The experimental results showed that the LiAlON also has a large range of solubility and its mechanical properties are better than those of AlON 16 .…”

“…Based on the Rietveld refinement, Chen et al. obtained the lattice constant of LiAlON and proposed that all the Li + enter into 16d sites, and a small number of cation vacancies are distributed at 8a positions 13,16 . However, the multi‐disordered structure from experimental determinations still makes it difficult to construct the reasonable supercell model for theoretical investigations 18,19 …”