Lithiumaluminiumamid, LiAl(NH₂)₄, Darstellung, röntgenographische Untersuchung, Infrarotspektrum und thermische Zersetzung

Abstract

Durch Umsetzung von Lithium und Aluminium mit flüssigem Ammoniak bei 80–100°C entsteht innerhalb weniger Tage LiAl(NH2)4. Kristalle für eine röntgenographische Strukturbestimmung erhält man bei langsamen Wachstum aus flüssigem NH3 ausgehend von feinteiligem ternären Amid. Die Aufklärung der Atomanordnung gelang einschließlich der H‐Lagenbestimmung an den NH2−‐Ionen. Raumgruppe: P21/n; Gitterkonstanten: a = 9,478(1) Å, b = 7,351(1) Å, c = 7,398(1) Å, β = 90,26(1)°; Z = 4, R‐Werte (ungew./gew. mit w = 1): 0,042/0,046. Die Atomanordnung von LiAl(NH2)4 kann formal als eine neue Variante der GaPS4‐Struktur aufgefaßt werden. Die Verbindung wird weiterhin durch ihr IR‐Spektrum charakterisiert. Bei der thermischen Zersetzung von LiAl(NH2)4 bildet sich bei 180°C amorphes Al2(NH)3 neben gut kristallinem LiNH2; bei 220°C liegt bereits feinteiliges AlN vor. Oberhalb 400°C reagiert dieses AlN mit LiNH2 bzw. Li2NH zu Li3AlN2.

Structural and thermal gas desorption properties of metal aluminum amides

“…Our results for M = Li and Na agree well with the previous reports, indicating that our Li-and Na-samples are actually LiAl(NH 2 ) 4 and NaAl(NH 2 ) 4 , respectively, where an Al atom is coordinated by four [NH 2 ] − units (i.e., anion complex [Al(NH 2 ) 4 ] − )[6,7,9,13]. On the other hand, the crystal symmetry for our K-sample was different from the previously reported crystal struc-…”

“…2 gives the FT-IR spectra in the frequency region of N-H stretching mode for M[Al(NH 2 ) 4 ] x together with those for M(NH 2 ) x as references. The FT-IR spectra of our Li-and Na-samples are very similar to those of LiAl(NH 2 ) 4 and NaAl(NH 2 ) 4 reported by Jacobs et al [7] and Lutz et al [13]. It should be noted that the N-H stretching vibration (3250-3450 cm −1 ) for LiAl(NH 2 ) 4 is higher in frequency than that (3200-3350 cm −1 ) for LiNH 2 .…”

“…In the MAS spectra, all the products have a peak in the chemical shift range of 100-130 ppm. We observed a single sharp signal for the Li-sample, being consistent with a single Al crystallographic site in LiAl(NH 2 ) 4 [6,7]. Taking account of the fact that NaAl(NH 2 ) 4 also has a single Al site [9,13], it is found that a small peak splitting observed for the Nasample is coming from the second-order quadrupolar interaction as is evidenced in Fig.…”

“…Actually, LiAl(NH 2 ) 4 releases 35 mass% of the NH 3 gas with a peak temperature of about 140 • C [5][6][7]. Moreover, Aliouane et al showed that NaAl(NH 2 ) 4 had 30 mass% NH 3 density and the NH 3 desorption peak temperature was about 100 • C [8] which was much lower than that of LiAl(NH 2 ) 4 [9,10].…”

Studies of covalent amides for hydrogen storage systems: Structures and bonding of the MAl(NH2)4 phases with M=Li, Na and K

“…The vertical line at 2Â = 45.9 • corresponds to a reflection of the sample holder Be window. Except this reflection, all the lines can be indexed in the unit cells already reported in the literature for the MAl(NH 2 ) 4 phases: monoclinic with P2 1 /n space group for LiAl(NH 2 ) 4 [32], monoclinic with P2 1 /c space group for NaAl(NH 2 ) 4 [33] and orthorhombic with C222 1 space group for KAl(NH 2 ) 4 [34]. In all cases, the unit cells contain four MAl(NH 2 ) 4 formula units (Z = 4).…”