LiLa4N2I7 as the first lithium-containing nitride halide of the lanthanides: Synthesis, crystal structure and spectroscopic characterization

Initially, the quaternary nitride iodide LiLa₄N₂I₇ was synthesized at 600 °C by a partial metathesis reaction of lithium nitride (Li₃N) and lanthanum triiodide (LaI₃) within 7 days. Single-phase samples were obtained through annealing of the corresponding binary compounds (LiI, LaN and LaI₃) for 20 days overall (4 days at 600 °C prior to 16 days at 500 °C). The title compound crystallizes in the orthorhombic space group Pnma with four formula units per unit cell (a = 1374.17(9), b = 1213.45(8), c = 1079.91(7) pm at 100 K). The dominating structural features are trans-edge shared [NLa₄]⁹⁺ tetrahedra building up ¹{[NLa_4/2^e]³⁺} ∞ 1 chains running along [010]. The linkage of these chains via iodide anions leads to a three-dimensional framework, which embeds isolated trigonal bipyramids [LiI₄₊₁]^4- with one long besides four short Li⁺-I^- contacts. Infrared spectra of crystalline samples show four main bands between 300 and 600 cm^-1, which can probably be assigned as vibrational modes of the condensed [NLa₄]⁹⁺ tetrahedra. ⁷Li-MAS NMR spectroscopy reveals a different coordination sphere of Li⁺ in rocksalt-type LiI than in LiLa₄N₂I₇, but no significant lithium-cation mobility.