Structural Investigation of Lithium–Ammonia Intercalation Compounds of TiS₂

The structures of two lithium–ammonia intercalation compounds of TiS₂ have been determined by Rietveld refinement of time-of-flight neutron powder diffraction data taken at 300 and 12 K. Li⁺_0.23(ND₃)_0.63TiS₂^0.23−and Li⁺_0.11(ND₄⁺)_0.11(ND₃)_0.54TiS₂^0.22− are best described as 3R polytypes in the space group throughout the temperature range investigated. The former compound has a = 3.4234 (1) Å and c = 26.8235 (13) Å at 300 K and a = 3.41213 (8) Å and c = 26.4921 (11) Å at 12 K, whereas the latter material has a = 3.42064 (9) Å and c = 26.7754 (12) Å at 300 K and a = 3.40928 (9) Å and c = 26.4471 (13) Å at 12 K. Available evidence strongly suggests lithium is coordinated to three ammonia molecules in the van der Waals gap of the former compound. The ammonia molecules are distorted from their normal pyramidal geometry in trigonal prismatic sites and have their pseudo-C₃ axes parallel to the TiS₂ sheets. The distance from nitrogen to the deuterium plane for ammonia increases with increasing Li⁺ concentration. The ammonium cation is spherically disordered in trigonal prismatic sites. The substitution of ND₄⁺ for Li⁺ results in a contraction of the TiS₂ octahedra parallel to the host layers and an elongation of the octahedra per¬pendicular to the host layers. These compounds exhibit an anisotropic strain of ~0.18% along the [001] direction and essentially no strain perpendicular to [001].