Abstract:
This work investigates the reactions and chemical modifications of WTe2 and related solid-state systems, with a particular focus on intercalation chemistry, halogen-induced transformations, cluster formation, and structure–property relationships in inorganic and hybrid materials. Hereby WTe2 is used as a chemically tunable layered host because its distorted Td-type structure, weak van der Waals (vdW) gaps, and characteristic W–W zigzag chains make it highly responsive to changes in electron count and interlayer bonding.
A central part of the work is about the intercalation of cationic and anionic guest species into WTe2. Low-temperature solution reactions with alkali naphthalenides yielded the ordered intercalation compounds A0.5WTe2 (A = K, Rb, Cs). These phases retain the layered host structure, but show a systematic expansion of the van der Waals gap, ordered alkalimetal arrangements, and distortions of the tungsten chains, thereby directly affecting one of the key structural motifs responsible for the characteristic properties of WTe2. The resulting A0.5WTe2 phases (A = K, Rb, Cs) are consistently described by optical measurements, electrical transport data, and density functional theory calculations as narrow-gap semiconductors with band gaps of approximately 0.35–0.48 eV. The oxidative intercalation chemistry of WTe2 was extended from iodine to bromine. WTe2I was confirmed as a reversible anionic intercalation compound and was structurally described in substantially greater detail. Instead of a simple average structure, it occurs in closely related commensurately and incommensurately modulated variants, both dominated by modulation of the iodine substructure. No phase transition was observed between 100K and 270 K, showing that the modulation is an intrinsic structural feature. Bromine intercalation gave access to the multistage series WTe2Brx with x = 0.5, 1.0, and 1.25. The compound WTe2Br0.5 contains bromine atoms in the van der Waals gap that form directional Te–Br contacts and exhibit pronounced positional disorder. At higher bromine content, WTe2Br1.25 adopts a staged bromine-rich structure with two different bromine interlayers, corresponding to 0.5 and 0.75 Br per WTe2 formula unit. The differences in the interlayer distances reflect the different filling levels. These results show that halogen intercalation in WTe2 combines reversible guest uptake, staging, modulation, and strong host–guest interactions. At elevated temperature or higher halogen activity, the chemistry of WTe2 shifts from topochemical intercalation to reconstructive halogenation. The previously reported assignment of WI4 was corrected and shown to correspond to the new metastable polymorph ζ-TeI4. In addition, the iodination of WTe2 at comparatively mild conditions yielded (TeI3)2[W6I14], a tungsten iodide cluster compound composed of [W6I14]2– anions and (TeI3)+ cations. These studies connect the intercalation chemistry of WTe2 with follow-up reactions leading to tellurium iodides and metal-halide clusters. Further contributions address transition-metal chalcohalide and oxyiodide cluster compounds. Ta4SBr11 was characterized as a corrugated van der Waals layered compound based on butterfly-shaped [Ta4S] clusters and was described as a likely cluster Mott insulator. The niobium oxyiodide family Nb4OI12–x (x = 0, 1, 2) establishes a structural sequence from molecular clusters to one-dimensional chains and two-dimensional layers. The compounds Li3Nb7O5I15 and Nb8O5I17(NbI5) further extend this chemistry from [Nb4O] and [Nb5O4] motifs to larger [Nb7O5] and [Nb8O5] cluster cores with multicenter Nb–Nb bonding. The work also includes melamine-based hybrid halides as SHG materials. HgCl2(C3N6H6) crystallizes in the polar space group C2 and combines a layered mercury chloride substructure with ordered melamine molecules, resulting in pronounced second-harmonic generation (SHG) activity. The related zinc halides (C3N6H7)ZnX3(C3N6H6) (X = Cl, Br) show how changes in symmetry, hydrogen bonding, and polarizability influence nonlinear optical response. Finally, a complete recovery cycle for spent graphitic Li-NMC battery material was developed, combining thermal treatment, leaching, impurity precipitation, copper electrolysis, and aqueous sol–gel synthesis to regenerate fresh Li-NMC cathode material. Overall, the results show how targeted synthesis, careful structural characterization, and complementary physical measurements reveal direct links between composition, guest ordering, cluster connectivity, symmetry, and functional properties in complex inorganic and hybrid materials.
WTe2