Abstract

Here, we establish a quantitative link between halide-driven Ni-X covalency and band-edge electronic structure evolution in low-dimensional halide perovskites. Hexagonal CsNiX3 (X = Cl, Br, I) compounds were synthesized by a solvent-free mechanochemical route under ambient conditions. Phase-pure materials were obtained after milling times between 4 and 7 h, depending on the halide, evidencing strong anion-dependent reaction kinetics. Structural analysis reveals halide-dependent crystallinity, microstrain, and agglomeration induced by the mechanochemical process. Optical spectroscopy combined with ligand-field analysis confirms high-spin octahedral Ni2+ and reveals a systematic reduction of ligand-field and Racah parameters from Cl to I, consistent with increasing metal-halide covalency. Diffuse reflectance measurements show a pronounced narrowing of the optical band gap from 3.51 to 1.47 eV and unusually large Urbach energies, indicative of significant electronic disorder and strong structural-electronic coupling. Density functional theory calculations reproduce the experimental band-gap trend and show band edges dominated by hybridized Ni 3d and halide p states. QTAIM analysis quantitatively confirms the progressive increase in Ni-X covalency across the series. VB-XPS enables estimation of band-edge positions, identifying CsNiBr3 as the composition exhibiting the most balanced relative band alignment within simplified heterostructure models. Device simulations using SCAPS-1D illustrate general transport limitations associated with defect density and interfacial band offsets in wide-bandgap halide semiconductors.