“Butterfly Effect” of Halogen Substitution on Phase Transition Features in One-Dimensional Spin-Peierls-type van der Waals Crystals

Designing phase transition (PT) materials and manipulating their properties are significant but challenging tasks. In this study, we explore a strategy to design molecule-based spin-Peierls-type PT materials and manipulate their PT features by modifying intermolecular van der Waals interactions within the crystal. We successfully synthesized three isomorphic one-dimensional spin-Peierls-type PT salts [Me₃NCH₂X][Ni(mnt)₂], where mnt^2- is maleonitriledithiolate ligand and Me₃NCH₂X⁺ represents halogen-substituted tetramethylammonium, with X = Cl (1), Br (2), and I (3). These salts were characterized in terms of their composition, thermal stability, and magnetic properties. Notably, the minor differences in the compositions of 1-3 result in significant distinctions in their PT properties. This “butterfly effect” of halogen substitution on PT features is further discussed using Hirshfeld surface analysis. Our results reveal that ubiquitous dispersion interactions in molecular crystals can be used as an efficient tool for manipulating the PT behaviors of molecule-based functional materials.