H···X Hydrogen Bonds Govern Chain Geometry, Thermal Expansion, and Phase Behavior in Crown-Ether-Based Lead Halide...

Hydrogen bonding is a central and versatile noncovalent interaction across nature and synthetic materials. However, a fundamental understanding of how specific hydrogen-bonding motifs dictate crystal structure and macroscopic properties remains limited. In this work, we demonstrate that H···X (X = halide) hydrogen bonds between organic cations and inorganic lead halide polyhedral chains provide a direct means to control structure, phase transitions, and thermal expansion in a series of crown-ether-based lead halide hybrids, MA(18-C-6)PbX₃ (MA⁺ = Methylamine; 18-C-6 = 18-Crown-6; X = Cl (1), Br (2), or I (3)). These directional interactions suppress the conformational and rotational freedom of the crown ether while simultaneously regulating the coordination geometry of the PbX_n polyhedra and their assembly into one-dimensional inorganic chains. Consequently, the materials exhibit distinct phase-transition behaviors and pronounced anisotropic thermal expansion, including negative and near-zero thermal expansion along the chain direction. These results establish hydrogen-bond-network engineering as a general strategy for programming structural responses and thermal properties in flexible hybrid materials.