Halide/Pseudohalide Anion Regulation of Polar Structure and Glass‐Forming Ability Toward Polar Hybrid Glass‐Ceramic

ABSTRACT Hybrid glasses have emerged as a promising class of functional materials with tunable physicochemical properties, yet systematic anion‐size engineering to modulate glass‐forming ability (GFA) remains challenging due to the lack of well‐defined comparative platforms. By assembling the (4‐fluorobenzyl)(triphenyl)phosphonium cation (FBTPP + ) with tetrahedral cadmium(II) anions, we present four new hybrids, (FBTPP) 2 [CdX 4 ] (X = Cl − , Br − , I − , NCS − for 1–4 ), forming a textbook‐style comparative system to elucidate crystal‐glass phase transformations. This series enables synergistic control over core properties: a centrosymmetric‐to‐noncentrosymmetric evolution ( Pbca → P 2 1 / c → Cc → Cc ), a 146 K melting point depression (515 → 369 K), a decreasing fragility index (41.8 → 30.3), and a marked GFA enhancement (0.70 → 0.83). These systematic trends originate from anion enlargement and electronic polarization, which weaken intermolecular contacts, reduce lattice cohesion, lower the melting barrier, and promote diffuse charge distributions, thus broadening structural relaxation near the glass transition and enhancing kinetic stability against recrystallization. The bulky, charge‐delocalized NCS − anion further induces a polar crystal packing with an open, loosened structure and greatly increased molecular separation. We further demonstrate a novel strategy that bypasses the kinetic limitation of recrystallization: a polar glass‐ceramic thin film ( 4‐GC ) was fabricated under ambient conditions via direct consolidation of its glassy phase with polycrystalline 4 above the vitrification temperature, exhibiting a 2.4‐fold enhanced second‐harmonic generation response. This work provides a programmable ion‐engineering strategy for high‐performance nonlinear‐optical hybrid glass‐ceramics.