Entropy-Driven Precursor Symmetry Breaking for Phase-Selective Synthesis of Noncentrosymmetric Rb₂CdCl₂I₂

Controllable synthesis of noncentrosymmetric (NCS) structures is a key challenge in infrared nonlinear optical (NLO) materials. We propose an entropy-driven precursor symmetry breaking strategy that directs crystallization of NCS phases under identical composition by tuning precursor symmetry and aggregation. In Rb₂CdCl₂I₂, hydrothermal conditions transform symmetric [CdCl₄I₂] octahedra into asymmetric configurations and larger clusters, while room-temperature evaporation yields symmetric small aggregates. Raman and dynamic light scattering show vibrational entropy drives this: at elevated temperatures, degeneracy lifting activates extra vibrational modes, giving asymmetric species an entropic advantage. Classical nucleation theory (CNT) reveals interfacial energy barrier inversion due to symmetry change, enabling selective crystallization of noncentrosymmetric I4mm phase over centrosymmetric I4/mmm. Variable-temperature X-ray diffraction (XRD) and differential scanning calorimetry (DSC) confirm a reversible displacive phase transition driven by softening/freezing of octahedral tilting modes, underscoring vibrational entropy's role in phase stability. The resulting Rb₂CdCl₂I₂ (I4mm) exhibits a bandgap of 3.70 eV, transparency from 0.48–83 µm, laser-induced damage threshold (LIDT) of 56.2 MW cm⁻² (19 × AgGaS₂), and phase-matchable second-harmonic generation (SHG) of 0.17 × KDP. This strategy offers a new paradigm for synthesizing NCS structures for infrared NLO materials.