The first polyanion-substitution-driven centrosymmetric-to-noncentrosymmetric structural transformation realizing an excellent nonlinear optical supramolecule [Cd4P2][CdBr4]

Abstract

Crystallographically, noncentrosymmetricity (NCS) is an essential precondition and foundation of achieving nonlinear optical (NLO), pyroelectric, ferroelectric, and piezoelectric materials. Herein, structurally, octahedral [SmCl₆]³⁻ is substituted by the acentric tetrahedral polyanion [CdBr₄]²⁻, which is employed as a templating agent to induce centrosymmetric (CS)-to-NCS transformation based on the new CS supramolecule [Cd₅P₂][SmCl₆]Cl (1), thereby providing the NCS supramolecule [Cd₄P₂][CdBr₄] (2). Meanwhile, this replacement further results in the host 2D _∞²[Cd₅P₂]⁴⁺ layers converting to yield the twisted 3D _∞³[Cd₄P₂]²⁺ framework, which promotes the growth of bulk crystals. Additionally, phase 2 possesses well-balanced NLO properties, enabling considerable second-harmonic generation (SHG) responses (0.8–2.7 × AgGaS₂) in broadband spectra, the thermal expansion anisotropy (2.30) together with suitable band gap (2.37 eV) primarily leading to the favorable laser-induced damage threshold (3.33 × AgGaS₂), broad transparent window, and sufficient calculated birefringence (0.0433) for phase-matching ability. Furthermore, the first polyanion substitution of the supramolecule plays the role of templating agent to realize the CS-to-NCS transformation, which offers an effective method to rationally design promising NCS-based functional materials.