Cd/V Ratio-Driven Centrosymmetric Transition of Crystal Structures in Cesium Cadmium Vanadate Oxychlorides...

Nonlinear optical (NLO) crystals are crucial components in advanced laser technologies. Based on the Rb₃CdV₄O₁₂Br matrix, a new mid-infrared (MIR) NLO material Cs₃CdV₄O₁₂Cl was designed and synthesized via isovalent substitution of Rb⁺¹ with Cs⁺¹ cations and Br^-1 with Cl^-1 anions. During the synthesis exploration, a novel transition metal vanadate oxychloride, CsCdV₂O₆Cl, was discovered. Cs₃CdV₄O₁₂Cl crystallizes in the orthorhombic system with space group Cmm2, featuring a characteristic 2D [CdV₄O₁₂Cl]_∞ layered structure. The fundamental building units of Cs₃CdV₄O₁₂Cl are CdO₄Cl square pyramids and VO₄ tetrahedra. The compound exhibits strong phase-matchable second-harmonic generation (SHG) effects (5 × KDP@1064 nm and 1.9 × AGS@2.1 μm) in the 180-280 μm particle size range, along with a wide band gap of 3.24 eV and far-infrared absorption cutoff edge (10.2 μm). In contrast, CsCdV₂O₆Cl crystallizes in the monoclinic centrosymmetric space group C2/c, featuring a 3D framework structure intertwined by Cd-O-Cl and V-O one-dimensional chains. Notably, the distinct Cd/V atom ratios for Cs₃CdV₄O₁₂Cl versus CsCdV₂O₆Cl act as the pivotal driver, triggering the remarkable differentiation in structural symmetry and thus the drastic divergence of their optical performances. This work enriches and expands the optical materials in the transition metal vanadate halides, shedding light on the structural design strategy via stoichiometric ratio regulation for functional NLO materials.