Cd/V Ratio-Driven Centrosymmetric Transition of Crystal Structures in Cesium Cadmium Vanadate Oxychlorides Cs3CdV4O12Cl and CsCdV2O6Cl.

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.