Two-Step Substitutions of Square Pyramidal VO₅ with CdO₄Br to Achieve Higher Laser-Induced DamageThreshold from Polar Oxybromides Rb₄CdV₅O₁₅Br to Rb₃CdV₄O₁₂Br

Mid-infrared (MIR) nonlinear optical (NLO) materials play a crucial role in laser technology applications, yet their rational structural design remains a significant challenge. In this article, guided by the aliovalent substitution strategy, two novel MIR NLO crystals, Rb₄CdV₅O₁₅Br and Rb₃CdV₄O₁₂Br, were designed and successfully synthesized. Rb₄CdV₅O₁₅Br crystallizes in noncentrosymmetric tetragonal space group <i>P</i>4, featuring the two-dimensional (2D) [CdV₅O₁₅Br]_∞ layer structure. The functional units of this structure include [CdO₄Br]/[VO₅] square pyramids and [VO₄] tetrahedra. Rb₃CdV₄O₁₂Br crystallizes in orthorhombic space group <i>Pba</i>2, and its 2D [CdV₄O₁₂Br]_∞ layered structure is composed of [CdO₄Br] square pyramids and [VO₄] tetrahedra. Rb₃CdV₄O₁₂Br exhibits strong phase-matched second harmonic generation (SHG) responses (7.7 × KDP@1064 nm and 0.9 × [email protected] μm). In contrast, Rb₄CdV₅O₁₅Br shows phase matching at 2.1 μm with a strong SHG response of 3.4 × [email protected] μm. Both compounds exhibit a high laser-induced damage threshold (LIDT) of 40 × AGS@1064 nm for Rb₄CdV₅O₁₅Br and 70 × AGS@1064 nm for Rb₃CdV₄O₁₂Br. Two-step substitutions of square pyramidal VO₅ → CdO₄Br achieve higher LIDT from the parent oxide Rb₂(VO)­(V₂O₇) to polar oxybromides Rb₄CdV₅O₁₅Br and Rb₃CdV₄O₁₂Br. These findings enrich the structure chemistry of vanadates and provide new insights into the design of infrared oxide crystals with strong SHG response, high LIDT, and broad spectral transparency.