Two-Step Substitutions of Square Pyramidal VO5 with CdO4Br to Achieve Higher Laser-Induced Damage Threshold 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 P4, 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 Pba2, 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 × AGS@2.1 μm). In contrast, Rb4CdV5O15Br shows phase matching at 2.1 μm with a strong SHG response of 3.4 × AGS@2.1 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.