CaM₃Si₄As₁₁ (M = Ga, In): arsenide infrared nonlinear optical crystals enforced by novel [As₂] linear motifs

Exploration of new arsenide-based infrared (IR) nonlinear optical (NLO) crystals has long been hindered by the narrow band gap limitations and the difficulty of achieving non-centrosymmetric (NCS) structures, despite their promising potential in the long-wave infrared region. By assembling MAs₄ (M = Ga, In) tetrahedra and novel [As₂] linear motifs, we designed and synthesized two new NCS arsenides, CaM₃Si₄As₁₁ (M = Ga, In), featuring quasi-T₃ M₃Si₄As₁₁[As₂]₆ supertetrahedra decorated by [As₂] motifs. CGSA and CISA exhibited strong SHG responses (1.7 and 2.5 × ZGP at 2900 nm), wide band gaps among arsenides (1.65 eV and 1.49 eV), broad IR transparency range (cutoff edge up to 12.5 µm), and large birefringence (0.122 and 0.128). Structure-property relationship analyses revealed that the wide band gap originated from the synergistic effect of the high average coordination number of As atoms induced by As-As bonding and the high excitation energy of SiAs₄ tetrahedra. Furthermore, the [As₂] motifs with highly polarizable π-interactions made a significant contribution to NLO effects and birefringence. This work demonstrates that arsenides with introduced [As₂] units are promising systems for long-wave IR NLO materials.