Stabilization of the Polar Structure and Giant Second-Order Nonlinear Response of Single Crystal γ-NaAs_0.95Sb_0.05Se₂

The dearth of suitable materials significantly restricts the practical development of infrared (IR) laser systems with highly efficient and broadband tuning. Recently, γ-NaAsSe₂ is reported, and it exhibits a large nonlinear second-harmonic generation (SHG) coefficient of 590 pm V⁻¹ at 2 µm. However, the crystal growth of γ-NaAsSe₂ is challenging because it undergoes a phase transition to centrosymmetric δ-NaAsSe₂. Herein, the stabilization of non-centrosymmetric γ-NaAsSe₂ by doping the As site with Sb, which results in γ-NaAs_0.95Sb_0.05Se₂ is reported. The congruent melting behavior is confirmed by differential thermal analysis with a melting temperature of 450 °C and crystallization temperature of 415 °C. Single crystals with dimensions of 3 mm × 2 mm are successfully obtained via zone refining and the Bridgman method. The purification of the material plays a significant role in crystal growth and results in a bandgap of 1.78 eV and thermal conductivity of 0.79 Wm⁻¹ K⁻¹. The single-crystal SHG coefficient of γ-NaAs_0.95Sb_0.05Se₂ exhibits an enormous value of |d₁₁| = 648 ± 74 pm V⁻¹, which is comparable to that of γ-NaAsSe₂ and ≈20× larger than that of AgGaSe₂. The bandgap of γ-NaAs_0.95Sb_0.05Se₂ (1.78 eV) is similar to that of AgGaSe₂, thus rendering it highly attractive as a high-performing nonlinear optical material.