The exploration of new infrared nonlinear optical crystals based on the polymorphism of BaGa4S7

Abstract

Balancing the key performance metrics, such as a large second harmonic generation (SHG) response and a wide band gap, is an extremely important but intractable challenge for the development of infrared (IR) nonlinear optical (NLO) materials. Herein, we report two new polymorphs of BaGa₄S₇, β- and γ-BaGa₄S₇, both of which crystallize in the noncentrosymmetric and polar space groups, Pmc2₁ (No. 26) for β-BaGa₄S₇ and Cm (No. 8) for γ-BaGa₄S₇. They show an interesting structural evolution from AA′ [Ga₆S₁₆]_∞ chains to AA′A′A [Ga₆S₁₆]_∞ chains to AAA′AAA′ [Ga₆S₁₆]_∞ chains as their structures show a transition from the high-temperature phase to the low-temperature phase. More importantly, they both exhibit excellent NLO properties. In particular, β-BaGa₄S₇ exhibits the best balance along with a large phase-matching SHG response (1.2 × AgGaS₂) and a wide band gap (3.94 eV). These indicate that β-BaGa₄S₇ will be a promising IR NLO crystal for high-power laser application. The research also provides an insight into exploring new IR NLO crystals based on structural rearrangement.