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

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, beta- and gamma-BaGa4S7, both of which crystallize in the noncentrosymmetric and polar space groups, Pmc2(1) (No. 26) for beta-BaGa4S7 and Cm (No. 8) for gamma-BaGa4S7. They show an interesting structural evolution from AA ' [Ga6S16](infinity) chains to AA ' A ' A [Ga6S16](infinity) chains to AAA ' AAA ' [Ga6S16](infinity) 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, beta-BaGa4S7 exhibits the best balance along with a large phase-matching SHG response (1.2 x AgGaS2) and a wide band gap (3.94 eV). These indicate that beta-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.

Automated summary

This study reports two new polymorphs of BaGa4S7 that have a large second harmonic generation response and a wide band gap, making them promising materials for infrared nonlinear optical applications. Among the two polymorphs, β-BaGa4S7 exhibits the best balance and is considered a potential infrared NLO crystal.