Ionothermal Synthesis of Metal Chalcogenides M2Ag3Sb3S7 (M = Rb, Cs) Displaying Nonlinear Optical Activity in the Infrared Region

Two interesting non-centrosymmetric metal chalcogenides, Rb 2 Ag 3 Sb 3 S 7 and Cs 2 Ag 3 Sb 3 S 7 , were synthesized by an ionothermal approach. Crystals of compounds Rb 2 Ag 3 Sb 3 S 7 and Cs 2 Ag 3 Sb 3 S 7 possess isomorphic configuration, consisting of two-dimensional (2D) anionic networks ∞ [Ag 3 Sb 3 S 7 ] 2- , which are split by alkali-metal M + cations. The band gaps are 2.11 and 2.02 eV for Rb 2 Ag 3 Sb 3 S 7 and Cs 2 Ag 3 Sb 3 S 7 , respectively. Second-harmonic generation (SHG) studies revealed that Rb 2 Ag 3 Sb 3 S 7 affords a powder SHG performance of ∼0.5 × AgGaS 2 with type-I phase matching, while Cs 2 Ag 3 Sb 3 S 7 shows a slightly stronger SHG performance of ∼0.6 × AgGaS 2 with type-I phase matching. Both compounds possess broad transparency ranges (∼0.6-20 μm), suggesting their potential as infrared (IR) nonlinear optical (NLO) materials. The laser damage thresholds (LDTs) of both Rb 2 Ag 3 Sb 3 S 7 and Cs 2 Ag 3 Sb 3 S 7 are about 2.3 × AgGaS 2 . The calculated birefringence indexes Δ n are 0.1885 and 0.1944 at 1.064 μm, respectively, which are sufficiently large enough to achieve phase matching. Theoretical studies using density functional theory have been implemented to further understand the relationship between their NLO properties and band structures.