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

Two interesting non-centrosymmetric metal chalcogenides, RbAgSbS and CsAgSbS, were synthesized by an ionothermal approach. Crystals of compounds RbAgSbS and CsAgSbS possess isomorphic configuration, consisting of two-dimensional (2D) anionic networks [AgSbS], which are split by alkali-metal M cations. The band gaps are 2.11 and 2.02 eV for RbAgSbS and CsAgSbS, respectively. Second-harmonic generation (SHG) studies revealed that RbAgSbS affords a powder SHG performance of ∼0.5 × AgGaS with type-I phase matching, while CsAgSbS shows a slightly stronger SHG performance of ∼0.6 × AgGaS 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 RbAgSbS and CsAgSbS are about 2.3 × AgGaS. The calculated birefringence indexes Δ 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.