[ASr4Cl][Ge3S10] (A = Na, K) and [KBa4Cl][Ge3S10]: new salt-inclusion infrared nonlinear optical crystals with zero-dimensional [Ge3S9] clusters

Abstract

Three new chalcohalide nonlinear optical crystals, [ASr₄Cl][Ge₃S₁₀] (A = Na, K) and [KBa₄Cl][Ge₃S₁₀], have been successfully designed and synthesized by combining the covalent [Ge₃S₉] clusters and ionic [AB₄Cl] frameworks. Their crystal structures were determined by single crystal X-ray diffraction, which showed that all three compounds crystallized in the noncentrosymmetric and polar space group of P6₃. Three structures are iso-structural and can be seen as salt-inclusion structures with the isolated [Ge₃S₉] clusters stacked alternately along the c axis and ¹_∞[ClNaSr(2)₃]⁶⁺ chains acting as scissors to cut the connection between the [Ge₃S₉] clusters, which is still rare in the known chalcohalides. The IR and Raman spectra indicate that [ASr₄Cl][Ge₃S₁₀] (A = Na, K) and [KBa₄Cl][Ge₃S₁₀] have wide IR transparent windows up to 20 μm. UV-Vis-NIR diffuse reflectance spectra manifest that all three compounds have large band gaps of 3.54, 3.51, and 3.57 eV, respectively. In addition, the title compounds also exhibit large laser damage thresholds (∼12 × AgGaS₂) and moderate SHG responses (1.08, 0.91, and 0.82 × AgGaS₂, respectively). These properties demonstrate their potential as IR NLO crystals. Furthermore, the influence of the (M + N)/Q ratios (M = IIIA or IVA, N = IB or IIB or cations coordinated as [M_xQ_y] anionic groups; Q = S, Se) on the anionic group structural dimensions was discussed, and the structure–property relationship was also studied based on the first-principles calculations. These may produce some insights for designing new chalcohalides.