The closely related phases alpha- and beta-A(2)Hg(3)M(2)S(8) (A = K, Rb; M = Ge, Sn) have been discovered using the alkali polychalcogenide flux method and are described in detail. They present new structure types with a polar noncentrosymmetric crystallographic motif and strong nonlinear second-harmonic generation (SHG) properties. The alpha-allotropic form crystallizes in the orthorhombic space group Aba2 with a = 19.082(2) A, b = 9.551(1) A, c = 8.2871(8) A for the K(2)Hg(3)Ge(2)S(8) analogue, and a = 19.563(2) A, b = 9.853(1) A, c = 8.467(1) A for the K(2)Hg(3)Sn(2)S(8) analogue. The beta-form crystallizes in the monoclinic space group C2 with a = 9.5948(7) A, b = 8.3608(6) A, c = 9.6638(7) A, beta = 94.637 degrees for the K(2)Hg(3)Ge(2)S(8) analogue. The thermal stability and optical and spectroscopic properties of these compounds are reported along with detailed solubility and crystal growth studies of the alpha-Kappa(2)Hg(3)Ge(2)S(8) in K(2)S(8) flux. These materials are wide gap semiconductors with band gaps at approximately 2.40 and approximately 2.64 eV for the Sn and Ge analogues, respectively. Below the band gap the materials exhibit a very wide transmission range to electromagnetic radiation up to approximately 14 microm. alpha-K(2)Hg(3)Ge(2)S(8) shows anisotropic thermal expansion coefficients. SHG measurements, performed with a direct phase-matched method, showed very high nonlinear coefficient d(eff) for beta-K(2)Hg(3)Ge(2)S(8) approaching 20 pm/V. Crystals of K(2)Hg(3)Ge(2)S(8) are robust to air exposure and have a high laser-damage threshold.

Optical propertiesOptical properties D 6000 α-and β-A 2 Hg 3 M 2 S 8 (A: K, Rb; M: Ge, Sn): Polar Quaternary Chalcogenides with Strong Nonlinear Optical Response. -The title compounds are synthesized from powders of M (M: Ge, Sn), A2S (A: K, Rb), HgS, and elemental sulfur using the alkali polychalcogenide flux method (400-500°C, 2-4 d, 64-85% yield). As revealed by single crystal XRD, the α-form of A2Hg3M2S8 crystallize in the orthorhombic space group Aba2 with Z = 4 and the β-form in the monoclinic space group C2 with Z = 2. Both forms possess polar noncentrosymmetric structures. The materials are wide gap semiconductors, possess high IR transmission, and exhibit strong second-harmonic generation, particularly in the midwave and far-wave IR region. The compounds easily crystallize im millimeter-sized crystals with high spectral transparency and high laser damage threshold. -(LIAO, J.-H.; MARKING, G. M.; HSU, K. F.; MATSUSHITA, Y.; EWBANK, M. D.; BORWICK, R.; CUNNINGHAM, P.; ROSKER, M. J.; KANATZIDIS*, M. G.; J. Am. Chem. Soc. 125 (2003) 31, 9484-9493; Dep. Chem., Mich. State Univ., East Lansing, MI 48824, USA; Eng.) -W. Pewestorf 42-016

The influence of the Lewis basicity of alkali polysulfide fluxes (A(2)S(x)) (A = alkali) as a function of x on the respective reactivities of Pb and Sn with Ge was studied and found to be strong. Cs(4)Pb(4)Ge(5)S(16), K(2)PbGe(2)S(6), and K(4)Sn(3)Ge(3)S(14) could be prepared only under low basicity with S/A(2)S ratios of > or =11. These compounds display complex frameworks and are semiconductors. Cs(4)Pb(4)Ge(5)S(16) is luminescent with red emission.

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