α- and β-A2Hg3M2S8 (A = K, Rb; M = Ge, Sn):  Polar Quaternary Chalcogenides with Strong Nonlinear Optical Response

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.