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 (x-allotropic form crystallizes in the orthorhombic space group Aba2 with a = 19.082(2) Angstrom, b = 9.551 (1) Angstrom, c = 8.2871 (8) Angstrom for the K2Hg3Ge2S8 analogue, and a = 19.563(2) Angstrom, b = 9.853(1) Angstrom, c = 8.467(1) Angstrom for the K2Hg3Sn2S8 analogue. The beta-form crystallizes in the monoclinic space group C2 with a = 9.5948(7) Angstrom, b = 8.3608(6) Angstrom, c = 9.6638(7) Angstrom, beta = 94.637degrees for the K2Hg3Ge2S8 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-K2Hg3Ge2S8 in K2S8 flux. These materials are wide gap semiconductors with band gaps at similar to2.40 and similar to 2.64 eV for the Sri and Ge analogues, respectively. Below the band gap the materials exhibit a very wide transmission range to electromagnetic radiation up to similar to14 mum. alpha-K2Hg3Ge2S8 shows anisotropic thermal expansion coefficients. SHG measurements, performed with a direct phase-matched method, showed very high nonlinear coefficient d(eff) for beta-K2Hg3Ge2S8 approaching 20 pm/V. Crystals of K2Hg3Ge2S8 are robust to air exposure and have a high laser-damage threshold.