α- and β-A₂Hg₃M₂S₈ (A = K, Rb; M = Ge, Sn): Polar quaternary chalcogenides with strong nonlinear optical response

The closely related phases α- and β-A₂Hg₃M₂S₈ (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 α-allotropic form crystallizes in the orthorhombic space group Aba2 with a = 19.082(2) Å, b = 9.551 (1) Å, c = 8.2871(8) Å for the K₂Hg₃Ge₂S₈ analogue, and a = 19.563(2) Å, b = 9.853(1) Å, c = 8.467(1) Å for the K₂Hg₃Sn₂S₈ analogue. The β-form crystallizes in the monoclinic space group C2 with a = 9.5948(7) Å, b = 8.3608(6) Å, c= 9.6638(7) Å, β = 94.637° 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 α-K₂Hg₃Ge₂S₈ in K₂S₈ flux. These materials are wide gap semiconductors with band gaps at ∼2.40 and ∼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 ∼14 μm. α-K₂Hg₃Ge₂S₈ shows anisotropic thermal expansion coefficients. SHG measurements, performed with a direct phase-matched method, showed very high nonlinear coefficient d_eff for β-K₂Hg₃Ge₂S₈ approaching 20 pm/V. Crystals of K₂Hg₃Ge₂S₈ are robust to air exposure and have a high laser-damage threshold.