Cs₂Hg₃S₄: A low-dimensional direct bandgap semiconductor

Cs₂Hg₃S₄ was synthesized by slowly cooling a melted stoichiometric mixture of Hg and Cs₂S₄. Cs₂Hg₃S₄ crystallizes in the Ibam spacegroup with a = 6.278(1) Å, b = 11.601(2) Å, and c = 14.431(3)Å; d_calc = 6.29 g/cm³. Its crystal structure consists of straight chains of [Hg₃S₄]_n^2n- that engage in side-by-side weak bonding interactions forming layers and are charge balanced by Cs⁺ cations. The thermal stability of this compound was investigated with differential thermal analysis and temperature dependent in situ synchrotron powder diffraction. The thermal expansion coefficients of the a, b, and c axes were assessed at 1.56 × 10^-5, 2.79 × 10^-5, and 3.04 × 10^-5 K^-1, respectively. Large single-crystals up to ∼5 cm in length and ∼1 cm in diameter were grown using a vertical Bridgman method. Electrical conductivity and photoconductivity measurements on naturally cleaved crystals of Cs₂Hg₃S₄ gave resistivity ρ of ≥10⁸ ω·cm and carrier mobility-lifetime (μδ) products of 4.2 × 10^-4 and 5.82 × 10^-5 cm² V^-1 for electrons and holes, respectively. Cs₂Hg₃S₄ is a semiconductor with a bandgap E_g ∼ 2.8 eV and exhibits photoluminescence (PL) at low temperature. Electronic band structure calculations within the density functional theory (DFT) framework employing the nonlocal hybrid functional within Heyd-Scuseria-Ernzerhof (HSE) formalism indicate a direct bandgap of 2.81 eV at. The theoretical calculations show that the conduction band minimum has a highly dispersive and relatively isotropic mercury-based s-orbital-like character while the valence band maximum features a much less dispersive and more anisotropic sulfur orbital-based band.