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 <i>Ibam</i> spacegroup with <i>a</i> = 6.278(1) Å, <i>b</i> = 11.601(2) Å, and <i>c</i> = 14.431(3)­Å; <i>d</i>_calc = 6.29 g/cm³. Its crystal structure consists of straight chains of [Hg₃S₄]_<i>n</i>^2<i>n</i>– 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 <i>a</i>, <i>b</i>, and <i>c</i> 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 (μτ) 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 <i>E</i>_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 Γ. 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