Cs₂Hg₃S₄: A Low-Dimensional Direct Bandgap Semiconductor

Cs 2 Hg 3 S 4 was synthesized by slowly cooling a melted stoichiometric mixture of Hg and Cs 2 S 4 . Cs 2 Hg 3 S 4 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/cm 3 . Its crystal structure consists of straight chains of [Hg 3 S 4 ] n 2 n – 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 Cs 2 Hg 3 S 4 gave resistivity ρ of ≥10 8 Ω·cm and carrier mobility-lifetime (μτ) products of 4.2 × 10 –4 and 5.82 × 10 –5 cm 2 V –1 for electrons and holes, respectively. Cs 2 Hg 3 S 4 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 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.