An unusual crystal growth method of the chalcohalide semiconductor, β-Hg₃S₂Cl₂: A new candidate for hard radiation detection

We assess the mercury chalcohalide compound, β-Hg₃S₂Cl₂, as a potential semiconductor material for X-ray and γ-ray detection. It has a high density (6.80 g/cm³) and wide band gap (2.56 eV) and crystallizes in the cubic Pm3n space group with a three-dimensional structure comprised of [Hg₁₂S₈] cubes with Cl atoms located within and between the cubes, featuring a trigonal pyramidal SHg₃ as the main building block. First-principle electronic structure calculations at the density functional theory level predict that the compound has closely lying indirect and direct band gaps. We have successfully grown transparent, single crystals of β-Hg₃S₂Cl₂ up to 7 mm diameter and 1 cm long using a new approach by the partial decomposition of the quaternary Hg₃Bi₂S₂Cl₈ compound followed by the formation of β-Hg₃S₂Cl₂ and an impermeable top layer, all happening in situ during vertical Bridgman growth. The decomposition process was optimized by varying peak temperatures and temperature gradients using a 2 mm/h translation rate of the Bridgman technique. Formation of the quaternary Hg₃Bi₂S₂Cl₈ followed by its partial decomposition into β-Hg₃S₂Cl₂ was confirmed by in situ temperature-dependent synchrotron powder diffraction studies. The single crystal samples obtained had resistivity of 10¹⁰ Ω·cm and mobility-lifetime products of electron and hole carriers of 1.4(4) × 10^-4 cm²/V and 7.5(3) × 10^-5 cm²/V, respectively. Further, an appreciable Ag X-ray photoconductivity response was observed showing the potential of β-Hg₃S₂Cl₂ as a hard radiation detector material.