Synthesis, Crystal Structure, and Transport in Ordered Vacancy Compound Hg₂SiTe₄

Hg₂SiTe₄ is found to be stable at room temperature and pressure and crystallizes in the I4̅ space group. The tetragonal compound (a = 6.04681(3) Å, c = 12.01208(9) Å, Z = 2) exhibits p-type electronic conduction and ultralow thermal conductivity (κ_total < 1 W/m K at 50 °C). The crystal structure is determined via charge flipping using TOPAS software to analyze X-ray diffraction data on powder samples. The compound is synthesized from elemental precursors and requires densification before annealing to form the ternary compound, unlike the similar compound Hg₂GeTe₄. Several synthetic procedures are tested to determine the best method to form the novel compound. Measured thermal and electronic property data are discussed along with first-principles defect calculations to inform future doping or optimization studies. Defect calculations suggest that ${\mathrm{Si}}_{\mathrm{Hg}}^{2+}$, $V_{\mathrm{Hg}}^{2-}$, and ${\mathrm{Hg}}_{\mathrm{Si}}^{2-}$ native antisite defects may pin the Fermi level deep within the bandgap. Throughout, we compare the crystallographic and electronic properties of Hg₂SiTe₄ to those of the similar, previously discovered compound Hg₂GeTe₄.