Tl ₂ Hg ₃ Q ₄ (Q = S, Se, and Te): High-density, wide-band-gap semiconductors

We present the synthesis, crystal structures, and physical properties of Tl ₂ Hg ₃ Q ₄ (Q = S, Se, and Te). The incongruently melting Tl ₂ Hg ₃ Q ₄ crystals were grown in a Tl _x Q flux. These compounds are isostructural and crystallize in a monoclinic cell with a layered structure, adopting the space group C2/c with a = 11.493(2) Å, b = 6.6953(13) Å, c = 12.937(3) Å, β = 114.98(3)° for Tl ₂ Hg ₃ S ₄ , a = 11.977(2) Å, b = 6.9264(14) Å, c = 13.203(3) Å, β = 116.36(3)° for Tl ₂ Hg ₃ Se ₄ and a = 12.648(3) Å, b = 7.3574(15) Å, c = 13.701(3) Å, β = 117.48(3)° for Tl ₂ Hg ₃ Te ₄ . The structures feature infinite chains of [Hg ₃ Q ₄ ] ^2- , which are linked into layers by charge balancing Tl atoms. The compounds have very high densities (>8.3 g/cm ³ ) with experimentally determined band gaps of 2.05, 1.57, and 0.90 eV for Q = S, Se, and Te, respectively. Using the refined crystal structures, we performed detailed band structure calculations at the density functional theory (DFT) level, using the screened-exchange local density approximation (sx-LDA). The results indicate that the compounds are semiconductors with the sulfur analog, having an indirect band gap, and the selenium and tellurium analogs, having direct energy band gaps. There is strong Hg 6s and Tl 6p orbital character in the conduction band minimum, while the valence band maximum has predominantly chalcogen p state character mixed in with a Tl 6s contribution. The band structure calculations support the experimental observation of a narrowing of the band gap in the series Q = S, Se, and Te, which results from the increasing extension of the outermost chalcogen p orbitals.