Thallium mercury chalcobromides, TlHg₆Q₄Br ₅ (Q = S, Se)

The new compounds TlHg₆Q₄Br₅ (Q = S, Se) are reported along with their syntheses, crystal structures, and thermal and optical properties, as well as electronic band structure calculations. Both compounds crystallize in the tetragonal I4/m space group with a = 14.145(1) Å, c = 8.803(1) Å, and d_calc = 7.299 g/cm³ for TlHg₆S₄Br₅ (compound 1) and a = 14.518(2) Å, c = 8.782(1) Å, and d_calc = 7.619 g/cm³ for TlHg₆Se₄Br₅ (compound 2). They consist of cuboid Hg₁₂Q₈ building units interconnected by trigonal pyramids of BrHg₃, forming a three-dimensional structure. The interstitial spaces are filled with thallium and bromide ions. Compounds 1 and 2 melt incongruently and show band gaps of 3.03 and 2.80 eV, respectively, which agree well with the calculated ones. First-principles electronic structure calculations at the density functional theory level reveal that both compounds have indirect band gaps, but there also exist direct transitions at energies similar to the indirect gaps.