Germanium Antimony Bonding in Ba4Ge2Sb2Te10 with Low Thermal Conductivity

Ba14Si4Sb8Te32(Te3): hypervalent Te in a new structure type with low thermal conductivity

Heavier pnictogen (Sb, Bi) containing chalcogenides are well known for their complex structures and semiconducting properties for numerous applications, particularly thermoelectric materials. Here, we report the syntheses of single crystals and polycrystalline phases of a new complex quaternary polytelluride, Ba14Si4Sb8Te32(Te3), via a high-temperature reaction of elements. A single-crystal X-ray diffraction study showed that it crystallizes in an unprecedented structure type with monoclinic symmetry (space group: P21/c). The crystal structure of Ba14Si4Sb8Te32(Te3) consists of one-dimensional ∞1[Si4Sb8Te32(Te3)]28- stripes, which are separated by the Ba2+ cations. Its complex structure features linear polytelluride units of Te34- having intermediate Te⋯Te interactions. A polycrystalline Ba14Si4Sb8Te32(Te3) sample shows a direct narrow bandgap of 0.8(2) eV, which indicates its semiconducting nature. The electrical resistivity of a sintered pellet of the polycrystalline sample exponentially decreases from ∼39.3 Ωcm to ∼0.57 Ωcm on heating it from 323 K to 773 K, confirming the sample's semiconducting nature. The sign of Seebeck coefficient values is positive in the 323 K to 773 K range confirming the p-type nature of the sintered sample. Interestingly, the sample attains an extremely low thermal conductivity of ∼0.32 Wm-1K-1 at 773 K, which could be attributed to the lattice anharmonicity caused by the lone pair effect of Sb3+ species in its complex pseudo-one-dimensional crystal structure. The electronic band structure of the title phase and the strength of chemical bonding of pertinent atomic pairs have been evaluated theoretically using the DFT method.

Ba4FeAgS6: a new antiferromagnetic and semiconducting quaternary sulfide

The single crystals of a quaternary sulfide, Ba₄FeAgS₆, have been synthesized by reacting elements at 873 K inside a sealed fused silica tube. The title phase is the first ordered quaternary compound of the Ba-Ag-Fe-S system. The crystal structure of Ba₄FeAgS₆ is characterized by a single-crystal X-ray diffraction study at 298(2) K. It crystallizes in the space group C52h - P2₁/n of the monoclinic crystal system with unit cell dimensions of a = 8.6367(5) Å, b = 12.0291(7) Å, c = 13.2510(7) Å, and β = 109.015(2)°. This compound is stoichiometric, and its structure contains twelve unique crystallographic sites: four Ba, one Fe, one Ag, and six S sites. All atoms of the structure occupy the general positions. The Ba₄FeAgS₆ structure consists of one-dimensional chains of 1∞[FeAgS₆]^8- that are extended in the [100] direction. The negative charges on these chains are counterbalanced by the filling of Ba²⁺ cations in between the 1∞[FeAgS₆]^8- chains. The Fe atoms are bonded to four S atoms that form a distorted tetrahedral geometry around the central Fe atom. Each Ag atom in this structure is coordinated with four S atoms in a distorted tetrahedral fashion. These FeS₄ and AgS₄ motifs are the main building blocks of the Ba₄FeAgS₆ structure. The corner-sharing of FeS₄ and AgS₄ tetrahedra creates one-dimensional chains of 1∞[FeAgS₆]^8-. This structure does not contain any homoatomic or metallic bonds and can be charge-balanced as (Ba²⁺)₄(Fe³⁺)₁(Ag¹⁺)₁(S^2-)₆. The optical absorption study performed on a polycrystalline Ba₄FeAgS₆ sample reveals a direct bandgap of 1.2(1) eV. The magnetic studies reveal the antiferromagnetic behavior of Ba₄FeAgS₆ below 50 K. The thermal conductivity and theoretical electronic structure of Ba₄FeAgS₆ are also studied in detail.

Five coordinated Mn in Ba4Mn2Si2Te9: synthesis, crystal structure, physical properties, and electronic structure

A new structure type Ba4Mn2Si2Te9 containing unique MnTe5 units is synthesized. The structure comprises two independent Mn atoms, each with 50% occupancy. It is a narrow bandgap semiconductor (Eg = 0.6(1) eV) consistent with the DFT studies.