Ba4Ge2Sb2Te8Se2 and Ba2Ge2Te4.5Se0.5: complex heteroanionic Ge-containing phases with extremely low thermal conductivities

Abstract

Herein, we report the structural aspects of two novel crystals, Ba₄Ge₂Sb₂Te₈Se₂ and Ba₂Ge₂Te_4.5Se_0.5, synthesized by high-temperature reactions of elements. Single-crystal X-ray diffraction studies of these crystals at room temperature (RT) revealed that the Ba₄Ge₂Sb₂Te₈Se₂ phase crystallizes in the centrosymmetric monoclinic P2₁/c space group in contrast to the Ba₂Ge₂Te_4.5Se_0.5 phase, which adopts the non-centrosymmetric orthorhombic Pna2₁ space group. The asymmetric unit of the Ba₄Ge₂Sb₂Te₈Se₂ structure features eighteen unique sites: four Ba, two Ge, two Sb, five Te, and five mixed Te/Se. Here, the Ge atoms in the structure form a distorted tetrahedral geometry, while the Sb atoms exhibit a seesaw configuration by bonding with the Te/Se atoms. These polyhedra are shared to form one-dimensional ¹_∞[Ge₂Sb₂Te₈Se₂]⁸⁻ strips separated by the Ba²⁺ cations. The Ba₂Ge₂Te_4.5Se_0.5 structure is built up by filling nine independent sites (two Ba, two Ge, four Te, and one mixed Te/Se), resulting in the formation of ¹_∞[Ge₂Te_4.5Se_0.5]⁴⁻ chains that feature Ge–Ge bonding. Monophasic polycrystalline phases with the loaded compositions of Ba₄Ge₂Sb₂Te₈Se₂ and Ba₂Ge₂Te_4.5Se_0.5 were prepared by heating the elements at high temperatures. No bandgap transition was observed for both title phases down to 0.52 eV from the optical absorption studies at RT. The p-type semimetallic or degenerate semiconducting nature of the Ba₄Ge₂Sb₂Te₈Se₂ and Ba₂Ge₂Te_4.5Se_0.5 samples was established by simultaneous measurements of electrical resistivity and the Seebeck coefficient values above RT. A semimetal-to-insulator transition below 20 K was found from the low-temperature resistivity study of the Ba₂Ge₂Te_4.5Se_0.5 phase. Thermal transport measurements showed ultralow thermal conductivity values (≤0.35 W m⁻¹ K⁻¹) at 673 K for the polycrystalline phases.