New Monoclinic Phase at the Composition Cu₂SnSe₃ and Its Thermoelectric Properties

A new monoclinic phase (<i>m2</i>) of ternary diamond-like compound Cu₂SnSe₃ was synthesized by reaction of the elements at 850 K. The crystal structure of <i>m2</i>-Cu₂SnSe₃ was determined through electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data (space group <i>Cc</i>, <i>a</i> = 6.9714(2) Å, <i>b</i> = 12.0787(5) Å, <i>c</i> = 13.3935(5) Å, β = 99.865(5)°, <i>Z</i> = 8). Thermal analysis and annealing experiments suggest that <i>m2</i>-Cu₂SnSe₃ is a low-temperature phase, while the high-temperature phase has a cubic crystal structure. According to quantum chemical calculations, <i>m2</i>-Cu₂SnSe₃ is a narrow-gap semiconductor. A study of the chemical bonding, applying the electron localizability approach, reveals covalent polar Cu–Se and Sn–Se interactions in the crystal structure. Thermoelectric properties were measured on a specimen consolidated using spark plasma sintering (SPS), confirming the semiconducting character. The thermoelectric figure of merit <i>ZT</i> reaches a maximum value of 0.33 at 650 K

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity

BaGe₆ and BaGe_6‑x: Incommensurately Ordered Vacancies as Electron Traps

We report the high-pressure high-temperature synthesis of the germanium-based framework compounds BaGe₆ (<i>P</i> = 15 GPa, <i>T</i> = 1073 K) and BaGe_6–<i>x</i> (<i>P</i> = 10 GPa, <i>T</i> = 1073 K) which are metastable at ambient conditions. In BaGe_6‑<i>x</i>, partial fragmentation of the BaGe₆ network involves incommensurate modulations of both atomic positions and site occupancy. Bonding analysis in direct space reveals that the defect formation in BaGe_6–<i>x</i> is associated with the establishment of free electron pairs around the defects. In accordance with the electron precise composition of BaGe_6‑<i>x</i> for <i>x</i> = 0.5, physical measurements evidence semiconducting electron transport properties which are combined with low thermal conductivity

Intermediate-Valence Ytterbium Compound Yb₄Ga₂₄Pt₉: Synthesis, Crystal Structure, and Physical Properties

The title compound was synthesized by a reaction of the elemental educts in a corundum crucible at 1200 °C under an Ar atmosphere. The excess of Ga used in the initial mixture served as a flux for the subsequent crystal growth at 600 °C. The crystal structure of Yb₄Ga₂₄Pt₉ was determined from single-crystal X-ray diffraction data: new prototype of crystal structure, space group <i>C</i>2<i>/m</i>, Pearson symbol <i>mS</i>74, <i>a</i> = 7.4809(1) Å, <i>b</i> = 12.9546(2) Å, <i>c</i> = 13.2479(2) Å, β = 100.879(1)°, <i>V</i> = 1260.82(6) ų, <i>R</i>_<i>F</i> = 0.039 for 1781 observed reflections and 107 variable parameters. The structure is described as an <i>ABABB</i> stacking of two slabs with trigonal symmetry and compositions Yb₄Ga₆ (<i>A</i>) and Ga₁₂Pt₆ (<i>B</i>). The hard X-ray photoelectron spectrum (HAXPES) of Yb₄Ga₂₄Pt₉ shows both Yb²⁺ and Yb³⁺ contributions as evidence of an intermediate valence state of ytterbium. The evaluated Yb valence of ∼2.5 is in good agreement with the results obtained from the magnetic susceptibility measurements. The compound is a bad metallic conductor