Cage-Forming Compounds in the Ba–Rh–Ge System: From Thermoelectrics to Superconductivity

Phase relations and solidification behavior in the Ge-rich part of the phase diagram have been determined in two isothermal sections at 700 and 750 °C and in a liquidus projection. A reaction scheme has been derived in the form of a Schulz–Scheil diagram. Phase equilibria are characterized by three ternary compounds: τ1-BaRhGe3 (BaNiSn3-type) and two novel phases, τ2-Ba3Rh4Ge16 and τ3-Ba5Rh15Ge36‑x, both forming in peritectic reactions. The crystal structures of τ2 and τ3 have been elucidated from single-crystal X-ray intensity data and were found to crystallize in unique structure types: Ba3Rh4Ge16 is tetragonal (I4/mmm, a = 0.65643(2) nm, c = 2.20367(8) nm, and RF = 0.0273), whereas atoms in Ba5Rh15Ge36–x (x = 0.25) arrange in a large orthorhombic unit cell (Fddd, a = 0.84570(2) nm, b = 1.4725(2) nm, c = 6.644(3) nm, and RF = 0.034). The body-centered-cubic superstructure of binary Ba8Ge43□3 was observed to extend at 800 °C to Ba8Rh0.6Ge43□2.4, while the clathrate type I phase, κI-Ba8RhxGe46–x–y□y, reveals a maximum solubility of x = 1.2 Rh atoms in the structure at a vacancy level of y = 2.0. The cubic lattice parameter increases with increasing Rh content. Clathrate I decomposes eutectoidally at 740 °C: κI ⇔ (Ge) + κIX + τ2. A very small solubility range is observed at 750 °C for the clathrate IX, κIX-Ba6RhxGe25–x (x ∼ 0.16). Density functional theory calculations have been performed to derive the enthalpies of formation and densities of states for various compositions Ba8RhxGe46–x (x = 0–6). The physical properties have been investigated for the phases κI, τ1, τ2, and τ3, documenting a change from thermoelectric (κI) to superconducting behavior (τ2). The electrical resistivity of κI-Ba8Rh1.2Ge42.8□2.0 increases almost linearly with the temperature from room temperature to 730 K, and the Seebeck coefficient is negative throughout the same temperature range. τ1-BaRhGe3 has a typical metallic electrical resistivity. A superconducting transition at TC = 6.5 K was observed for τ2-Ba3Rh4Ge16, whereas τ3-Ba5Rh15Ge35.75 showed metallic-like behavior down to 4 K.