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

2013, Inorganic Chemistry

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: τ 1-BaRhGe 3 (BaNiSn 3-type) and two novel phases, τ 2-Ba 3 Rh 4 Ge 16 and τ 3-Ba 5 Rh 15 Ge 36-x , both forming in peritectic reactions. The crystal structures of τ 2 and τ 3 have been elucidated from singlecrystal X-ray intensity data and were found to crystallize in unique structure types: Ba 3 Rh 4 Ge 16 is tetragonal (I4/mmm, a = 0.65643(2) nm, c = 2.20367(8) nm, and R F = 0.0273), whereas atoms in Ba 5 Rh 15 Ge 36−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 R F = 0.034). The body-centeredcubic superstructure of binary Ba 8 Ge 43 □ 3 was observed to extend at 800°C to Ba 8 Rh 0.6 Ge 43 □ 2.4 , while the clathrate type I phase, κ I-Ba 8 Rh x Ge 46−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: κ I ⇔ (Ge) + κ IX + τ 2. A very small solubility range is observed at 750°C for the clathrate IX, κ IX-Ba 6 Rh x Ge 25−x (x ∼ 0.16). Density functional theory calculations have been performed to derive the enthalpies of formation and densities of states for various compositions Ba 8 Rh x Ge 46−x (x = 0−6). The physical properties have been investigated for the phases κ I , τ 1 , τ 2 , and τ 3 , documenting a change from thermoelectric (κ I) to superconducting behavior (τ 2). The electrical resistivity of κ I-Ba 8 Rh 1.2 Ge 42.8 □ 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. τ 1-BaRhGe 3 has a typical metallic electrical resistivity. A superconducting transition at T C = 6.5 K was observed for τ 2-Ba 3 Rh 4 Ge 16 , whereas τ 3-Ba 5 Rh 15 Ge 35.75 showed metallic-like behavior down to 4 K.