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

Abstract

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.

Preparation, Crystal Structure and Physical Properties of the Superconducting Cage Compound Ba3Ge16Ir4

“…From both ratios we conclude that Ba 3 Ge 16 Rh 4 is a superconductor with moderately strong e-ph coupling. This is in contrast to the results of Falmbigl et al, [14] where a value Δc p /γ N T c ≈ 1.38 is reported indicating weak e-ph coupling. For Ba 3 Ge 16 Ir 4 , due to the lower T c and the smaller Sommerfeld coefficient γ N , the electronic specific heat data are less accurate than for the Rh compound.…”

“…The structure is described by three kinds of fundamental polyhedral building blocks, the centers of which arrange in www.zaac.wiley-vch.de the BaAl 4 motif ( Figure 3). As described previously, [14] an eye-catching feature of the structure type is the condensation of elongated rhombic dodecahedra to layers perpendicular to the c axis, which is also reminiscent of the BaAl 4 type. [16] The dodecahedra (4 8 squares resemble the characteristic building unit of skutterudites BaT 4 Ge 12 .…”

“…Samples of the isotypic compound Ba 3 Ge 16 Rh 4 were prepared analogously, yet with annealing at 700°C for 14 d. Products obtained this way contained small amounts of elemental Ge and traces of an unidentified phase. Previously, Ba 3 Ge 16 Rh 4 was obtained at 770°C [14] . …”

“…Ba 3 Ge 16 Ir 4 crystallizes isotypically to Ba 3 Ge 16 Rh 4 ; [14] the atomic parameters were refined from single-crystal X-ray diffraction data (Table 1 and Table 2). The atomic arrangement in Ba 3 Ge 16 Rh 4 represents a hierarchical derivative of BaAl 4 type lattice.…”

“…During preparation of this manuscript, the crystal structure of Ba 3 Ge 16 Rh 4 and its superconducting properties were reported. [14] Moreover, the existence of the isotypic Ba 3 Ge 16 Ir 4 was indicated by X-ray powder diffraction. [15] Experimental Section Synthesis: Sample preparation was performed in an argon atmosphere.…”