In the present work we report on the synthesis, crystal structure, and physical properties (resistivity, magnetization, heat capacity) in combination with density functional theory (DFT) calculations of the electronic structure and phonon properties for the intermetallic compound LaPtSi. LaPtSi crystallizes in its own noncentrosymmetric structure type (space group I 4 1 md; a = 0.42502(1) nm and c = 1.4525(5) nm), which is an ordered ternary derivative of the centrosymmetric α-ThSi 2 -structure. The weakly correlated compound LaPtSi (Sommerfeld value γ = 6.5 mJ/molK 2 ) exhibits superconductivity below T c = 3.35 K and appears to be a fully gapped, weakly coupled s-wave BCS superconductor. The experimental observations are supported by DFT calculations which show that, despite a substantial spin-orbit splitting of the Fermi surfaces, a spin-singlet pairing is prevalent.
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.
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