Composition dependent polymorphism and superconductivity in Y₃₊{Rh,Ir}₄Ge_13-

Area of Science:

• Solid State Chemistry
• Materials Science
• Condensed Matter Physics

Background:

• The Y₃₊Rh₄Ge_13- and Y₃₊Ir₄Ge_13- series exhibit polymorphism, a phenomenon where a solid material can exist in more than one crystal structure.
• Understanding structural transformations is crucial for predicting and tuning material properties, particularly in intermetallic compounds.
• The Remeika prototype (Yb₃Rh₄Sn₁₃) serves as a reference for the structural variations observed in these Y-based germanides.

Purpose of the Study:

• To investigate the structural polymorphism in the Y₃₊Rh₄Ge_13- and Y₃₊Ir₄Ge_13- series as a function of Yttrium content.
• To explore the superconducting properties and thermal conductivity of these compounds.
• To analyze the relationship between structural changes, electronic structure, and potential phase transitions like charge density waves.

Main Methods:

• X-ray diffraction (XRD) was used to determine the crystal structures and lattice parameters of the synthesized compounds.
• Superconducting transition temperatures (T_c) were measured to characterize their superconducting behavior.
• Thermal conductivity and Seebeck coefficients were measured, and electronic structure calculations were performed.

Main Results:

• Polymorphism was observed, with Y_3.6Rh₄Ge_12.4 adopting a primitive cubic structure (Pm3̄n), while lower Y-content compounds (x=0.4, 0) transitioned to body-centered cubic (I4₁32) and tetragonal (I4₁/amd) structures, respectively.
• Similar structural symmetry lowering was found in the Y₃₊Ir₄Ge_13- series.
• Compounds exhibit weak-coupled BCS-like superconductivity with T_c values up to 1.25 K, low thermal conductivity (<1.5 W K^-1 m^-1), and small Seebeck coefficients. Y₃Rh₄Ge₁₃ shows a first-order phase transition at 177 K, indicative of a charge density wave.

Conclusions:

• The Y-content significantly influences the crystal structure of Y-Rh-Ge and Y-Ir-Ge compounds, leading to a series of related structures derived from the Remeika prototype.
• These materials are weakly-coupled superconductors with properties suitable for exploring low-temperature phenomena.
• The observed phase transition in Y₃Rh₄Ge₁₃ suggests an electronic instability, further supported by electronic structure calculations indicating the instability of idealized structures.