Clathrate formation in the systems Sr–Cu–Ge and (Ba,Sr)–Cu–Ge

In the ternary system Sr–Cu–Ge, a novel clathrate type-I phase was detected, Sr₈Cu_xGe_46−x (5.2≤x<5.4), which exists close to the Zintl limit in a small temperature interval. Sr₈Cu_5.3Ge_40.7 decomposes eutectoidally on cooling at 730±3 °C into (Ge), SrGe₂ and τ₁-SrCu_2−xGe_2+x. Phase equilibria at 700 °C have been established for the Ge rich part and are characterized by the appearance of only one ternary compound, τ₁-SrCu_2−xGe_2+x, which crystallizes with the ThCr₂Si₂ structure type and forms a homogeneity range up to x=0.4 (a=0.42850(4), c=1.0370(1) nm). Additionally, the extent of the clathrate type-I solid solution Ba_8−ySr_yCu_xGe_46−x (0≤y≤∼5.6; 5.2≤x≤5.4, from as cast alloys) has been studied at various temperatures. The clathrate type-I crystal structure (space group Pm3^¯n) has been proven by X-ray single crystal diffraction on two single crystals with the composition (from refinement): Sr₈Cu_5.36Ge_40.64 (a=1.06368(2) nm at 300 K) and Ba_4.86Sr_3.14Cu_5.36Ge_40.64 (a=1.06748(2) nm at 300 K) measured at 300, 200 and 100 K. From the temperature dependence of the lattice parameters and the atomic displacement parameters, thermal expansion coefficients, Debye- and Einstein-temperatures and the speed of sound have been determined. From heat capacity measurements of Sr₈Cu_5.3Ge_40.7 at low temperatures the Sommerfeld coefficient (γ=24 mJ/mol K²) and the Debye temperature (Θ_D^LT=273 K) have been extracted. From a detailed analysis of these data at higher temperatures, Einstein branches of the phonon dispersion relation have been derived and compared to those obtained from the atomic displacement parameters. Electrical resistivity measurements of Sr₈Cu_5.3Ge_40.7 reveal a rather metallic behavior in the low temperature range (<300 K). Density function theory calculations provide densities of states, electronic resistivity and Seebeck coefficient as well as the vibrational spectrum and specific heat. - Graphical abstract: Clathrate type-I unit cell including the coordination polyhedra for the Sr (or Ba) atoms. - Highlights: • A new Sr-based clathrate type-I compound Sr₈Cu_5.3Ge_40.7 has been prepared. • The solid solution Ba_8−xSr_xCu_∼5.3Ge_40.7 has been investigated. • Physical properties of Sr₈Cu_5.3Ge_40.7 were measured at low temperatures. • DFT calculations have been performed