Clathrate formation in the systems Sr–Cu–Ge and {Ba,Sr}–Cu–Ge

Abstract

In the ternary system Sr–Cu–Ge, a novel clathrate type-I phase was detected, Sr8CuxGe46−x (5.2≤x<5.4), which exists close to the Zintl limit in a small temperature interval. Sr8Cu5.3Ge40.7 decomposes eutectoidally on cooling at 730±3°C into (Ge), SrGe2 and τ1-SrCu2−xGe2+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-SrCu2−xGe2+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 Ba8−ySryCuxGe46−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): Sr8Cu5.36Ge40.64 (a=1.06368(2) nm at 300K) and Ba4.86Sr3.14Cu5.36Ge40.64 (a=1.06748(2) nm at 300K) measured at 300, 200 and 100K. 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 Sr8Cu5.3Ge40.7 at low temperatures the Sommerfeld coefficient (γ=24mJ/molK2) and the Debye temperature (ΘDLT=273K) 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 Sr8Cu5.3Ge40.7 reveal a rather metallic behavior in the low temperature range (<300K).Density function theory calculations provide densities of states, electronic resistivity and Seebeck coefficient as well as the vibrational spectrum and specific heat.