  In the ternary system Sr-Cu-Ge, a clathrate type-I phase, Sr8Cu5.3Ge40.7 (a =
1.06311(3), exists close to the Zintl limit in a small temperature interval.
Sr8Cu5.3Ge40.7 decomposes eutectoidally on cooling at 730{\deg}C into (Ge),
SrGe2 and tau1-SrCu2-xGe2+x. Phase equilibria at 700{\deg}C have been
established for the Ge rich part and are characterized by the appearance of
only one ternary compound, tau1-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-xSrxCuyGe46-y (5.2 < y < 5.4) has been studied at various
temperatures. The clathrate type-I crystal structure (space group ) has been
proven by X-ray single crystal diffraction on two single crystals with
composition Sr8Cu5.3Ge40.7 (a = 1.06368(2) nm) and Ba4.9Sr3.1Cu5.3Ge40.7 (a =
1.06748(2) nm) measured at 300, 200 and 100 K. From the temperature dependency
of the lattice parameters and the atomic displacement parameters, the thermal
expansion coefficients, the 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 and the Debye temperature have
been extracted, whereas from a detailed analysis of these data at higher
temperatures, Einstein branches of the phonon dispersion relation have been
derived and compared with those obtained from the atomic displacement
parameters. Electrical resistivity measurements of Sr8Cu5.3Ge40.7 reveal a
rather metallic behaviour in the low temperature range (< 300 K).
