Synthesis and Superconductivity of a Strontium Digermanide SrGe_2-δ with ThSi₂ Structure

We have succeeded in crystallizing a new strontium digermanide (SrGe_2-δ) with the ThSi₂-type structure (tetragonal SrGe₂), which is theoretically predicted to compete with the EuGe₂-type one (trigonal SrGe₂) under pressure. The tetragonal SrGe₂ appeared as a metastable phase in samples at approximately 900 °C under a pressure of 2 GPa. X-ray diffraction studies show that the tetragonal SrGe₂ is formed by the reaction between trigonal SrGe₂ and excess Sr. The composition of the tetragonal SrGe₂ was analyzed to be SrGe_1.66(4). Lattice parameters for the tetragonal SrGe₂ are determined to be a = 4.559(4) Å and c = 14.42(1) Å. The tetragonal SrGe₂ shows metallic resistivity behavior and exhibits superconductivity with a critical temperature (T_c) of 7.3 K, which is the highest among compounds with the ThSi₂-type structure. Superconducting properties of the tetragonal SrGe₂, such as the upper critical field, and the effect of pressure on T_c, are presented and superconductivity is discussed on the basis of electronic band structure calculations.