Synthesis and Superconductivity of a Strontium Digermanide SrGe with ThSi Structure.

We have succeeded in crystallizing a new strontium digermanide (SrGe) with the ThSi-type structure (tetragonal SrGe), which is theoretically predicted to compete with the EuGe-type one (trigonal SrGe) under pressure. The tetragonal SrGe 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 SrGe is formed by the reaction between trigonal SrGe and excess Sr. The composition of the tetragonal SrGe was analyzed to be SrGe. Lattice parameters for the tetragonal SrGe are determined to be a = 4.559(4) Å and c = 14.42(1) Å. The tetragonal SrGe shows metallic resistivity behavior and exhibits superconductivity with a critical temperature (T) of 7.3 K, which is the highest among compounds with the ThSi-type structure. Superconducting properties of the tetragonal SrGe, such as the upper critical field, and the effect of pressure on T, are presented and superconductivity is discussed on the basis of electronic band structure calculations.