Structural instability and superconductivity in the solid solution SrNi₂(P_1−xGe_x)₂

The solid solution SrNi₂(P_1−xGe_x)₂ with ThCr₂Si₂-type structure has been synthesized over the whole composition range by high-temperature solid state reactions. Structure investigations were performed using X-ray diffraction (XRD) methods. In addition, in situ temperature-dependent powder XRD measurements down to 10 K were carried out for two SrNi₂(P_1−xGe_x)₂ samples with x = 0.110(3) and 0.212(3), which indicate a temperature-induced collapse of the structure of the first compound around 200 K. For both samples, superconductivity was detected above 1.8 K by magnetic, resistivity, and heat capacity measurements (T_C ∼ 3.0 and 2.4 K, respectively). The crystal chemistry and the chemical bonding in the superconducting compounds are discussed in terms of linear muffin-tin orbital (LMTO) band structure calculations including density of states (DOS), crystal orbital Hamilton populations (COHPs), and a topological analysis using the electron localization function (ELF).