NbOsSi and TaOsSi - Two new superconducting ternary osmium silicides

The new equiatomic silicides NbOsSi and TaOsSi as well as ZrOsSi, TIrSi (T = Zr, Hf, Nb, Ta) and TPtSi (T = Nb, Ta) were prepared from the elements by arc-melting. These silicides crystallize with the orthorhombic TiNiSi type structure, space group Pnma. Irregularly shaped crystals of ZrOsSi, NbOsSi, TaOsSi, ZrIrSi and HfIrSi were separated from the annealed samples and investigated by single-crystal X-ray diffraction (a = 640.46(7), b = 404.07(5), c = 743.66(8) pm, wR2 = 0.0285, 390 F2 values, 20 variables for ZrOsSi; a = 629.78(6), b = 388.72(4), c = 727.48(7) pm, wR2 = 0.0350, 397 F2 values, 20 variables for NbOsSi, a = 626.80(6), b = 389.36(4), c = 726.22(7) pm, wR2 = 0.0501, 385 F2 values, 20 variables for TaOsSi, a = 653.48(8), b = 395.35(4), c = 739.19(8) pm, wR2 = 0.0427, 413 F2 values, 20 variables for ZrIrSi and a = 646.34(12), b = 393.57(7), c = 736.8(14) pm, wR2 = 0.0582, 371 F2 values, 20 variables for HfIrSi). The striking structural motifs in the new osmium compounds are three-dimensional [OsSi] networks (Os-Si: 240-251 pm) in which the osmium atoms have strongly distorted tetrahedral silicon coordination. High-pressure/high-temperature experiments (9.5 GPa/1520 K) on TaOsSi gave no hint for a structural phase transition. Temperature dependent measurements of the magnetic susceptibility and the electrical conductivity of NbOsSi and TaOsSi showed superconductivity below TC = 3.5 and 5.5 K, respectively. 29Si solid state MAS NMR investigations of the prepared silicides approved the structural models and showed a correlation between the observed 29Si resonance shifts and the electronegativity of the involved refractory metal.