Phase Stability and Superconducting Properties of AlB₂-Type YbGa_xSi_2−x(1.12 ≤x≤ 1.49)

We investigated the phase stability and superconducting properties of YbGaxSi2-x. Polycrystalline samples were synthesized by argon arc melting and subsequent annealing. The composition range of the phase with the AlB2-type structure was determined to be 1.12(1) <= x <= 1.49(3) by powder X-ray diffraction and electron probe microanalysis. The lattice constants, both a and c, increase linearly with x throughout the single-phased region. Electrical resistivity showed metallic behavior for all samples, and the superconducting transition was observed to occur at critical temperatures ranging from T-C = 2.4 K for x = 1.15 to less than 1.8 K for x = 1.41. The extent of the decrease in T-C with x is comparable to that in MGaxSi2-x (M = Ca, Sr, Bit) for x > 1. The negative sign of the Seebeck coefficient measured for x = 1.20 indicates that the dominant carriers are electrons. The temperature dependence of the magnetic susceptibility below room temperature showed almost nonmagnetic behavior suggesting the nearly divalent state of Yb. However, Yb was found to be in the mixed valent state Yb2.3+ based on a precise measurement using X-ray absorption spectroscopy in the partial fluorescence yield mode at the Yb L-III absorption edge. Comparison is made with the nonsuperconducting YbGaGe from the viewpoint of the relation of the crystal structure and superconductivity.