Physical properties, crystal and magnetic structure of layered [Fe.sub.1.11] [Te.sub.1-x] [Se.sub.x] superconductors

The physical and structural properties of [Fe.sub.1.11] Te and [Fe.sub.1.11] [Te.sub.0.5] [Se.sub.0.5] have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the [Fe.sub.1.11] Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting [Fe.sub.1.11] [Te.sub.0.5] [Se.sub.0.5] compound, the superconducting transition with [T.sub.c] = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field [H.sub.c2] is obtained by measuring the resistivity under different magnetic fields. The Kim's critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that [Fe.sub.1.11] [Te.sub.0.5] [Se.sub.0.5] crystalizes in tetragonal structure at 300 K as in the parent compound [Fe.sub.1.11] Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.