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It has been already shown that the almost ideal single crystal of FeTe_0.65Se_0.35 exhibits a greater width of superconducting transition and a considerably smaller value of the critical current density in comparison with non-uniform sample of the same compound. Resistivity results confirmed that the inhomogeneous spatial distribution of ions and presence of small hexagonal-like phase in chalcogenides with nanoscale phase separation seem to enhance the superconductivity in this system. Under ambient pressure the weakening of superconducting state properties was observed in Fe_0.994Ni_0.007Te_0.66Se_0.34 crystal, with disorder introduced by Ni substitution, as compared with those in Fe_0.99Te_0.66Se_0.34. For Fe_0.994Ni_0.007Te_0.66Se_0.34, the x-ray diffraction studies have revealed a degradation of crystal quality under applied elevated pressure. Comprehensive studies of impact of pressure on crystal structure and on superconducting state properties confirmed that enhancement of superconductivity under pressure correlates with appearance of mosaicity. It was shown that thermal diffusion of hydrogen into the crystals causes significant structural changes, leads to degeneration of crystal quality, and significantly alters superconducting properties, especially enhances the critical current density.

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