Abstract
The effect of the Te for Se substitution in the Fe7Se8 compound having a layered NiAs-type structure has been studied by means of X-ray diffraction, scanning electron microscopy, thermal expansion, electrical resistivity and magnetization measurements. The growth of Te tellurium content in Fe7(Se1−yTey)8 above y ∼ 0.15 is found to result in the phase segregation; besides the NiAs-type phase the superconducting tetragonal PbO-type phase appears in these samples. It has been observed that the crystal structure and composition of the hexagonal Fe7(Se,Te)8 phase (P63/mmc) transforms with increasing Te content to the CdI2-type phase (P3¯m1) and then to the monoclinic Fe3(Se,Te)4-type phase (I12/m1). The concentration intervals in which these NiAs-type phases coexist with the tetragonal phase have been determined. At y ≥ 0.4, the Fe7(Se1−yTey)8 samples show bulk superconductivity with higher superconducting transition temperatures (Tc up to 17.8 K) compared with single-phase samples of Fe(Se,Te). Such an enhancement is suggested to result from the interactions between the superconducting and non-superconducting phases coexisting in Fe7(Se1−yTey)8.
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