The structural features and physical properties of the antiferromagnetic K0.8Fe1.6Se2 (so-called K2Fe4Se5 phase) have been studied in the temperature range from 300 K up to 600 K. Resistivity measurements on both single crystal and polycrystalline samples reveal a semiconducting behavior. Structural investigations of K2Fe4Se5 by means of transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) demonstrate the presence of a well-defined superstructure within the a-b plane originating from a Fe-vacancy order along the [1 −10] direction (indexed based on the supercell with space group of I4/m). Moreover, in situ heating structural analysis shows that K0.8Fe1.6Se2 undergoes a transition of the Fe-vacancy order to disorder at about 600 K. The phase separation and the Fe-vacancy ordering in the superconducting materials of KxFe2−ySe2 (0.2 y 0.3) has been briefly discussed.
The structural features of the antiferromagnetic K0.8Fe1.6S2 have been studied in the temperature range from 300 K up to 700 K by means of in situ transmission electron microscopy (TEM). The superstructure with a wave vector originating from a Fe-vacancy order has been clearly observed; moreover, the structural analysis shows that K0.8Fe1.6S2 undergoes a transition from the Fe-vacancy order to disorder at about 585 K. The S substitution effect on the phase separation and superconductivity in the K0.8Fe1.75Se2−ySy materials has been systematically investigated by SEM and TEM structural analyses, as well as by electrical resistivity measurements. Our experimental results reveal that the S element adopts a homogeneous distribution in all investigated materials, and the essential phase-separation nature is very similar to what was observed in the K0.8Fe1.75Se2 superconductor. A phase-separated state formed by the coexistence of two Fe-vacancy orders with wave vectors and in K0.8Fe1.5+xS2 (0 < x < 0.1) has been briefly discussed.
Superconductivity and structural properties of Ti70−xZr30Nbx have been systematically investigated for x ranging from 0 to 60. Superconductivity is observed in the cubic β-phase with 10 x 60. Moreover, evident modifications in superconductivity and superstructure, being interpreted as Nb and Ti local orders, are discovered in samples with 25 x 50. This superstructure phase in general coexists with the cubic β-phase and yields two superconducting transitions in the superconducting materials. Electronic structure calculations reveal a relatively higher density of states for the superstructure phase at the Fermi level, therefore a strong electron-phonon coupling and higher superconducting Tc are expected in agreement with the experimental data.
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