The effect of sorption of air and hydrogen components on the structural characteristics of superconducting FeTe0.65Se0.35 single crystals

Abstract

X-ray studies of iron chalcogenide FeTe0.65Se0.35 single crystals were performed after they were exposed in air and hydrogen atmospheres at room and high temperatures, for long periods of time. It is found that impurity components have a significant effect on the structural characteristics of the diffraction pattern for this layered tetragonal superconducting material. It is shown that molecular impurities do not change the matrix symmetry of the tetragonal substitution solutions. However, when hydrogen molecules dissociate close to 200 °C under the thermocatalytic effect of Fe atoms, the tetragonal lattice of the matrix becomes unstable. Symmetry is reduced against the background of an increased concentration of atomic hydrogen and strengthening of the chemical bonds in the crystals. The crystal density sharply increases, and a FeTe0.65Se0.35 + H interstitial solution based on an orthorhombic lattice is formed. It is shown that the structural tetra-ortho phase transition in such solutions occurs near 200 °C.

The Effect of Hydrogenation on Structure and Superconducting Properties of FeTe0.65Se0.35 Single Crystals

“…The transition is accompanied by strong lattice compression at which the crystal density increases by almost 15%. 19 22 In contrast to this expected behavior, we found that after the hydrogenation at temperatures of 180-250 °C, relaxation was practically absent (Fig. 2b).…”

“…Since the volume of the crystal lattice decreases by about 15% after hydrogenation, 19 this leads to large chemical pressure effect. Hence, observed increase in the critical current density can be explained by the appearance of additional pinning centers due to significant mechanical stresses associated with the rearrangement of the crystal lattice after hydrogenation.…”

“…The transition is accompanied by strong lattice compression at which the crystal density increases by almost 15%. 19 The thermally activated creep of individual vortices and their bundles can lead to the redistribution and weakening of bulk superconducting currents. In this case, the integral magnetic moment m(t) may decrease.…”

“…15 Annealing of the Fe-Te-Se compound in an oxygen atmosphere at temperatures of 300-400 °C leads to a strong changes in superconducting properties 16 and subsequent annealing of this compound in hydrogen causes an increase in the steepness of the superconducting transition. 17 Recently performed x-ray structural studies of Fe-Te-Se showed 19 that air molecules change the interlayer interaction in the tetragonal phase of the Fe-Te-Se and come up with a significant effect on the structural parameters of this superconductor. Pronounced structural changes occur under the influence of diffuse penetration of hot hydrogen (at the temperature of about 200 o C) into the sample.…”

“…17 Finally, the chemical combination of FeSe with tetrabutyl ammonium (TBA), each molecule of which contains 36 hydrogen atoms, shifted a critical temperature in (TBA) 0.3 FeSe to the level of 50 K (from T c = 8 K for FeSe), what is currently record-breaking for bulk single crystals of this family of ICSs. 18 Recently performed x-ray structural studies of Fe-Te-Se showed 19 that air molecules change the interlayer interaction in the tetragonal phase of the Fe-Te-Se and come up with a significant effect on the structural parameters of this superconductor. Pronounced structural changes occur under the influence of diffuse penetration of hot hydrogen (at the temperature of about 200 o C) into the sample.…”