Fe(Se,Te) from melting routes: The influence of thermal processing on microstructure and superconducting properties

Superconducting properties of the Fe(Se,Te) system are directly related to its microstructure. However, in the framework of a not well assessed phase diagram, a large spread of chemico-physical and functional properties is reported for compounds characterized by similar overall chemical compositions. This is in particular observed in materials produced by melting routes, where chemical inhomogeneities and multiple phases are commonly obtained. Both composition and morphology of these inhomogeneities seem to play a role in the behaviour of the superconducting phases, pointing out the need to assess the dependence of phase composition and therefore of the material performance on the sample thermal history. In this work, the effect of different post-synthesis annealing treatments is studied. A melting route has been adopted to synthesize polycrystalline samples, subsequently subjected to thermal treatments at different temperatures. Morphology and microstructure were characterized by means of x-ray diffraction and microscopy techniques. The superconducting properties have been evaluated by means of electrical and magnetic measurements. It is observed how microstructure, chemical composition and morphology of the obtained products strictly depend on the annealing temperature. Moreover, a crucial role of the cooling step is evidenced: in fact, a rapid cooling procedure from high temperature leads to the formation of high temperature (Tconset > 19 K) superconducting phases, but the overall bulk superconducting performance is negatively affected. Other annealing treatments can promote instead an enhancement in the critical current at high fields with respect to pristine sample, suggesting that a careful engineering of a post-synthesis thermal step can represent a useful tool to optimize the material properties