Impact of impurity phases and superstoichiometric iron on the critical temperature of iron chalcogenides

The results of transport and magnetic measurements of iron chalcogenide crystals are analyzed taking into account the data of the chemical composition and impurity phases obtained by means of X-ray diffractometry. It has been found that ions of excess (superstoichiometric) iron, as well as impurity phases, produce stresses in the crystal lattice, and this leads to an increase in the superconducting transition temperature. The amount of impurity inclusions increases with time owing to spinodal decomposition, and they also promote the critical temperature rise.