Phase segregation and alteration in superconducting properties caused by substitution of palladium for iron in Fe$_{1.02}$Se$_{0.5}$Te$_{0.5}$

A series of polycrystalline samples with nominal compositions Fe$_{1.02}$Se$_{0.5}$Te$_{0.5}$ ($x$ = 0, 0.02, 0.05, 0.1, 0.15, 0.2) has been synthesized and studied by means of the X-ray diffraction, scanning electron microscopy, electrical resistivity and magnetization measurements. An increase in the Pd content above $x$ = 0.02 leads to the phase segregation and the appearance of a non-superconducting tetragonal FeSe-like phase and a PdTe-type hexagonal phase along with the superconducting FeTe-like phase. The unit cell volume of the FeTe-like and FeSe-like phases is observed to increase and decrease, respectively, when the Pd content increases. Such a behavior is ascribed to the redistribution of selenium and tellurium between coexisting phases. It has been shown that the revealed changes in the composition and volume fraction of the superconducting FeTe-like phase with increasing Pd content are the main reason for the decrease in the critical temperature and critical current density in Fe$_{1.02}$Se$_{0.5}$Te$_{0.5}$.