Effect of Ag Addition on Microstructure and Raman Vibrational Modes of Bulk FeSe

We have synthesized polycrystalline bulk FeSe materials with various Ag concentrations through double-sintering solid-state reaction. All the samples exhibited similar onset critical temperature of around 8 K. Atomic force microscopic observation revealed that the grain connectivity was improved through Ag addition. Raman spectroscopic studies confirmed that pristine and Ag-doped FeSe compounds have superconducting tetragonal structure with high crystalline quality. There was a clear red shift and peak broadening in Fe-related Raman modes when the Ag concentration is increased, which might be ascribed to the incorporation of Ag+ ions into the FeSe lattice. The critical current density decreased with Ag addition, although onset critical temperature remained constant.