Evolution of structure, magnetic and transport properties of Fe$_{1-x}$Mn$_{x}$Se

The present paper seeks to investigate effect of Mn doping in superconducting FeSe. It is found that over the entire doping range in Fe$_{1-x}$Mn$_x$Se ($0 \le x \le 1$), Mn does not substitute Fe in the superconducting tetragonal phase. Instead two impurity phases, NiAs type hexagonal phase and NaCl type cubic phase grow with increasing Mn content. Initially, hexagonal phase has a higher content than the cubic phase but beyond $x$ = 0.5, the cubic phase grows rapidly and for $x \ge 0.8$, the sample is monophasic with cubic NaCl type structure. The superconducting tetragonal phase content steadily decreases with increasing Mn concentration and completely disappears beyond $x = 0.5$. The premise that Mn never replaces Fe in the superconducting phase is further strengthened by observation of a sharp drop in AC susceptibility akin to superconducting transition at the T$_c$ of FeSe up to $x$ = 0.5. EXAFS studies at the Fe K edge also show that the Fe has a four coordinated tetragonal local structure in all compositions below $x = 0.5$, similar to that in FeSe and it gradually changes to a six coordinated one as is expected for a NaCl type cubic phase for $x \ge 0.5$.