  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 $x \ge 0.5$.
