  Single crystals of Ba(Fe_(1-x)Mn_x)_2As_2, 0<x<0.148, have been grown and
characterized by structural, magnetic, electrical transport and thermopower
measurements. Although growths of single crystals of Ba(Fe_(1-x)Mn_x)_2As_2 for
the full 0<=x<=1 range were made, we find evidence for phase separation
(associated with some form of immiscibility) starting for x>0.1-0.2. Our
measurements show that whereas the structural/magnetic phase transition found
in pure BaFe_2As_2 at 134 K is initially suppressed by Mn substitution,
superconductivity is not observed at any substitution level. Although the
effect of hydrostatic pressure up to 20 kbar in the parent BaFe_2As_2 compound
is to suppress the structural/magnetic transition at the approximate rate of
0.9 K/kbar, the effects of pressure and Mn substitution in the x=0.102 compound
are not cumulative. Phase diagrams of transition temperature versus
substitution concentration, x, based on electrical transport, magnetization and
thermopower measurements have been constructed and compared to those of the
Ba(Fe_(1-x)TM_x)_2As_2 (TM=Co and Cr) series.
