  Neutron and x-ray diffraction studies of Ba(Fe{1-x}Mn{x})2As2 for low doping
concentrations (x <= 0.176) reveal that at a critical concentration, 0.102 < x
< 0.118, the tetragonal-to-orthorhombic transition abruptly disappears whereas
magnetic ordering with a propagation vector of (1/2 1/2 1) persists. Among all
of the iron arsenides this observation is unique to Mn-doping, and unexpected
because all models for "stripe-like" antiferromagnetic order anticipate an
attendant orthorhombic distortion due to magnetoelastic effects. We discuss
these observations and their consequences in terms of previous studies of
Ba(Fe{1-x}TM{x})2As2 compounds (TM = Transition Metal), and models for magnetic
ordering in the iron arsenide compounds.
