  The crystal structure, magnetic, electronic, and thermal properties of
Mn-doped SrFe2As2 crystals have been systematically investigated. A miscibility
gap is found in the system from x = 0.4362(4) to x = 0.9612(9). For x <
0.2055(2), the single crystals holding tetragonal structure (space group I4/mmm
(no. 139)) have a continuously enlarged lattice parameter c, followed by a
phase separation with crystals holding both tetragonal and trigonal structures
up to x = 0.4362(4). Beyond the miscibility gap, the crystals with x >
0.9612(9) hold the trigonal structure (space group P-3m1 (no. 164)). Moreover,
the spin density wave ordering for x = 0 to x = 0.0973(1) is suppressed,
followed by an abnormal and broadened increase of the ordering temperature for
x = 0.0973(1) to x = 0.2055(2). Eliminating the possibility of real-space phase
separation of Mn and Fe based on the results of X-ray diffraction, this novel
phenomenon can be attributed to a possible intrinsic Griffiths phase. No any
superconducting signals are observed down to 2 K in the whole composition range
with 0 < x < 1. A phase diagram with multi-critical points of Mn-doped SrFe2As2
system is established accordingly.
