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Non-stoichiometry and the magnetic structure of Sr2CrO3FeAs

The iron arsenide Sr2CrO3FeAs with the tetragonal Sr2GaO3CuS-type structure was synthesized and its crystal structure re-determined by neutron powder diffraction. In contrast to previous X-ray crystallographic studies, a mixed occupancy of chromium and iron was found within the FeAs4/4 layer (93+/-1% Fe : 7+/-1% Cr). We suggest that the partial Cr-doping at the Fe site is the reason for the absence of a spin-density wave anomaly and superconductivity in this compound. Additional experiments via neutron polarization analysis revealed short-range spin correlations below ~100 K and long-range antiferromagnetic ordering below T_N = 36 K with a magnetic propagation vector of q = (1/2, 1/2, 0). The Cr3+ ions form a collinear magnetic structure of the C-type in the magnetic space group C_Pmma' (a' = a-b, b'=a+b, c'=c), where Cr3+-ions occupy the 4g (0, 1/4, z) Wyckoff position. The magnetic moments are aligned along the orthorhombic a'-axis. At 3.5 K, an ordered magnetic moment of 2.75+/-0.05 mu_B for the Cr3+-sublattice was refined.