Non-stoichometry 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 TN=36 K with a magnetic propagation vector of ). The Cr3+-ions form a collinear magnetic structure of the C-type in the magnetic space group CPmma' (a'=a-b, b'=a+b, c'=c), where Cr3+-ions occupy the 4g () 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 μB for the Cr3+-sublattice was refined.