Non-stoichometry and the magnetic structure of Sr 2 CrO 3 FeAs

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 FeAs 4/4 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 q = ( 1 2 , 1 2 , 0).The Cr 3+ -ions form a collinear magnetic structure of the C-type in the magnetic space group CP mma (a = a -b, b = a + b, c = c), where Cr 3+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 µB for the Cr 3+sublattice was refined.This paper is dedicated to Dr Klaus Römer on the occasion of his 70th birthday.