Non-stoichometry and the magnetic structure of Sr₂CrO₃FeAs

¹  Department Chemie, Ludwig-Maximilians-Universität München - Butenandtstraße 5-13 (Haus D), 81377 München, Germany, EU
²  Jülich Centre for Neutron Science, IFF, Forschungszentrum Jülich, Outstation at FRM II Lichtenbergstraße 1, D-85747 Garching, Germany, EU
³  Jülich Centre for Neutron Science, Forschungszentrum Jülich, Outstation at Institut Laue-Langevin BP 156, 38042 Grenoble Cedex 9, France, EU
⁴  Institut Laue-Langevin - BP 156, 38042 Grenoble Cedex 9, France, EU

Corresponding author: johrendt@lmu.de

Received: 31 October 2009
Accepted: 19 January 2010

Abstract

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 T_N = 36 K with a magnetic propagation vector of q = (, , 0). The Cr³⁺-ions form a collinear magnetic structure of the C-type in the magnetic space group (a' = a-b, b' = a+b, c' = c), where Cr³⁺-ions occupy the 4g (0, , 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  for the Cr³⁺-sublattice was refined.