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.Comment: 6 pages, 6 figure

Suppression of superconductivity by V-doping and possible magnetic order in Sr2VO3FeAs

Superconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal structures and exact stoichiometries are determined by combined neutron- and x-ray powder diffraction. Sr2VO3FeAs is shown to be very sensitive to Fe/V mixing, which interferes with or even suppresses superconductivity. This inhomogeneity may be intrinsic and explains scattered reports regarding Tc and reduced superconducting phase fractions in Sr2VO3FeAs. Neutron diffraction data collected at 4 K indicates incommensurate mag- netic ordering of the V-sublattice with a propagation vector q = (0,0,0.306). This suggests strongly correlated vanadium, which does not contribute significantly to the Fermi surface of Sr2VO3FeAs.Comment: text revised, magnetic q-vector added, one reference added 4 pages, 4 figure

Spin density wave anomaly at 140 K in the ternary iron arsenide BaFe2As2

The ternary iron arsenide BaFe2As2 with the tetragonal ThCr2Si2-type structure exhibits a spin density wave (SDW) anomaly at 140 K, very similar to LaFeAsO, the parent compound of the iron arsenide superconductors. BaFe2As2 is a poor Pauli-paramagnetic metal and undergoes a structural and magnetic phase transition at 140 K, accompanied by strong anomalies in the specific heat, electrical resistance and magnetic susceptibility. In the course of this transition, the space group symmetry changes from tetragonal (I4/mmm) to orthorhombic (Fmmm). 57Fe Moessbauer spectroscopy experiments show a single signal at room temperature and full hyperfine field splitting below the phase transition temperature (5.2 T at 77 K). Our results suggest that BaFe2As2 can serve as a new parent compound for oxygen-free iron arsenide superconductors.Comment: 4 pages, 6 figures, submitted to PR