Structure, properties and chemistry of layered oxypnictides
Keywords: superconductor
iron
oxypnictide

This thesis reports the synthesis and characterisation of the layered oxypnictides Sr2MO3FeAs (M = Sc, V and Cr) and CeOMnAs. In these materials the choice of transition metal cation at the tetrahedral site in the arsenide layer chiefly dictates the physical properties that are observed.
The bulk of this work has focussed on the development of a new family of iron arsenide superconductor with the general formula Sr2MO3FeAs (M = Sc, V, Cr). This structure is comprised of anti-PbO-type [FeAs]- layers which alternate with insulating [Sr2MO3]+ oxide fragments that resemble a portion of the K2NiF4 structure. In contrast to other FeAs parent materials, no member of the Sr2MO3FeAs family exhibits any strong evidence for long range Fe order or a tetragonal to orthorhombic distortion upon cooling. Attempts to electron and hole dope Sr2ScO3FeAs into the superconducting regime have as yet been unsuccessful.
Although Sr2ScO3FeAs shows no evidence for Fe ordering, a checkerboard arrangement of Cr3+ spins in the ab-plane is observed below 40 K (k = (½, ½, 0)) analogous to that seen in Pr2CuO4. The partial substitution of Fe2+ (d6) by Co2+ (d7) in Sr2CrO3Fe1-xCoxAs has been shown to be a fruitful strategy for electron-doping this material into the superconducting regime with Tc maximised at 18 K in Sr2CrO3Fe0.92Co0.08As. It is also established that this substitution influences the ordering on the Cr sub-lattice with a doubling in the size of the magnetic cell along the c axis (k = (½, ½, ½)).
Sr2VO3FeAs, a rare example of an “undoped” superconductor (Tc = 25 K), is shown to be electron-doped by mixed valence vanadium +3.13(5). Magnetometry measurements also reveal a series of magnetic transitions in Sr2VO3FeAs, however μSR and powder neutron diffraction studies suggest that this system is some way from commensurate long range order.
In contrast to Sr2CrO3FeAs, electron-doping strategies in Sr2VO3FeAs have the effect of decreasing Tc and ultimately suppressing superconductivity entirely as Sr2V1-xTixO3FeAs and Sr2VO3Fe1-xCoxAs materials are over electron-doped. Sr2V1-xMgxO3FeAs samples were also prepared, but rather than this strategy hole-doping the FeAs layer it preferentially oxidises vanadium towards V4+. This substitution also has a considerable effect on the superconducting critical temperature (Tc) which is raised as high as 31 K in Sr2V0.775Mg0.225O3FeAs. The isovalent substitution of Sr2+ by Ca2+ in Sr2-xCaxVO3FeAs has been shown to strongly influence the superconducting properties of this material and a clear correlation between the evolution of Tc and the shape of the FeAs4 tetrahedron has been established. These results demonstrate that superconductivity in iron-based superconductors is extremely sensitive to both electron count and the crystal structure.
Finally, investigations into the manganese oxide arsenide CeOMnAs reveal room temperature ordering of Mn2+ spins and a spin reorientation transition of Mn moments at 36 K. This transition is concomitant with Ce ordering and an apparent weak structural distortion, demonstrating that f electrons are able to dictate the orientation of Mn moments.