Effects of compensating doping or chemical pressure in 122-type superconductors and structure-property relations of superconducting platinum-iron arsenides with layered structures

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In this project we are studying subtle lattice effects and the crystal chemistry of iron arsenide superconductors. We expect new insights in the interplay of electronic and lattice degrees of freedom as well as a widening of the material basis of iron arsenides. These recently discovered materials represent a new class of high-Tc superconductors after the cuprates. Superconductivity takes place in FeAs layers, separated by spacers acting as charge reservoirs. The current material basis consists of three well known structures types with similar FeAs layers, but different separating layers. It is generally accepted, that superconductivity is unconventional in iron arsenides, but the role of the lattice for the pairing mechanism is far from being clear. Following a first hint at lattice anomalies in LaFeAsO close to Tc, we will search for deviations from the conventional three-dimensional crystal structure by using high resolution single crystal X-ray diffraction. By exploiting the full capacity of this method, we will obtain detailed information about anharmonic atomic displacements, modulations, or superlattices. We will also map the experimental electron density of CaFe2As2. The second branch of the project is devoted to increase the Tc of these materials by increasing their anisotropy. We will explore and characterize FeAs compounds with big separating blocks like Sr2VO3FeAs (Tc = 37 K) and also search for multilayer-structures containing non-equivalent FeAs layers. The materials will be studied by X-ray and neutron diffraction, magnetic measurements and Mössbauer spectroscopy.