Variation of FeAs-superconductors through replacement of As by Si or Ge and Fe by Co or Ni

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Recently, superconducting iron pnictides have been studied intensively with respect to relationships between their structures, magnetism, composition and superconducting properties. Whereas the hole or electron doping is a very important strategy to induce superconductivity in these compounds, no attempts of direct charge carrier doping by substitution at the As sites in the FeAs layer have been made. Therefore, we plan to study whether doping with p-element such as Si or Ge at the As site can induce superconductivity in FeAs-based compounds as well as the influence of such substitutions on the structure stability and on magnetic ordering. Besides, during simultaneous Fe/Co(Ni) and As/Si(Ge) substitution the formation of new compounds with isolated FeAs and Ni(Co)Ge(Si) layers can be expected. Different methods will be applied for the preparation of the desired phases, e. g. based on solid-state reactions at high temperatures in closed containers. Obtaining highquality large single crystals for further investigations will be an important task of our work. The chances to discover new interesting materials in these systems are expected to be very high.