Nonstoichiometry and bond character in unconventional superconductors

Abstract

The presence of mixed valency in unconventional superconductors is a necessary consequence of the fact that there is chemical equilibrium between the atoms of a transition element having different oxidation states. Consequently, phases described by stoichiometric formulas such as FeSe, Ba(FeAs) 2 , La 2 CuO 4 etc. deviate in fact from their nominal compositions. These deviations can be considered as doping by replacing some of the cationic atoms Fe II and Cu II in the conducting units by small amounts of Fe and Cu in higher, sometimes in lower oxidation state. From a consideration of the electronegativity values of the constituents of a selection of widely different unconventional superconductors and corresponding parent-compounds, it is evident that bonds between atoms of the conducting units have higher covalency than bonds between conducting units and charge reservoirs which are mixed ionic/covalent, and bonds within the charge reservoirs which are strongly ionic. In compounds without charge reservoirs, the bonds between neighbouring conducting units have van der Waals character.