Quest for Superconductivity in Compounds of Divalent Silver: 8 Years after the Prediction

Fluorine is the most electronegative among all chemical bond–forming elements. Because of this, the vast majority of binary and higher inorganic fluorides are high–melting large–band gap electronic insulators, which are transparent in the visible region of the electromagnetic spectrum. Rare examples of metallic fluorides are known, but valence orbitals of F marginally participate in the electronic transport in these compounds. In this account we describe recent theory–driven attempts to turn a family of TM fluorides into a novel class of high–temperature superconductors. Substantial mixing of F’s 2p orbitals and metal valence functions (‘covalence’) as well as partial band occupation are needed to generate band crossing judged responsible for unusually strong vibronic coupling and for the appearance of superconductivity. This precondition is satisfied for unusual fluorides of divalent silver – fluoroargentates (II). These fascinating materials share lots of common features with oxides of Cu(I). Unfortunately, in all known layered fluorides of silver (II) the unpaired d electrons of silver order ferromagnetically. This is in contrast to antiferromagnetic ordering observed for undoped oxocuprates. In this account attempts will be described of crystal-engineering of fluoroargentates (II) which target a layered antiferromagnetic precursor of a superconductor.