Real and Hypothetical Intermediate-Valence AgII /AgIII and AgII /AgI Fluoride Systems as Potential Superconductors.

Abstract

With the aim of gauging their potential as conducting or superconducting materials, we examine the crystal structures and magnetic properties of the roughly one hundred binary, ternary, and quaternary AgII and AgIII fluorides in the solid state reported up to date. The AgII cation appears in these species usually in a distorted octahedral environment, either in an [AgF]+ infinite chain or as [AgF2 ] sheets. Sometimes one finds discrete square-planar [AgF4 ]2- ions. The AgIII cation occurs usually in the form of isolated square-planar [AgF4 ]- ions. Systems containing AgIII (d8 ) centers are typically diamagnetic. On the other hand, the rich spectrum of AgII (d9 ) environments in binary and ternary fluorides leads to most diverse magnetic properties, ranging from paramagnetism, through temperature-independent paramagnetism (characteristic for half-filled band and metallic behavior) and antiferromagnetism, to weak ferromagnetism. AgII and AgIII have the same d-electron count as CuII (d9 ) and CuIII (d8 ), respectively. F- and O2- ions are isoelectronic, closed-shell (s2 p6 ) species; both are weak-field ligands. Led by these similarities, and by some experimental evidence, we examine analogies between the superconducting cuprates (CuII /CuIII -O2- and CuII /CuI -O2- systems) and the formally mixed-valence AgII /AgIII -F- and AgII /AgI -F- phases. For this purpose we perform electronic-structure computations for a number of structurally characterized binary and ternary AgI , AgII , and AgIII fluorides and compare the results with similar calculations for oxocuprate superconductors. Electronic levels in the vicinity of the Fermi level (x2 -y2 or z2 ) have usually strongly mixed Ag(d)/F(p) character and are Ag-F antibonding, thus providing the potential of efficient vibronic coupling (typical for d9 systems with substantially covalent bonds). According to our computations this is the result not only of a coincidence in orbital energies; surprisingly the Ag-F bonding is substantially covalent in AgII and AgIII fluorides. The electron density of state at the Fermi level (DOSF ) for silver fluoride materials and frequencies of the metal-ligand stretching modes have values close to those for copper oxides. The above features suggest that properly hole- or electron-doped AgII fluorides might be good BCS-type superconductors. We analyze a comproportionation/disproportionation equilibrium in the hole-doped AgII fluorides, and the possible appearance of holes in the F(p) band. It seems that there is a chance of generating an AgIII -F- /AgII -F0 "ionic/covalent" curve crossing in the hole-doped AgII -F- fluorides, significantly increasing vibronic coupling.