The Unexpected Role of Fluorine in the Band Gap Narrowing of Silver Niobium and Tantalum Pyrochlore Oxyfluorides

Mixed anion compounds have attracted growing interest in solid-state chemistry as a way to tailor physical properties. In this work, we synthesized new silver niobium and tantalum pyrochlore oxyfluorides by an ion-exchange reaction from Na₂M₂O₅F₂ (<i>M</i> = Nb or Ta). Instead of a classical Na⁺/Ag⁺ cation exchange, a less conventional dual cation and anion exchange reaction (2 Na⁺ + F^-)/(Ag⁺ + H₂O) takes place. Indeed, chemical and thermal analyses, as well as Rietveld refinement and ¹⁹F NMR, reveal the formation of AgTa₂O₅F·H₂O and Na_0.4Ag_0.8Nb₂O₅F_1.2·0.8H₂O leading to a significant band gap narrowing of approximately 0.4 eV, as determined by diffuse reflectance spectroscopy. DFT calculations show that Ag 4d-O 2p states are located at the edge of the valence band and that the presence of fluorine in the coordination sphere of Ag promotes the hybridization and hence contributes to the band gap narrowing.