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 NaMOF ( = Nb or Ta). Instead of a classical Na/Ag cation exchange, a less conventional dual cation and anion exchange reaction (2 Na + F)/(Ag + HO) takes place.