The Unexpected Role of Fluorine in the Band Gap Narrowing of Silver Niobium and Tantalum Pyrochlore Oxyfluorides
Celine Moussa (Université de Lille)
Frédérique Pourpoint (Université de Lille, Institut Universitaire de France)
Sébastien Saitzek (Université de Lille)
Theodosios Famprikis (TU Delft - Applied Sciences)
Pascal Roussel (Université de Lille)
Houria Kabbour (Université de Lille, Université de Nantes)
Edouard Boivin (Institut des Molécules et Matériaux Du Mans , Université de Lille)
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Abstract
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 (M = 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 Na0.4Ag0.8Nb2O5F1.2·0.8H2O 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.