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

Celine Moussa; Frédérique Pourpoint; Sébastien Saitzek; Theodosios Famprikis; Pascal Roussel; Houria Kabbour; Edouard Boivin

doi:10.1021/acs.inorgchem.4c05119

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

Celine Moussa, Frédérique Pourpoint, Sébastien Saitzek, Theodosios Famprikis, Pascal Roussel, Houria Kabbour, Edouard Boivin^*

^*Corresponding author for this work

RST/Storage of Electrochemical Energy

Research output: Contribution to journal › Article › Scientific › peer-review

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 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.

Original language	English
Pages (from-to)	3898-3907
Number of pages	10
Journal	Inorganic chemistry
Volume	64
Issue number	8
DOIs	https://doi.org/10.1021/acs.inorgchem.4c05119
Publication status	Published - 2025

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Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care
Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.

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

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.",

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AU - Pourpoint, Frédérique

AU - Saitzek, Sébastien

AU - Famprikis, Theodosios

AU - Roussel, Pascal

AU - Kabbour, Houria

AU - Boivin, Edouard

N1 - Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.

PY - 2025

Y1 - 2025

N2 - 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.

AB - 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.

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

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