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Alain DemourguesICMCB-CNRS-University of BordeauxPolyanionic Structures And Competitive Bonds Around Fluorine : Tuning The Band Gap For New Applications In Solid State Chemistry Tressaud International Symposium on Solid State Chemistry for Applications and Sustainable Development Back to Plenary Lectures » |
Abstract:Around various transition metals (Ti and Fe), p-block elements (In) or rare-earths (Ce) associated to at least two different anions (F, O, S), original networks can be designed associated to specific optical and electronic properties and various elemental rules can be established. First, the Ti-based oxy-hydroxy-fluorides with the HTB network were prepared by the solvothermal route and the original structure has been determined. The optical band gap can be tuned in UV range as a function of the O/F atomic ratio, leading to the design of UV-absorbers with low refractive index. V or Mo partial substitution contributes to shift the band gap in the visible range. The heat-treatment of Fe fluoride trihydrate up to T=350°C under Ar leads to stability for the first time where Fe oxy-fluoride has anionic vacancies in the HTB network. The formation of structural units containing 5-fold coordinated Fe atoms in this HTB network leads to a strong reduction of the optical band gap from 4.05 eV in FeF3, 3H2O to 2.05 eV in the Fe oxyfluoride thanks to the occurrence of 2p-oxygen states on the top of the valence band. The reaction between anhydrous In fluoride and water at 400°C under Ar (InF3 + H2O → InOF + 2HF) leads to In oxyfluoride which adopt a derived fluorite-type structure with O/F ordering. InOF can be considered as a transparent conductive oxyfluoride with band gap energy of 3.7 eV. It can also be considered as a smaller work function compared to In2O3 due to the destabilization of conduction band thanks to the structural features of InOF. With these unusual optical and electronic properties, numerous In-based oxyfluorides can be designed. The reduction under H2 at 700°C of Ce fluoro-carbonate CeFCO3 leads to the preparation of pure CeOF with O/F ordering which exhibits a blue-grey coloration. The H2S treatment at T=700°C-800°C of CeIII fluoride, oxyfluoride or fluorocarbonate, allows obtainment of Ce fluoro-sulfide CeSF which adopts a 2D network like CeOF, derived from the fluorite-type structure. The band gap strongly varies from UV in CeOF to visible range in CeSF with around 2eV associated to a red coloration. The key features of mixed anion systems will be highlighted from the analysis of reactivity, structure and local environments to tune the band gap. References:[1] A. Demourgues, N. Penin, F. Weill, D. Dambournet, N. Viadere and A. Tressaud. Chem. Mater. 21, (2009) 1275-1283 |
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