First Evidence of Aliovalent Fluorine Doping in Barium Stannate—Combined Experimental and Theoretical Analysis

Fluorine-doped barium stannate nanoparticles (BaSnO₃:F) are synthesized using a previously established solution combustion method.¹ The incorporation of fluorine into the lattice depends on how the fluorine source is introduced during synthesis. A synthesis protocol is developed wherein the addition of the fluorine source to peroxo precipitates, followed by annealing in a reducing environment, yields not only phase-pure BaSnO₃ but also an optical response characteristic of Drude behavior in aliovalently doped BaSnO₃. X-ray photoelectron spectroscopy (XPS) and STEM-EDS confirm the presence of fluorine in both synthesized nanoparticles and pressed pellets. Analysis of fluorine binding energy and shifts in cation binding energy due to fluorine incorporation elucidates bonding and defects within the system, revealing that fluorine binds to metal cations in the bulk with no indications of interstitial or surface fluorine defects. Hall measurements on the pellets indicate n-type conductivity with mobility around ∼10¹ cm² V^-1 s^-1, albeit with significantly fewer carriers than anticipated from complete fluorine ionization. Fluorine substitution at oxygen sites may be counterbalanced by electron localization and, consequently, reduced Sn valence, while fluorine occupying oxygen vacancies complicates electron concentration dynamics. DFT calculations of fluorine-doped BaSnO₃, both with and without oxygen vacancies clarify the atomic orbital contributions to the electronic states in the conduction band, corroborating the XPS findings.<br/><br/>¹Chawla, S.; Aggarwal, G.; Kumar, A.; Singh, A. J.; Woodward, P. M.; Balasubramaniam, K. R. Low-temperature synthesis of transparent conducting La-doped BaSnO₃<i>via</i> rejuvenation of the dried peroxo-precursor.<i> J. Solid State Chem</i>. 2024, 333, 124620