Sequential double anionic substitution through synthesis of perovskite-type AB(O,N,F)3 with A = Ca, Sr, Ba and BTi, Zr

For the synthesis of perovskite-type oxynitrides AB(O,N)3 in most cases cationic substitution is applied for charge compensation, while anionic substitution, e.g., using fluoride is by far less investigated. Herein, we have investigated the possibility to use sequential double anionic substitution for charge compensation while forming perovskite-type oxynitride fluorides AB(O,N,F)3 with A = Ca, Sr, Ba and BTi, Zr. As expected from Goldschmidt's tolerance factor, the cubic SrTi(O,N,F)3 and BaZr(O,N,F)3 were found to be easily formed. The synthesis required a lower number of rigorous thermal treatment cycles to obtain single-phase materials. The non-cubic crystal structures favored a factor six to ten higher nitrogen content of e.g., up to 0.06 per formula unit in SrZr(O,N,F)3, while the fluoride content did not change within measurement uncertainty. Overall, the amount of anionic substitution remained low and a presence of oxygen vacancies in the range of 0.1 per formula unit was indicated in all cases. In comparison to the corresponding oxides the double anionic substituted oxynitride fluorides displayed a band gap reduction by about 0.7 eV on average, however, the obtained band gap values of around 3 eV were still too large for a potential application in solar water splitting (SWS). Nonetheless, a clear correlation of the nitrogen content and the band gap reduction was obtained, e.g., SrZr(O,N,F)3 revealing the highest nitrogen content also showing the highest band gap reduction. While no band gap change was observed in CaZr(O,N,F)3 showing a negligible nitrogen amount. The main drawback of a sequential double anionic substitution strategy using consecutive heterovalent substitution by F− and N3− is found to be the need of intermediate charge compensation. In parallel the Fermi level is altered. Both allowed only for a very limited substitution of the anionic sublattice. Instead, alternative synthesis pathways enabling a simultaneous anionic double substitution will be needed to realize increased partial substitution of nitride and fluoride in perovskite-type AB(O,N,F)3 reducing the band gaps to values suitable for SWS.