Bandgap tuning in SrTi(N,O,F)3SrTi(N,O,F)_{3} by anionic-lattice variation

Polycrystalline SrTiO3 and SrTi(O,F)3 powders were synthesized by a solid-state reaction. A partial substitution of oxygen by nitrogen was subsequently carried out using thermal ammonolysis resulting in SrTi(N,O)3 and SrTi(N,O,F)3. Powder X-ray diffraction (XRD) revealed a cubic perovskite structure with space group Pm-3m for all samples. The thermal ammonolysis slightly increased the lattice parameters, crystallite sizes and strain. As a result from the co-substitution of oxygen with nitrogen and fluorine for SrTi(N,O,F)3, highly distorted TiO6 octahedra were detected using X-ray absorption near edge structure (XANES) spectroscopy. The weakening of all active modes of the Raman spectra after thermal ammonolysis also indicated enhanced distortions in the local crystal structure. SrTi(N,O,F)3 has the largest amount of nitrogen as well as fluorine among all four samples as determined by thermogravimetric analysis (TGA), elemental analysis and X-ray photoelectron spectroscopy (XPS). In the UV–vis spectra a distinctive shift of the absorption-edge energy was observed exclusively for the SrTi(N,O,F)3 sample from 390 to 510 nm corresponding to a bandgap narrowing from 3.18 to 2.43 eV