Perovskite-type SrTi1−xNbx(O,N)3 compounds: Synthesis, crystal structure and optical properties

The synthesis, crystal structure, thermal stability and absorbance spectra of perovskite-type oxynitrides with the general formula SrTi1−xNbx(O,N)3 (x=0.05, 0.10, 0.20, 0.50, 0.80, 0.90, 0.95) have been investigated. Oxide samples were prepared by a polymerized complex synthesis route and post-treated under ammonia at 850 °C for 24 h to substitute nitrogen for oxygen. Synchrotron X-ray powder diffraction (XRD) evidenced that the mixed oxide phases were all transformed into oxynitrides with perovskite-type structure during a thermal ammonolysis. SrTi1−xNbx(O,N)3 with compositions x≤0.80 crystallized in a cubic and samples with x≥0.90 in a tetragonal structure. The Rietveld refinement indicated a continuous enlargement of the lattice parameters towards higher niobium content of the samples. Thermogravimetric analysis (TGA) and hotgas extraction revealed the dependence of the nitrogen incorporation upon the degree of niobium substitution. It showed that more nitrogen was detected in the samples with higher niobium content. Furthermore, TGA disclosed stability for all oxynitrides at T≤400 °C. Diffuse reflectance spectroscopy indicated a continuous decrease of the band gap’s width from 3.24 eV (SrTi0.95Nb0.05 (O,N)3) to 1.82 eV (SrTi0.05Nb0.95(O,N)3) caused by the increasing amount of nitrogen towards the latter composition.