Syntheses and characterizations of complex perovskite oxynitrides (Ca, Sr, Ba)TaO 2 N

The syntheses, crystal structures, thermal decomposition, and optical absorbance spectra of the tantalum-based complex perovskite oxynitrides Ca 1/2 Sr 1/2 TaO 2 N, Sr 1/2 Ba 1/2 TaO 2 N, and Ca 1/3 Sr 1/3 Ba 1/3 TaO 2 N were investigated. Synchrotron X-ray Rietveld refinement showed that Ca 1/2 Sr 1/2 TaO 2 N, Sr 1/2 Ba 1/2 TaO 2 N, and Ca 1/3 Sr 1/3 Ba 1/3 TaO 2 N were tetragonal, cubic, and orthorhombic, respectively. Additionally, the lattice volume and Ta − O/N bond lengths varied in accordance with the mean ionic radius of the A-site cations. As determined using thermogravimetry and differential scanning calorimetry, the above complex perovskite oxynitrides underwent a two-step thermal decomposition consisting of O 2 -uptake at 800–900 K with Δ H = − 2620 to − 2400 kJ/mol and the N 2 release at 1300–1450 K with Δ H = − 45 to −66 kJ/mol. The optical band gaps were estimated from diffuse reflectance spectra as follows: Ca 1/2 Sr 1/2 TaO 2 N (light orange), 2.35 eV; Ca 1/3 Sr 1/3 Ba 1/3 TaO 2 N (light brown), 2.0 eV; Sr 1/2 Ba 1/2 TaO 2 N (red brown), 1.95 eV.