Abstract
The syntheses, crystal structures, thermal decomposition, and optical absorbance spectra of the tantalum-based complex perovskite oxynitrides Ca1/2Sr1/2TaO2N, Sr1/2Ba1/2TaO2N, and Ca1/3Sr1/3Ba1/3TaO2N were investigated. Synchrotron X-ray Rietveld refinement showed that Ca1/2Sr1/2TaO2N, Sr1/2Ba1/2TaO2N, and Ca1/3Sr1/3Ba1/3TaO2N 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 O2-uptake at 800–900 K with ΔH = − 2620 to − 2400 kJ/mol and the N2 release at 1300–1450 K with ΔH = − 45 to −66 kJ/mol. The optical band gaps were estimated from diffuse reflectance spectra as follows: Ca1/2Sr1/2TaO2N (light orange), 2.35 eV; Ca1/3Sr1/3Ba1/3TaO2N (light brown), 2.0 eV; Sr1/2Ba1/2TaO2N (red brown), 1.95 eV.
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