Crystal structures and color properties of new complex perovskite oxynitrides AMg0.2Ta0.8O2.6N0.4 (A = Sr, Ba)
New complex perovskite oxynitrides AMg0.2Ta0.8O2.6N0.4 (A = Sr, Ba) were synthesized by reacting A5Ta4O15 with MgCl2 in flowing NH3. The formation of AMg0.2Ta0.8O2.6N0.4 can be described as the cooperative insertion of Mg2+ + 2N3− and the release of 2O2− from the layered oxide, A5Ta4O15. Rietveld refinement of the neutron and synchrotron X-ray diffraction patterns indicated that BaMg0.2Ta0.8O2.6N0.4 is simple cubic (Pmm) and SrMg0.2Ta0.8O2.6N0.4 is body-centered tetragonal (I4/mcm), isostructural with BaM0.2Ta0.8O2.8N0.2 and SrM0.2Ta0.8O2.8N0.2 (M = Li, Na), respectively. The nitrogen in SrMg0.2Ta0.8O2.6N0.4 is partially ordered favoring the c-axial site over the ab-plane of the tetragonal cell, which is a different O/N order pattern from that of SrLi0.2Ta0.8O2.8N0.2 and SrNa0.2Ta0.8O2.8N0.2. The group of simple and complex perovskites, ATaO2N, ALi0.2Ta0.8O2.8N0.2, ANa0.2Ta0.8O2.8N0.2, and AMg0.2Ta0.8O2.6N0.4 (A = Sr, Ba) covers the band gap range, 1.9–2.4 eV, and the color ranges from yellow to dark brown.