Crystal structures and color properties of new complex perovskite oxynitrides AMg0.2Ta0.8O2.6N0.4 (A = Sr, Ba)

Abstract

New complex perovskite oxynitrides AMg_0.2Ta_0.8O_2.6N_0.4 (A = Sr, Ba) were synthesized by reacting A₅Ta₄O₁₅ with MgCl₂ in flowing NH₃. The formation of AMg_0.2Ta_0.8O_2.6N_0.4 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 BaMg_0.2Ta_0.8O_2.6N_0.4 is simple cubic (Pmm) and SrMg_0.2Ta_0.8O_2.6N_0.4 is body-centered tetragonal (I4/mcm), isostructural with BaM_0.2Ta_0.8O_2.8N_0.2 and SrM_0.2Ta_0.8O_2.8N_0.2 (M = Li, Na), respectively. The nitrogen in SrMg_0.2Ta_0.8O_2.6N_0.4 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 SrLi_0.2Ta_0.8O_2.8N_0.2 and SrNa_0.2Ta_0.8O_2.8N_0.2. The group of simple and complex perovskites, ATaO₂N, ALi_0.2Ta_0.8O_2.8N_0.2, ANa_0.2Ta_0.8O_2.8N_0.2, and AMg_0.2Ta_0.8O_2.6N_0.4 (A = Sr, Ba) covers the band gap range, 1.9–2.4 eV, and the color ranges from yellow to dark brown.