Crystal structure and optical property of complex perovskite oxynitrides ALi0.2Nb0.8O2.8N0.2, ANa0.2Nb0.8O2.8N0.2, and AMg0.2Nb0.8O2.6N0.4 (A = Sr, Ba)

New complex perovskite oxynitrides AM0.2Nb0.8O3−xNx (A = Sr, Ba; M = Li, Na, Mg; x = 0.2, 0.4) were prepared via 2D → 3D lattice conversion of the hexagonal layered perovskite oxides.308