Principles and Applications of Anion Order in Solid Oxynitrides

Metal oxynitrides are emerging materials that may combine the advantages of oxides and nitrides. Anion order is important for controlling and tuning properties, and neutron diffraction provides good O/N contrast for experimental determinations of local or long-range O/N order in solids. Differences between oxide and nitride in charge, size, and covalent bonding are the important factors that drive anion order. An important example is the robust partial anion order in SrMO 2 N (M = Nb, Ta) and related oxynitride perovskites driven by covalency that results in disordered zigzag MN chains which segregate into planes within the perovskite lattice. This leads to unusual subextensive scaling of entropy, described as “open order”. Local anion order is important to optical materials. Size mismatch between host and dopant cations leads to local O/N clustering that tunes photoluminescence shifts systematically in M 1.95 Eu 0.05 Si 5– x Al x N 8– x O x phosphors, leading to a red shift when the M = Ba and Sr host cations are larger than the Eu 2+ dopant but a blue shift when the M = Ca host is smaller.