Thermally Robust Anion-Chain Order in Oxynitride Perovskites

The presence and thermal stability of anion order in the oxynitride perovskites SrTaO2N and LaTaON2 have been determined using high resolution powder neutron and electron diffraction data. Partial order of oxide and nitride anions consistent with the formation of planes of disordered cis-anion chains is observed in both materials, with a chemical symmetry between distributions in SrTaO2N and LaTaON2. No loss of anion order is observed up to 1100 degrees C and extrapolations based on lattice strains show the order to be stable to remarkably high temperatures >2000 degrees C, demonstrating that anions are segregated when the materials are synthesized. SrTaO2N has an apparent tetragonal I4/mcm superstructure at room temperature due to ordered octahedral tilts, but anion order lowers symmetry to an orthorhombic Fmmm supercell (with lattice parameters a = 8.0657(8), b = 8.0614(7), and c = 8.0775(4) angstrom). Anion order also lowers the symmetry of LaTaON2 from apparent orthorhombic Imma to monoclinic I2/m (a = 5.7140(6), b = 8.0595(6), c = 5.7506(5) angstrom, and beta = 90.239(4)degrees at 20 degrees C) and this superstructure persists up to 1100 degrees C with an extrapolated loss of tilting at 1540 degrees C. Anion order appears to direct octahedral tilting such that the more rigid Ta-N-Ta bridges retain bond angles closer to 180 degrees than the Ta-O-Ta connections in these superstructures.