Thermally Robust Anion-Chain Order in Oxynitride Perovskites

The presence and thermal stability of anion order in the oxynitride perovskites SrTaO 2 N and LaTaON 2 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 SrTaO 2 N and LaTaON 2 . No loss of anion order is observed up to 1100 °C and extrapolations based on lattice strains show the order to be stable to remarkably high temperatures >2000 °C, demonstrating that anions are segregated when the materials are synthesized. SrTaO 2 N has an apparent tetragonal I 4/ 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) Å). Anion order also lowers the symmetry of LaTaON 2 from apparent orthorhombic Imma to monoclinic I 2/ m ( a = 5.7140(6), b = 8.0595(6), c = 5.7506(5) Å, and β = 90.239(4)° at 20 °C) and this superstructure persists up to 1100 °C with an extrapolated loss of tilting at 1540 °C. Anion order appears to direct octahedral tilting such that the more rigid Ta–N–Ta bridges retain bond angles closer to 180° than the Ta–O–Ta connections in these superstructures.