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 <i>cis</i>-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 °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. SrTaO₂N has an apparent tetragonal <i>I</i>4/<i>mcm</i> superstructure at room temperature due to ordered octahedral tilts, but anion order lowers symmetry to an orthorhombic <i>Fmmm</i> supercell (with lattice parameters <i>a</i> = 8.0657(8), <i>b</i> = 8.0614(7), and <i>c</i> = 8.0775(4) Å). Anion order also lowers the symmetry of LaTaON₂ from apparent orthorhombic <i>Imma</i> to monoclinic <i>I</i>2/<i>m</i> (<i>a</i> = 5.7140(6), <i>b</i> = 8.0595(6), <i>c</i> = 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