Thermally Robust Anion-Chain Order in Oxynitride Perovskites

Abstract

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 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) Å). Anion order also lowers the symmetry of LaTaON 2 from apparent orthorhombic Imma to monoclinic I2/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.

High pressure densification and dielectric properties of perovskite-type oxynitride SrTaO2N

“…TG-MS measurement in Ar indicated SrTaO 2 N released N 2 gas above 1120 °C 4 and the color was changed from orange to dark orange with black particles, although the perovskite phase was retained, as confirmed by X-ray diffraction (XRD) measurement. Thermal annealing of SrTaO 2 N in an evacuated silica tube also partially released nitrogen and a secondary phase of rock-salt type TaO 0.9 was formed together with the perovskite phase at 1100 °C [6].…”

“…Two independent reports in 2011 indicate the presence of cis-type TaO 4 N 2 in SrTaO 2 N from neutron diffraction data [2,3]. The cis-configuration of TaO 4 N 2 was maintained up to 1100 °C from high temperature neutron diffraction measurements [4].…”

Processing of dielectric oxynitride perovskites for powders, ceramics, compacts and thin films

“…These results indicate that the crystal structure of SrTaO 2 N is likely to change after heating at 1200 °C with the partial loss of Sr (or SrO) and nitrogen from the compound, which should be compensated to maintain the original composition during the fabrication of ceramics. On the other hand, the anion order of SrTaO 2 N as a function of temperature was investigated using high resolution neutron and electron diffraction 20 . No loss of anion order was observed up to 1100 °C and the order was expected to remain stable at temperatures as high as 2000 °C 20 .…”

“…On the other hand, the anion order of SrTaO 2 N as a function of temperature was investigated using high resolution neutron and electron diffraction 20 . No loss of anion order was observed up to 1100 °C and the order was expected to remain stable at temperatures as high as 2000 °C 20 . These contrasting results indicate that further investigations are required to understand the crystal structures of partially-decomposed SrTaO 2 N and the related oxynitride perovskites, including the nonstoichiometry, and the physical and chemical properties.…”

“…The local anion order was described as forming a three-dimensional disordered zigzag Ta-N chain 5a,19 . The presence and thermal stability of anion order in the SrTaO 2 N and LaTaON 2 oxynitride perovskites was investigated using high resolution powder neutron and electron 2 | Dalton Trans., 2012, 00, [1][2][3] This journal is © The Royal Society of Chemistry 2012 diffraction 20 . The anion order was maintained up to 1100 °C and was expected to be stable up to significantly higher temperatures greater than 2000 °C.…”