Orléans, France
Orléans, France

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Xu J.,Sun Yat Sen University | Xu J.,Guilin University of Technology | Kuang X.,Sun Yat Sen University | Kuang X.,Guilin University of Technology | And 9 more authors.
Inorganic Chemistry | Year: 2014

The solubility of Ce in the La1-xCexSrGa3O7+δ and La1.54-xCexSr0.46Ga3O7.27+δ melilites was investigated, along with the thermal redox stability in air of these melilites and the conductivity variation associated with oxidization of Ce3+ into Ce4+. Under CO reducing atmosphere, the La in LaSrGa3O7 may be completely substituted by Ce to form the La1-xCexSrGa3O7+δ solid solution, which is stable in air to 600 °C when x 0.6. On the other side, the La1.54-xCexSr0.46Ga3O7.27+δ compositions displayed much lower Ce solubility (x 0.1), irrespective of the synthesis atmosphere. In the as-made La1-xCexSrGa3O7+δ, the conductivity increased with the cerium content, due to the enhanced electronic conduction arising from the 4f electrons in Ce3+ cations. At 600 °C, CeSrGa3O7+δ showed a conductivity of 10-4 S/cm in air, nearly 4 orders of magnitude higher than that of LaSrGa3O7. The oxidation of Ce3+ into Ce4+ in CeSrGa3O7+δ slightly reduced the conductivity, and the oxygen excess did not result in apparent increase of oxide ion conduction in CeSrGa3O7+δ. The Ce doping in air also reduced the interstitial oxide ion conductivity of La1.54Sr0.46Ga3O7.27. Neutron powder diffraction study on CeSrGa3O7.39 composition revealed that the extra oxygen is incorporated in the four-linked GaO4 polyhedral environment, leading to distorted GaO5 trigonal bipyramid. The stabilization and low mobility of interstitial oxygen atoms in CeSrGa3O7+δ, in contrast with those in La1+xSr1-xGa3O7+0.5x, may be correlated with the cationic size contraction from the oxidation of Ce3+ to Ce4+. These results provide a new comprehensive understanding of the accommodation and conduction mechanism of the oxygen interstitials in the melilite structure. © 2014 American Chemical Society.

Kuang X.,Guilin University of Technology | Kuang X.,Sun Yat Sen University | Pan F.,Sun Yat Sen University | Cao J.,Sun Yat Sen University | And 5 more authors.
Inorganic Chemistry | Year: 2013

New insight into the defect chemistry of the tetragonal tungsten bronze (TTB) Ba0.5-xTaO3-x is established here, which is shown to adapt to a continuous and extensive range of both cationic and anionic defect stoichiometries. The highly nonstoichiometric TTB Ba0.5-xTaO 3-x (x = 0.25-0.325) compositions are stabilized via the interpolation of Ba2+ cations and (TaO)3+ groups into pentagonal tunnels, forming distinct Ba chains and alternate Ta-O rows in the pentagonal tunnels along the c axis. The slightly nonstoichiometric Ba 0.5-xTaO3-x (x = 0-0.1) compositions incorporate framework oxygen and tunnel cation deficiencies in the TTB structure. These two mechanisms result in phase separation within the 0.1< x < 0.25 nonstoichiometric range, resulting in two closely related (TaO) 3+-containing and (TaO)3+-free TTB phases. The highly nonstoichiometric (TaO)3+-containing phase exhibits Ba2+ cationic migration. The incorporation of (TaO)3+ units into the pentagonal tunnel and the local relaxation of the octahedral framework around the (TaO)3+ units are revealed by diffraction data analysis and are shown to affect the transport and polarization properties of these compositions. © 2013 American Chemical Society.

Tao F.,Guilin University of Technology | Genevois C.,UPR3079 CEMHTI | Genevois C.,University of Orléans | Lu F.,Guilin University of Technology | And 8 more authors.
Chemistry of Materials | Year: 2016

Formation of hexagonal perovskite with mixed cubic and hexagonal stacking of AO3 layers becomes more and more difficult when the number of layers in the stacking repeating unit increases. So far, the highest number of layers reported for twinned hexagonal perovskite is 12, with alternative 5 consecutive cubic layers and one hexagonal layer in the (ccccch)2 sequence. Here, we present the unexpected formation of a 14-layer twinned hexagonal perovskite with a stacking sequence (cccccch)2 for the BaO3 layers on the Ba14Mn1.75Ta10.5O42 (Ba8MnTa6O24) composition, the first example of twinned hexagonal perovskite with a periodicity exceeding 12-layers. The B-cation and vacancy distributions are characterized by multiple efficient and complementary techniques including neutron and synchrotron powder diffraction, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, and electron energy loss spectroscopy (EELS) and X-ray energy dispersive spectroscopy (EDS) elemental mapping. Atomic-resolution STEM-HAADF imaging and EELS/EDS elemental mapping enables direct observation of high-spin d5 Mn2+ cation ordering in the d0 Ta5+ host, thus demonstrating the great potential of this technique for probing cation ordering and performing structure determination. Moreover, atomic mapping allows for the observation of local defect structure variants, which can be a powerful tool for future new material design. The large high-spin Mn2+ cation and Ta-vacancy pair formation in face-sharing octahedral sites play key roles on both the stabilization of this 14-layer twinned hexagonal perovskite structure and the Mn2+ ordering in the central corner-sharing octahedral (CSO) positions within the five-consecutive CSO layers. Compared with the 8-layer twinned Ba8ZnTa6O24 material, the low quality factor in microwave frequency and enhanced ultraviolet and visible light absorption of Ba14Mn1.75Ta10.5O42 as well as the photocatalytic activity on water splitting are discussed in terms of the presence of high-spin Mn2+ cations in the structure. © 2016 American Chemical Society.

Wang Q.,Guilin University of Technology | Liu S.,Guilin University of Technology | Wang X.,CAS Beijing National Laboratory for Molecular | Fu H.,CAS Beijing National Laboratory for Molecular | And 8 more authors.
Solid State Ionics | Year: 2015

Ba2TiSi2O8 fresnoite, containing a layered mixed TiO5 and SiO4 polyhedral network with pentagonal tunnels similar to melilite, displayed extremely limited solubility for both K+ and La3+ in Ba2+ sites. Atomistic static lattice simulations revealed high energetic costs for K+ and La3+ substitutions for Ba2+ in Ba2TiSi2O8. These results emphasize the rigidity of the mixed layered polyhedral network of Ba2TiSi2O8 fresnoite, which is not suitable for stabilizing the oxygen vacancies and interstitials. In contrast with the La-substituted compositions forming crystalline mixtures, the Ba2 - xKxTiSi2O8 - 0.5x compositions showed phase separation into crystalline Ba2TiSi2O8 and amorphous K2TiSi2O7. However, the addition of potassium enhanced the ionic conductivity of Ba2 - xKxTiSi2O8 - 0.5x compositions, which mainly arises from the potassium conduction in the glass component K2TiSi2O7 as the oxide ion conduction was found to be limited. Measurements on the pure K2TiSi2O7 glass entirely reproduced the electrical and crystallization behaviors observed in Ba2 - xKxTiSi2O8 - 0.5x composites, confirming experimentally the responsibility of the glassy material for the enhanced potassium ionic conductivity in Ba2 - xKxTiSi2O8 - 0.5x composites. This study contributes to the further understanding of oxide ionic conductivity of SrSiO3-based materials, which is currently under debate. © 2015 Elsevier B.V. All rights reserved.

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