We report a very simple precipitation route to prepare a layered perovskite-type structure, tungsten trioxide hydrate (TTH), with the nominal chemical formula of WO(3) x 1.3 H(2)O (identical with 1/2H(2)W(2)O(7) x 1.6 H(2)O), using aqueous Na(2)WO(4) and SrCl(2). Our investigation shows that the concentration of HCl used to dissolve the SrCl(2) plays a crucial role in the stabilization of different structure types of layered TTHs. Highly acidic SrCl(2) (dissolved in 9 M HCl) solution yields an orthorhombic layered TTH of WO(3) x 2 H(2)O, while SrCl(2) dissolved in 3 M HCl appears to give an A-site-deficient Ruddlesdon-Popper (RP) related double-perovskite-type layered structure (DOLS-TTH). A well-known scheelite-type structure is obtained under weakly basic conditions (pH = 10.3 for Na(2)WO(4(aq)), 7.0 for SrCl(2(aq))). Previously, RP-type a DOLS of H(2)W(2)O(7) x 0.58 H(2)O was prepared, using an acid-leaching method, from the corresponding n = 2 member of the layered Aurivillius phase (AP) Bi(2)W(2)O(9). Powder X-ray diffraction showed the formation of layered RP DOLS with a large d spacing approximately 12.5 A, which is consistent with acid-leaching (Kuto et al. Inorg. Chem. 2003, 42, 4479-4484; Wang et al. J. Solid State Chem. 2007, 180, 1125-1129) and exfoliation (Schaak et al. Chem. Commun. 2002, 706-707) methods for synthesized TTHs. The proposed DOLS-TTH structure of newly prepared TTHs was further confirmed by an intercalation reaction using n-octylamine (C8A). A transmission electron microscopy study showed the formation of nanosized particles, and scanning electron microscopy coupled with energy dispersive X-ray analysis showed the absence of Na and Sr in the air-dried, as-precipitated products under acidic conditions. The bulk electrical (proton) conductivity of presently prepared TTHs was found to be on the order of 10(-4)-10(-3) S/cm at room temperature in wet N(2).
The chemical stability and electrical properties of three promising perovskite-related structures BaCe0.8Gd0.15Pr0.05normalO3−δ , BaCe0.85Sm0.15normalO3−δ , and BaCe0.85Eu0.15normalO3−δ were tested in air, humidified normalN2 and normalH2 , as well as in normalD2O+normalN2 . Powder X-ray diffraction studies confirmed the formation of a cubic perovskite-like structure. The change in the lattice constant was consistent with B-site substitution in BaCeO3 . All the investigated compounds formed barium carbonate in CO2 at elevated temperatures and were found to be chemically unstable in boiling normalH2O . The data showed that these three compounds are chemically stable in humidified CH4 at 800°C ; however, at 600°C , the formation of barium carbonate was observed. The electrical conductivity in wet normalN2 and/or normalH2 was found to be higher than that in the normalD2O -containing atmosphere, confirming proton conduction in the doped BaCeO3 . The Gd+Pr co-doped BaCeO3 showed the highest total conductivity of 2.58×10−2Scm−1 in normalH2+3%normalH2O at 700°C with an activation energy of 0.36 eV in the temperature range of 450–700°C .
We report the first in-situ powder X-ray diffraction (PXRD) study of the BaCO(3)-CeO(2)-In(2)O(3) and CeO(2)-In(2)O(3) systems in air over a wide range of temperature between 25 and 1200 degrees C. Herein, we are investigating the formation pathway and chemical stability of perovskite-type BaCe(1-x)In(x)O(3-delta) (x = 0.1, 0.2, and 0.3) and corresponding fluorite-type Ce(1-x)In(x)O(2-delta) phases. The potential direct solid state reaction between CeO(2) and In(2)O(3) for the formation of indium-doped fluorite-type phase is not observed even up to 1200 degrees C in air. The formation of the BaCe(1-x)In(x)O(3-delta) perovskite structures was investigated and rationalized using in-situ PXRD. Furthermore the decomposition of the indium-doped perovskites in CO(2) is followed using high temperature diffraction and provides insights into the reaction pathway as well as the thermal stability of the Ce(1-x)In(x)O(3-delta) system. In CO(2) flow, BaCe(1-x)In(x)O(3-delta) decomposes above T = 600 degrees C into BaCO(3) and Ce(1-x)In(x)O(2-delta). Furthermore, for the first time, the in-situ PXRD confirmed that Ce(1-x)In(x)O(2-delta) decomposes above 800 degrees C and supported the previously claimed metastability. The maximum In-doping level for CeO(2) has been determined using PXRD. The lattice constant of the fluorite-type structure Ce(1-x)In(x)O(2-delta) follows the Shannon ionic radii trend, and crystalline domain sizes were found to be dependent on indium concentration.
We report the effect of donor-doped perovskite-type BaCeO(3) on the chemical stability in CO(2) and boiling H(2)O and electrical transport properties in various gas atmospheres that include ambient air, N(2), H(2), and wet and dry H(2). Formation of perovskite-like BaCe(1-x)Nb(x)O(3±δ) and BaCe(0.9-x)Zr(x)Nb(0.1)O(3±δ) (x = 0.1; 0.2) was confirmed using powder X-ray diffraction (XRD) and electron diffraction (ED). The lattice constant was found to decrease with increasing Nb in BaCe(1-x)Nb(x)O(3±δ), which is consistent with Shannon's ionic radius trend. Like BaCeO(3), BaCe(1-x)Nb(x)O(3±δ) was found to be chemically unstable in 50% CO(2) at 700 °C, while Zr doping for Ce improves the structural stability of BaCe(1-x)Nb(x)O(3±δ). AC impedance spectroscopy was used to estimate electrical conductivity, and it was found to vary with the atmospheric conditions and showed mixed ionic and electronic conduction in H(2)-containing atmosphere. Arrhenius-like behavior was observed for BaCe(0.9-x)Zr(x)Nb(0.1)O(3±δ) at 400-700 °C, while Zr-free BaCe(1-x)Nb(x)O(3±δ) exhibits non-Arrhenius behavior at the same temperature range. Among the perovskite-type oxides investigated in the present work, BaCe(0.8)Zr(0.1)Nb(0.1)O(3±δ) showed the highest bulk electrical conductivity of 1.3 × 10(-3) S cm(-1) in wet H(2) at 500 °C, which is comparable to CO(2) and H(2)O unstable high-temperature Y-doped BaCeO(3) proton conductors.
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