Good Oxide Ion Conductors Research Articles

Preparation, characterization and properties of LnSmWO$$_{6}$$ (Ln = Nd and Dy) nanofunctional ceramics

Isovalent substitution of lanthanide ions in \(\hbox {Ln}_{{2}}\hbox {WO}_{6 }\) (Ln = lanthanides) nanoceramics can probe into multifunctional applications due to their unique structural and electronic properties. In this work \(\hbox {Sm}^{3+}\) ions in the \(\hbox {Sm}_{{2}}\hbox {WO}_{{6}}\) nanoceramics were partially replaced with \(\hbox {Nd}^{3+}\) and \(\hbox {Dy}^{3+}\) ions and their structural, optical and ionic transport properties were studied. The size and structure of the nanocrystalline \(\hbox {LnSmWO}_{{6}}\) (Ln = Nd and Dy) compounds prepared through a combustion method were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy and transmission electron microscopy (TEM). UV–Visible spectroscopy and photoluminescence spectroscopy were used for the investigation of optical and electronic properties of the nanoceramics. Bulk ceramics prepared from the nanoparticles achieved high-density during the sintering process and the surface morphology of the dense \(\hbox {NdSmWO}_{{6}}\) was imaged using scanning electron microscopy. The electrical properties of the dense ceramics were analysed using impedance spectroscopy. XRD analysis carried out on the prepared materials showed a single-phase monoclinic structure for the nanoceramics. A combined analysis of FTIR and Raman studies showed the presence of Ln–O and O–W–O vibrations, which confirm the monoclinic structure. Particulate properties investigated through the TEM imaging showed that the prepared materials are polycrystalline aggregates. Optical studies carried out on the nanoparticles showed absorption in the UV region and emission in the visible region. Impedance spectroscopic studies conducted on bulk ceramics imply that these are good oxide ion conductors at high-temperature.

Crystal structure and electrical conductivity of Ba<i>R</i><sub>2</sub>ZnO<sub>5</sub> (<i>R</i> = Sm, Gd, Dy, Ho, and Er)—a new structure family of oxide-ion conductors

Metal oxides containing the divalent zinc cation (Zn2+, d10 electronic configuration) as an essential element are promising candidates for oxide-ion conductors, because there are several good oxide-ion conductors containing other d10 cations, such as Ga3+ and Ge4+. In the present study, we have found a new structure family of oxide-ion conductors, BaY2CuO5-type (BaCuY2O5-type) BaR2ZnO5 compounds (R = rare earths). BaR2ZnO5 compounds with a BaY2CuO5-type structure were found to be good candidates for oxide-ion conductors by the bond-valence method. BaR2ZnO5 compounds (R = Sm, Gd, Dy, Er, and Ho) were synthesized by solid-state reactions, and their electrical conductivities and crystal structures were investigated. Rietveld analyses of BaR2ZnO5 (R = Sm, Gd, Dy, Er, and Ho) using synchrotron X-ray powder diffraction data gave good fitting for the BaY2CuO5-type structure. BaHo2ZnO5 had the highest total electrical conductivity in air among BaR2ZnO5 compounds (R = Sm, Gd, Dy, Er, Ho). Oxide-ion conduction was confirmed for BaHo2ZnO5 by electrical conductivity measurements conducted under various oxygen partial pressures. Neutron powder diffraction data of BaHo2ZnO5 were analyzed by the Rietveld method, and the oxide-ion conduction paths were investigated by the bond-valence method.

Influence of La2Mo2O9 on the sintering behavior and electrochemical properties of gadolinium-doped ceria

Abstract Gadolinium-doped ceria (known as CGO or GDC) is a good oxide-ion conductor which presents potential application for solid-state electrochemical devices. However, one of the main drawbacks of this material is the high sintering temperature required to obtain dense ceramic. In this paper, we report a new way to reduce densification temperature of CGO by adding x weight percent of La 2 Mo 2 O 9 material (0≤ x ≤10). This latter is also a good oxide-ion conductor at high temperature (>580 °C) which allows the preservation of the electrochemical properties of CGO as shown by impedance spectroscopy. The sintering behavior is then studied using dilatometric measurements and mechanism of densification is discussed.

Synthesis and characterisation of new Bi(iii)-containing apatite-type oxide ion conductors: the influence of lone pairs.

Lone-pair cations are known to enhance oxide ion conductivity in fluorite- and Aurivillius-type materials. Among the apatite-type phases, the opposite trend is found for the more widely studied silicate oxide ion conductors, which exhibit a dramatic decrease in conductivity on Bi(iii) incorporation. In this work, the influence of lone-pair cations on the properties of apatite-type germanate oxide ion conductors has been investigated by preparing and characterising seven related compositions with varying Bi(iii) content, by X-ray and neutron powder diffraction and impedance spectroscopy. All materials are very good oxide ion conductors (with conductivities of up to 1.29 × 10-2 S cm-1 at 775 °C). Increasing Bi(iii) content leads to increases in conductivity by up to an order of magnitude, suggesting significant differences in the oxide-ion conduction mechanisms between lone-pair-containing apatite-type germanate and silicate solid electrolytes.

Enhanced bulk conductivity of A-site divalent acceptor-doped non-stoichiometric sodium bismuth titanate

Bismuth-deficient sodium bismuth titanate (nominally Na0.5Bi0.49TiO2.985, NB0.49T) is a good oxide-ion conductor. Here we report the influence of A-site divalent ions, M2+=Ca2+, Sr2+ and Ba2+, on the electrical properties of NB0.49T. A-site divalent doping for Bi3+ enhances the bulk (grain) conductivity by ~one order of magnitude without changing the conduction mechanism, which is attributed to an increase in the oxygen vacancy concentration based on the doping mechanism Bi3++½ O2−→M2+. Among these three dopants, Sr2+ is the most effective in increasing the bulk conductivity due to a combination of its smaller mismatch in ion size with Bi3+, its intermediate polarisability and lower bond strength to oxygen compared to Ca2+ and Ba2+. Doping strategies for further improvements to bulk conductivity of NBT materials are discussed based on these results. Comparison with other oxide-ion conductors and initial stability test under reducing atmosphere show the doped non-stoichiometric NBT materials are promising for low and intermediate temperature applications.

Sintering and electrical conductivity of the GdSmZr2O7 ceramic with and without ZnO sintering aid

The effects of zinc oxide (ZnO) additive on the crystal structure, densification, and electrical conductivity of the GdSmZr2O7 ceramic have been examined. The GdSmZr2O7 ceramic with and without the addition of 1 wt.% ZnO sintered at temperatures between 1773 and 1973 K exhibits a single cubic pyrochlore-type phase. ZnO is very effective as a sintering aid for the GdSmZr2O7 ceramic, and it reduces the sintering temperature by about 200 K with a positive effect on the total conductivity. The total conductivity of the ZnO-modified GdSmZr2O7, as measured by A.C. impedance spectroscopy under an air atmosphere, is higher than that of the unmodified GdSmZr2O7 at identical temperature levels. At 1173 K, the highest total conductivity of the ZnO-modified GdSmZr2O7 is about fifteen percent higher than that of the unmodified GdSmZr2O7. The 1 wt.% ZnO-modified GdSmZr2O7 is one of good oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10−4 to 1.0 atm at different test temperatures.

Oxide Ion Conductivity, Phase Transitions, and Phase Separation in Fluorite-Based Bi38−xMo7+xO78+1.5x

We present, for the first time, the ionic conductivity properties of two different, but closely related, bismuth molybdates: Bi38Mo7O78 and Bi37.5Mo7.5O78.75. Both are good oxide ion conductors, with the latter being comparable to yttria-stabilized zirconia. We show that the structure of Bi38Mo7O78 is more complex than previously reported, and that this compound is a 5 × 3 × 6 fluorite superstructure with slight monoclinic distortion. In addition to being a good oxide ion conductor, the material is noncentrosymmetric-polar and second harmonic generation (SHG) active. The second phase, orthorhombic Bi37.5Mo7.5O78.75, reported for the first time, is an excellent oxide ion conductor. The materials have been characterized by impedance spectroscopy, variable-temperature synchrotron, neutron and laboratory powder X-ray diffraction, electron diffraction, and SHG measurements.

Synthesis and electrical transport properties of Lu 2+ xTi 2− xO 7− x/2 oxide-ion conductors

The Lu 2+ x Ti 2− x O 7− x/2 ( x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu 2O 3) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10 − 3 – 2.5 × 10 − 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO 2/Y 2O 3 ceramics. The conductivity of Lu 2+ x Ti 2− x O 7− x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu 2+ x Ti 2− x O 7− x/2 ( x = 0.096; 35.5 mol% Lu 2O 3) material containing ∼ 4.8 at.% Lu Ti anti-site defects.

Synthesis and characterisation of lanthanum germanate-based apatite phases

La germanate apatite-based materials, La 9.33 + x (GeO 4) 6O 2 + 1.5x , are good oxide ion conductors but there are uncertainties over the solid solution limits, the defect crystal structure and conductivity variations with composition and temperature. Volatilisation of GeO 2 during synthesis is increasingly important for temperatures above ∼1300 °C. The solid solution range at 1100 °C is ∼0.17 ≤ x ≤ ∼0.50 at which temperature GeO 2 loss appears not to be significant. With increasing x, the symmetry changes from hexagonal to triclinic; for high x compositions, high temperature X-ray diffraction shows a gradual triclinic to hexagonal transition over the range 600–800 °C. Conductivity data of the high x solid solutions also show evidence of the phase transition region, with lower activation energy and higher conductivity in the high temperature hexagonal structure. The extent of, and structural changes within, the apatite domain in the LaO 1.5–GeO 2–SrO ternary system at 1100 °C have been studied and single-phase samples are obtained for La 9.33 + x − 2y / 3 Sr y(GeO 4) 6O 2 + 1.5x y = 1.0 with x = 0.17 and 0.34. The hexagonal to triclinic transition is clearly associated with increasing oxygen content rather than filling the La sites by addition/substitution of Sr into the structure.

New monoclinic compounds, Bi 3.24Ln2W 0.76O 10.14, having a pseudo-orthohexagonal cell based on a pseudo-fcc subcell in the systems Bi 2O 3– Ln2O 3–WO 3 ( Ln=La, Pr, and Nd)

New compounds, Bi 3.24 Ln 2W 0.76O 10.14, have been found in the systems Bi 2O 3– Ln 2O 3–WO 3 ( Ln=La, Pr, and Nd). They crystallize ostensibly in the orthorhombic (or orthohexagonal) symmetry, e.g. with a=6.9694(1) A ̊ , b=4.0242(1) A ̊ , c=9.3335(1) A ̊ , and Z=1 for Ln=La. At the same time, the lattice forms a superstructure based on a pseudo-fcc subcell with a′≈5.6 A ̊ , where the transformation matrix is (1/2,1/2,−1)/(−1/2,1/2,0)/(1,1,1). However, the cation configuration has proved that the true symmetry is monoclinic ( C2/ m) with β≈90°. All new compounds undergo two reversible polymorphic transitions at about 980°C and at 1097–1210°C depending on Ln. Their electrical conductivity exhibited lower values (about 10 −4 S cm −1 at 500°C) despite the pseudo-fcc subcell in relation to the δ-Bi 2O 3 type which is the well-known good oxide-ion conductor.

Structural study of the trigonal Bi2.34U0.33La0.33O5 oxide ion conductor: Rietveld refinement of X-ray and neutron powder diffraction data

The structure of Bi2.34U0.33La0.33O5 has been investigated by Rietveld refinement of powder X-ray and neutron diffraction data. The compound is trigonal, space group P, Z = 1, and lattice parameters aH = 4.01395(8), cH = 9.5423(2) A. X-Ray diffraction data are in favour of a model in which U and La atoms are situated in different crystallographic positions: i.e. 1a and 2d of the P space group respectively, forming mixed Bi/U and Bi/La layers which alternate along the c axis. The structure of this bismuth-based mixed oxide can be described as built up of interlocked edge-sharing (Bi/U)O8 and (Bi/La)O8 polyhedra. The co-ordination polyhedron around the Bi/U atoms is a hexagonal bipyramid, the equatorial plane being a puckered hexagon. The Bi/La atoms are co-ordinated to eight oxygen atoms forming a slightly distorted cube. Electron diffraction studies showed that the structure can be described as a fluorite-type superstructure, the relation to the fluorite sub-cell is a*H ≈ ⅓[4]a*C, b*H ≈ ⅓[ 22]a*C, c*H ≈ ⅓[111]a*C. This compound is a good oxide ion conductor (σ300 °C = 2.5 × 10–5 S cm–1). On the basis of the anion vacancies distribution determined from neutron diffraction refinement, a pathway for the oxide ion conduction is proposed.

Ionic Conductivity and Structural Phase Transformations for Hexagonal and Cubic Bi1.33U0.33La0.33O3.5 Polymorphs

The hexagonal and cubic Bi1.33U0.33La0.33O3.5 polymorphs have been synthesized by heating at 950 °C a mixture of Bi2O3 and LaUO4+x. When the reaction mixture is slowly cooled to room temperature, the hexagonal phase with cell parameters aH = 4.0285(3) Å, cH = 9.5494(8) Å is formed. Quenching of the mixture from 950 °C leads to the formation of the cubic phase with a fluorite-type structure, aC = 5.641(1) Å. The hexagonal and cubic materials are good oxide ion conductors, the conductivity being higher for the cubic polymorph. The conductivity vs temperature for both polymorphs shows several linear dependencies, which are ascribed to different crystalline phases identified by X-ray powder diffraction. An intermediate hexagonal phase different from the starting hexagonal one is found. The schemes of the structural transformations with temperature are established, and the relationships between conductivity properties and phase transformations are pointed out.

Bi2W1–xCuxO6–x(0.7⩽x⩽0.8): a new oxide-ion conductor

Anion-deficient Aurivillius phases of the general formula, Bi2W1–xCuxO6 – 2x, possessing orthorhombic/tetragonal Bi2WO6-like structures, have been synthesized by quenching the oxide melts. The tetragonal phase stabilized for the compositions 0.7⩽x⩽0.8 is a good oxide-ion conductor in the temperature range 500–900 K, the x= 0.7 composition exhibiting the highest conductivity in the series.