Susana García-Martín

Lithium-ion conductivity in the novel La1/3-xLi3xNbO3 solid solution with perovskite-related structure

Abstract The stoichiometric range, crystal chemistry, ionic conductivity and electrochemical window of the La 1/3− x Li 3 x NbO 3 solid solution with a perovskite-related structure have been studied. The range of existence of the solid solution appears to be 0≤x≤0.06. These niobates have a basic diagonal unit cell a ≈√2 a p b √2 a p c ≈2 a p . Ionic conductivity of the materials and its dependence with the composition and temperature have been examined. We have found that the highest conductivity value is 4.3×1O −5 S cm −1 at 300 K for x =0.04. The electrochemical window of the compounds has been investigated by potentiostatic discharge and charge. Electrochemical experiments show that the use of the materials as solid electrolytes in secondary batteries is limited down to 1.75 V using Li metal as anode.

Octahedral tilt twinning and compositional modulation in NaLaMgWO6

A combination of selected-area electron diffraction (SAED), neutron powder diffraction (NPD) and high-resolution transmission electron microscopy (HRTEM) reveals a complex superstructure in the ordered perovskite NaLaMgWO(6). Through indexing of SAED patterns the unit-cell dimensions are found to be 46.8 x 7.8 x 7.9 A, which corresponds to a 12a(p) x 2a(p) x 2a(p) superstructure of the simple Pm3m perovskite unit cell. HRTEM images reveal the formation of an unmistakable stripe contrast that repeats with the same periodicity. Doubling of the b and c axes is brought about by a combination of layered ordering of Na and La, rock-salt ordering of Mg and W, and octahedral tilting. The a axis repeat distance results from a one-dimensional twinning of the octahedral tilts in combination with a compositional modulation. Modeling of the NPD pattern shows that the underlying tilt system is a(-)a(-)c(0) with tilt angles of approximately 8 degrees about the a and b axes. The octahedral tilt-twin boundaries run perpendicular to the a axis and are separated by 6a(p). Simulated HRTEM images show that octahedral tilt twinning alone cannot explain the stripes seen in the HRTEM images, rather a compositional modulation involving the A-site cations is necessary to explain the experimental images.

Structural studies on the optimisation of fast oxide ion transport

Extensive regions of cubic fluorite solid solution formation are observed in the Y 2 O 3 -ZrO 2 , Y 2 O 3 -ZrO 2 -TiO 2 and Y 2 O 3 -ZrO 2 -Nb 2 O 5 systems. Ionic conductivity studies indicate that the activation energy for conduction increases rapidly whilst the high temperature conductivity decreases as yttria-content increases. At high defect concentrations, however, activation energies tend towards a constant value. The increase in activation energy correlates with an increase in the extent of diffuse reflections in neutron diffraction experiments and with increases in the isotropic temperature factor associated with the anion sublattice. Significant negative correlation was observed between conductivity prefactor and oxygen temperature factors. Electron diffraction experiments also show diffuse elements in addition to the cubic fluorite structure, indicating the presence of short-range order related to C-type and distorted pyrochlore types.

Evidence of three types of short range ordered fluorite structure in the (1 − x) Y0.15Zr0.85O1.93–x Y0.75Nb0.25O1.75(0 ≤x≤ 1) system

The microstructure of the high-temperature quenched oxides of the (1 − x) Y0.15Zr0.85O1.93 – x Y0.75Nb0.25O1.75 (0 ≤ x ≤ 1) system has been investigated by selected area electron diffraction and high resolution transmission electron microscopy. We have found three types of fluorite structure: the x = 0 end member of the solid solution (Y0.15Zr0.85O1.93) shows evidence of tetragonal fluorite-type structure; we have also observed a modulated fluorite-type structure due to short-range ordering of the oxygen vacancies. The microdomain modulated structure gradually evolves from a distorted pyrochlore-type to a type intermediate between C-type and distorted pyrochlore as x decreases from 1 to 0.3.

Strontium Superstoichiometry and Defect Structure of SrCeO3 Perovskite

Strontium cerate (SrCeO(3)) is the parent phase of a family of prototype proton-conducting perovskites with important potential applications as electrolytes in protonic ceramic fuel cells, hydrogen-separation membranes, and sensors for hydrogen and humidity. Apparent nonstoichiometric behavior and the microstructure of SrCeO(3) have been investigated. Phase analysis by X-ray diffraction indicates that single-phase material in the system Sr(1+x)CeO(3+)delta is obtained for compositions x = 0.02-0.03 and that nominally stoichiometric SrCeO(3) (x = 0) synthesized by either solid-state reaction or the citrate method is Sr-rich. Selected area electron diffraction confirms that the system crystallizes with the GdFeO(3)-type orthorhombic perovskite structure (space group Pnma). Structural defects characterized by high-resolution transmission electron microscopy include twin domain boundaries and SrO-rich, Ruddlesden-Popper-type planar defects. Magnetic susceptibility measurements down to 2 K indicate that the Ce(3+) content is minor ( approximately 0.01 mol per formula unit for slow-cooled material) and does not influence the observed nonstoichiometry.

Giant barrier layer capacitance effects in the lithium ion conducting material La0.67Li0.25Ti0.75Al0.25O3

High dielectric permittivity (e′∼500000) has been observed in polycrystalline samples of La0.67Li0.25Ti0.75Al0.25O3 over a large frequency range (∼10<f<∼103Hz) and different temperatures. Complex impedance spectroscopy measurements demonstrate that the origin of the high dielectric constant can be attributed to a barrier layer capacitor associated with grain boundary effects in the ion conducting material.

A-site-cation deficiency in the SrCe0.9Yb0.1O3−δ perovskite: effects of charge-compensation mechanism on structure and proton conductivity

Cation non-stoichiometry is now recognised to have a significant influence on the electrical-transport properties of proton-conducting oxides. The effects of A-site-cation deficiency in the Sr1−xCe0.9Yb0.1O3−δ perovskite system with potential use in electrochemical devices are examined. Electron diffraction is used to verify the orthorhombic perovskite structure (space group, Pnma) of compositions 0 ≤ x ≤ 0.06. Combined Rietveld refinements of high-resolution neutron and X-ray diffraction data confirm a previously reported anomalous evolution of the lattice parameters within the series, with a maximum in specific free volume observed for x = 0.04. Analysis of the ion-site occupancies indicates that A-site and oxygen vacancies form up to x = 0.04, with partial partitioning of the Yb dopant over the Sr and Ce sites taking place for higher levels of Sr deficiency. The change in defect chemistry is likely to be responsible for the anomalous evolution in structural parameters, and may arise from the alleviation of strain associated with substantial octahedral tilting which occurs for a high A-site vacancy content. In addition to lattice parameters, electrical transport and stability in CO2 are also affected by dopant partitioning. The proton conductivity appears to be well correlated with both the nominal oxygen-vacancy content and the specific free volume. The stability of the series towards carbonation is not correlated to the Sr chemical potential but, instead, is lowest for the composition of greatest specific free volume.

Influence of the crystal microstructure on the dielectric response of the La0.67Li0.2Ti0.8Al0.2O3

Polycrystalline samples of La0.67Li0.2Ti0.8Al0.2 with different crystal microstructures have similar general dielectric behavior. The materials show very high dielectric permittivity (e′≈300000) over relatively wide frequency and temperature ranges with no apparent dependence on the microstructure. The impedance spectroscopy study indicates that the origin of the high dielectric constant is a barrier layer capacitor effect due to depletion of Li ions at the grain boundaries of these Li-conducting materials. However, influence of the three dimensional microstructure domains on bulk properties such as electrical resistance and capacitance is observed.

Dielectric response and ionic conductivity of Cs(NbTe)O6

Abstract A study of the dielectric response in the frequency range 10 Hz to 10 7 Hz has been conducted on Cs(NbTe)O 6 defect pyrochlore oxides. Two different behaviours are identified: one characteristic of a volume conduction process by Cs + ions and other due to a contact effect. Ionic conductivity is derived from complex impedance plots and an activation energy of 0.67 eV is deduced for ion motion.

Novel defect pyrochlores M0.5Tl0.5 (NbTe)O6 : preparation and structural characterization

Abstract New mixed oxides of composition M0.5Tl0.5(NbTe)O6 have been synthesized by reacting of Nb2O5, Te(OH)6, TlNO3 and MNO3 (M= K, Rb, Cs) in appropiate molar ratio at 973K. The characterization of these materials has been carried out by X-ray powder diffraction, giving a cubic pyrochlore structure (space group Fd3m) and lattice parameters a=10.252(8)A (M=K), a=10.255(3)A (M=Rb) and a=10.279(1)A (M=Cs). The atomic positions and R discrepancy factors were also calculated.