Structural Disorder in Doped Zirconias, Part I: The Zr0. 8Sc0. 2− x Y x O1. 9 (0.0≤ x≤ 0.2) System (original) (raw)
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Local structure and ionic conductivity in the Zr2Y2O7Y3NbO7 system
Journal of Physics: …, 2009
The Zr 0.5−0.5x Y 0.5+0.25x Nb 0.25x O 1.75 solid solution possesses an anion-deficient fluorite structure across the entire 0 x 1 range. The relationship between the disorder within the crystalline lattice and the preferred anion diffusion mechanism has been studied as a function of x, using impedance spectroscopy measurements of the ionic conductivity (σ ), powder neutron diffraction studies, including analysis of the 'total' scattering to probe the nature of the short-range correlations between ions using reverse Monte Carlo (RMC) modelling, and molecular dynamics (MD) simulations using potentials derived with a strong ab initio basis. The highest total ionic conductivity (σ = 2.66 × 10 −2 −1 cm −1 at 1473 K) is measured for the Zr 2 Y 2 O 7 (x = 0) end member, with a decrease in σ with increasing x, whilst the neutron diffraction studies show an increase in lattice disorder with x. This apparent contradiction can be understood by considering the local structural distortions around the various cation species, as determined from the RMC modelling and MD simulations. The addition of Nb 5+ and its stronger Coulomb interaction generates a more disordered local structure and enhances the mobility of some anions. However, the influence of these pentavalent cations is outweighed by the effect of the additional Y 3+ cations introduced as x increases, which effectively trap many anions and reduce the overall concentration of the mobile O 2− species.
Cation composition effects on oxide conductivity in the Zr2Y2O7Y3NbO7 system
Journal of Physics: …, 2009
Realistic, first-principles-based interatomic potentials have been used in molecular dynamics simulations to study the effect of cation composition on the ionic conductivity in the Zr2Y2O7-Y3NbO7 system and to link the dynamical properties to the degree of lattice disorder. Across the composition range, this system retains a disordered fluorite crystal structure and the vacancy concentration is constant. The observed trends of decreasing conductivity and increasing disorder with increasing Nb 5+ content were reproduced in simulations with the cations randomly assigned to positions on the cation sublattice. The trends were traced to the influences of the cation charges and relative sizes and their effect on vacancy ordering by carrying out additional calculations in which, for example, the charges of the cations were equalised. The simulations did not, however, reproduce all the observed properties, particularly for Y3NbO7. Its conductivity was significantly overestimated and prominent diffuse scattering features observed in small area electron diffraction studies were not always reproduced. Consideration of these deficiencies led to a preliminary attempt to characterise the consequence of partially ordering the cations on their lattice, which significantly affects the propensity for vacancy ordering. The extent and consequences of cation ordering seem to be much less pronounced on the Zr2Y2O7 side of the composition range. PACS numbers: 31.15.xv Molecular dynamics and other molecular methods 66.30.H-Self-diffusion and ionic conduction in non-metals 66.30.Dn Theory of diffusion and ionic conduction in solids
Vacancy ordering effects on the conductivity of yttria-and scandia-doped zirconia
Arxiv preprint arXiv: …, 2010
Polarizable interaction potentials, parametrized using ab initio electronic structure calculations, have been used in molecular dynamics simulations to study the conduction mechanism in Y 2 O 3 -and Sc 2 O 3 -doped zirconias. The influence of vacancy-vacancy and vacancy-cation interactions on the conductivity of these materials has been characterised. While the latter can be avoided by using dopant cations with radii which match those of Zr 4+ (as is the case of Sc 3+ ), the former is an intrinsic characteristic of the fluorite lattice which cannot be avoided and which is shown to be responsible for the occurrence of a maximum in the conductivity at dopant concentrations between 8 and 13 %. The weakness of the Sc-vacancy interactions in Sc 2 O 3 -doped zirconia suggests that this material is likely to present the highest conductivity achievable in zirconias.
Structure–property correlation: the ionic conductivity of selected zirconates
Acta Crystallographica Section A Foundations and Advances, 2017
Binary mixed metal oxides described by the chemical formal A2B2O7, or A2B2O6O' to highlight the uniqueness of one of the oxygen atoms, mostly adopts one of two major structure types, Pyrochlore or Defect Fluorite. Materials with both structures types have been investigated as electrolytes in solid oxide fuel cells (SOFCs) due to their ability to conduct ions and possibly low thermal expansion coefficients. Due to the need improve performance temperatures many investigation into doped pyrochlores have been under taken. Whittle et al. found that adding increasing concentrations of yttrium to a pyrochlore type zirconates makes the structure tend toward the fluorite structure. We report the results from our studies into the structure changes in materials of the form Ln2-xYxZr2O7 (where Ln = La and Sm) using variable temperature PXRD and Rietveld refinements. The ionic conductivity of these solid solutions was also studied as a function of the composition, using electrochemical impedance spectroscopy.
Journal of Physics: Condensed Matter, 2007
We have prepared different compositions in the Gd 2−y La y Zr 2 O 7 solid solution by mechanically milling stoichiometric mixtures of the corresponding oxides. Irrespective of their lanthanum content, as-prepared powder samples consist of single-phase anion-deficient fluorite materials, although the long-range ordering of cations and anion vacancies characteristic of pyrochlores was observed for y 0.4 after post-milling thermal treatments at 1200 • C. Ionic conductivity was found to be thermally activated and almost independent of La content for 0 y 1, since the pre-exponential factor decreases as structural ordering increases; however, there is a concomitant decrease of the activation energy E dc for oxide-ion diffusion, from E dc = 1.13 ± 0.02 eV for the anion-deficient fluorite Gd 2 Zr 2 O 7 to E dc = 0.85 ± 0.03 eV for the partially ordered pyrochlore-type Gd 1.2 La 0.8 Zr 2 O 7 . Electrical conductivity relaxation is well described by a Kohlrausch-Williams-Watts (KWW) function of the form = exp(−(t/τ ) 1−n ), where the fractional exponent n decreases as the La content (ordering) increases. These results are explained in terms of weaker ion-ion interactions in the better ordered structure and highlight the importance of structural ordering/disordering in determining the dynamics of mobile oxygen ions.
Oxygen vacancy ordering and the conductivity maximum in Y2O3-doped CeO2
The defect structure and ionic diffusion processes within the anion-deficient, fluorite structured system Ce1−xYxO2−x/2 have been investigated at high temperatures (873 K – 1073 K) as a function of dopant concentration, x, using a combination of neutron diffraction studies, impedance spectroscopy measurements of the ionic conductivity and molecular dynamics (MD) simulations using interionic potentials developed from ab initio calculations. Particular attention is paid to the short-range ion-ion correlations, with no strong evidence that the anion vacancies prefer, at high temperature, to reside in the vicinity of either cationic species. However, the vacancy-vacancy interactions play a more important role, with preferential ordering of vacancy pairs along the 111 directions, driven by their strong repulsion at closer distances, becoming dominant at high values of x. This effect explains the presence of a maximum in the ionic conductivity in the intermediate temperature range (873 K – 1073 K) as a function of increasing x. The wider implications of these conclusions for understanding the structure-property relationships within anion-deficient fluorite structured oxides are briefly discussed, with reference to complementary studies of yttria and/or scandiaium doped zirconia Zr1−xYxO2−x/2 and Zr1−xScxO2−x/2 published previously.
Relaxation dispersion of ionic conductivity in a Zr0·85Ca0·15O1·85 single crystal
Journal of the European Ceramic Society, 1993
a Abstract The dynamic behavior of the oxygen ion conductivity (?[a cubic Zro.ssCao.150~.8s single co,stal has been investigated with AC impedance spectroscopy and a dynamic pulse method as a function of both temperature and frequency between 450 and 1200 K and 20 and 10 ~ Hz. This is the frequeno,-temperature range where the relaxation dispersion o[the ionic conductivity can be observed. From the temperature dependence o[ the relaxation frequency, the diffusion coefficient and the mobility o['oxygen vacancies were determined. In the entire temperature range investigated, the temperature dependence of the ionic conductivi O, o[a Zro.8.sCao. ~sO~.ss single crystal arises exclusively Ji'om the temperature dependence of the mobility o[" oxygen vacancies, the concentration of which remains constant with temperature and is equal to the concentration of all extrinsic oxygen vacancies created by calcia stabilizing. No transition in the Arrhenius plot of the ionic conductivity due to a gradual dissociation o[oxygen vacancy-defect cation associates, as proposed in the literature, has been observed. A simple model .for the temperature dependence of the ionic conductivity of solid electroh'tes in terms of the parallel and serial combination of" RC-elements is given. Journal gfthe European Ceramic Society
Ionics, 1996
bz situ neutron diffraction studies of CaO and Y203 stabilized zirconia single crystals were performed at elevated temperatures and simultaneously applied DC electric field, i.e. lasting ionic current. Bragg data from Zro.85Ca0.15Oi.85 (CSZ15) were collected at room temperature without electric field, at 1170 K and 1370 K without and with 3.5 V and 1.8 V, respectively, (field vector Eli[Ill]), which generated a current of 60 mA in each case. In case of Zr0.70Y0.30Ol.85 (YSZ15) the electric field vector was directed along [001]. At 1170 K three data sets were collected: without field, with 1.5 V (I =60 mA), and with 2.5 V (I = 120 mA). Atomic displacement parameters (a.d.p.'s) were derived in the frame of a non-Gaussian Debye-Waller factor formalism for the oxygens. Corresponding probability density function (p.d.f.) maps and pseudo potential maps were calculated. Most probable curved diffusion pathways run close to < 100 >, independent of the external field direction, applied voltage and the kind of dopant. With lasting ionic current the potential corresponding to p.d.f. > 1% is lowered by about 0.06-0.07 eV.
THE RELAX.A TION DISPERSION OF IONIC CONDUCTIVITY IN CUBIC Zro.85Cao.1501.85 SINGLE CRYSTALS
The relaxation dispersion of ionic conductivity has been studied in cubic Zro.35Ceo.150i.35 single crystal at 450-1200 Kasa function of the applied electrical field frequency between de and 108 Hz. From the temperature dependence of the relaxation frequency, both the diffusion coefficient and the mobility of oxygen vacancies have been derived. The concentration of mobile charge carriers corresponds to the concentration of all extrinsic oxygen vacancies created by the calcium substitution and remains constant with temperature. No gradual dissociation of immobile defect complexes with temperature, as proposed in the literature, could be established in this temperature region.