Defect structure of yttria-stabilized zirconia and its influence on the ionic conductivity at elevated temperatures (original) (raw)
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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.
Structural Disorder in Doped Zirconias, Part I: The Zr0. 8Sc0. 2− x Y x O1. 9 (0.0≤ x≤ 0.2) System
Chemistry of …, 2011
The influence of local ordering of the anion vacancies and cation-anion vacancy interactions on the ionic conductivity of the aniondeficient fluorite Zr 0.8 Sc 0.2-x Y x O 1.9 (0.0 e x e 0.2) system have been investigated using impedance spectroscopy, molecular dynamics (MD) simulations, and reverse Monte Carlo (RMC) analysis of neutron powder diffraction data. At 1000 K, the ionic conductivity decreases by a factor of ∼2 as x increases from 0.0 to 0.2, while the oxygen anion partial radial distribution function, g OO (r), remains similar across the entire solid solution, even though the cation-oxygen interactions change with increasing Y 2 O 3 content. These experimental data are used to validate the MD simulations, which probe the details of the vacancy-vacancy interactions within the x = 0.0 and x = 0.2 end members. Both possess similar vacancy-vacancy ordering that favors the formation of pairs along AE111ae directions. Significantly, an increased proportion of the oxygen vacancies are associated with the Zr 4þ cations in Zr 0.8 Y 0.2 O 1.9 , while in Zr 0.8 Sc 0.2 O 1.9 they show no significant preference for being nearest neighbor to a Sc 3þ or a Zr 4þ cation. Thus, it is concluded that the lower ionic conductivity at x = 0.2 is predominantly a consequence of the larger size of the Y 3þ cation, which induces strain in the lattice and hinders diffusion of the O 2-, rather than changes in the local ordering of the anion vacancies.
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
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.
SOLID STATE IONIC MATERIALS World Scientific Global Publicii'tions Services
AOSTRACT Tk electrical conduc.tivity of sin1crcd Zro.ss~1t-O. l~01. 8 s ceramics anoC31od for 0-52 hc~rs at lOOO °C has been in\'CStiJPllcd by complex impedance spectroscopy bct\\ttn 450· tl 70 K. The annc111in& at 10()0 • c cau$C$ the dco;in\position or the cubic phase. The procipitatcs of the second phase suppress 1hc local conductivity of the ocramics at high 1ernpctalurcs. A cha. ngc in the slope or the Arthcnius ploLS of 1hc to1al conductivity can be obscMXl 11tis behaviour of tho ccr:unic wmpositc t3R be modcUod by a series c«mbiu.alion or two resistors. I. Jntroduction 155 The Arrhenius plots of the tolal (de) conductivity of zirconia·based solid eleclrolytes often show a bend al 800· 1200 K due to the decrease in the activation energy al high tempera1urcs (1·2]. To explain this behaviour, ii has been suggested in lhe literature (3] tl1at the concentration of charge carriers in z.irconia electrolytes is diminished al low temperatures due to the association of mobile oxygen vacancies with dopant atoms. However, in our studies [ 4-6) of the relaxation dispersion of the ionic conduc1ivi1y of zirconia electrolytes, it has been shown that the concentration of mobile extrinsic oxygen vacancies rcn1ains constant with temperature and is equaJ co the concentration of all cxtrinsk vacancies created by doping. Slope changes in Arrhenius plots may also be associated with the presence of a second phase. Ciacchi and Oadwal (7] has shown that even in the 8mol% yllria stabilised zirconia a redistribution of the dopant level allows the precipita1ion of the tetragonal phase on annealing. Ikeda et al. [8) have analysed the equivalent circuit fo r lhe two-phase mixture of cubic and tetragonal phases. Jn ZiQ2-MgO system, quenched cubic solid solutions decompose on annealing below t'1c eutcctoid temperature of l400 °C [9]. The cubic phase which is in equilibrium with telragonal precipitates at the eutectoid temperature conlains 14 molo/o MgO. According to the Viechnicki and Stubican [ I OJ. the decomposition of the cubic phase compri • cd formation of nuclei of the tetragonal phase and diffusion of Mg atoms out of the regions of the growth. Although the decomposition rate reaches a maximum al 1200 • c. a gradual formation of the tetragonal modificalion was observed even at 500 • c.
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.
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
Chemistry of …, 2011
Polarizable interaction potentials, parametrized using ab initio electronic structure calculations, have been used in molecular dynamics simulations to study the conduction mechanism in doped zirconias. The influence of vacancy-vacancy and vacancy-cation interactions on the conductivity of these materials has been characterized. Although the latter can be minimized by using dopant cations with radii which match those of Zr 4þ (as in the case of Sc 3þ ), the former appears as an intrinsic characteristic of the fluorite lattice that cannot be avoided and that 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 confirms that this material is likely to present the highest conductivity achievable in zirconias.
Oxide ion transport in highly defective cubic stabilized zirconias
Ionics, 1995
Ac impedance spectroscopy and neutron powder diffraction have been used to study the high temperature behaviour of defective fluorite solid electrolytes. In yttria-stabilised zirconia with an yttrium content of 15 mol% YOI.5 there is a marked change in conductivity behaviour at around 650 ,)(2, with a decrease in activation energy of 0.15 eV. Structural studies confirm that this is due to a change in the bulk of lhe sample with the disappearance of diffuse scattering peaks and marked changes in the behaviour of the isotropic temperature factors at the same temperature. These results indicate that the change in activation energy of yttria-stabilised zirconia at 650 ~ is due to an order-disorder transition involving local defect dusters. In studies of zirconia co-doped with yttrium and niobium, activation energy fox conduction is found to rapidly increase with the concentration of the trivalent yttrium Saturation doping is reached at about 20-30 % of yttrium and activation energy only increases slightly with doping. Introduction of pentavalent niobium at this level of doping serves to decrease activation energy, although it also decreases conductivity slightly. The low and high temperature activation energies converge as the saturation regime is approached. These observations seem to suggest that ordering of defect clusters into microdomains increases activation energy for ionic motion. At low defect concentrations and high temperatures, this local ordering breaks down and the activation energy for conduction decreases.