Activation Energy For Oxygen Migration Research Articles

Pyrochlore Compounds From Atomistic Simulations.

Pyrochlore compounds (A 2 B 2O7) have a large applicability in various branches of science and technology. These materials are considered for use as effective ionic conductors for solid state batteries or as matrices for immobilization of actinide elements, amongst many other applications. In this contribution we discuss the simulation-based effort made in the Institute of Energy and Climate Research at Forschungszentrum Jülich and partner institutions regarding reliable computation of properties of pyrochlore and defect fluorite compounds. In the scope of this contribution, we focus on the investigation of dopant incorporation, defect formation and anion migration, as well as understanding of order-disorder transitions in these compounds. We present new, accurate simulated data on incorporation of U, Np, Pu, Am and Cm actinide elements into pyrochlores, activation energies for oxygen migration and radiation damage-induced structural changes in these materials. All the discussed simulation results are combined with available experimental data to provide a reliable description of properties of investigated materials. We demonstrate that a synergy of computed and experimental data leads to a superior characterization of pyrochlores, which could not be easily achieved by either of these methods when applied separately.

Oxygen migration in doped BaGdInO4

Doped-BaGdInO4 is a promising material for many applications including solid oxide fuel cells due to its high oxide-ion conductivity. Here we employ atomistic simulations to show that the activation energy for oxygen migration in Ca-doped BaGdInO4 is lower as compared to undoped BaGdInO4 and importantly divalent doping will form oxygen vacancies. The results are consistent with recent experimental work that determined an increase in the diffusivity of Ca-doped BaGdInO4 as compared to the undoped case. Additionally, the most thermodynamically feasible defect in this material is the GdIn anti-site. Promising isovalent dopants on the Ba, Gd and In were found to be the Sr, Al and Sc respectively. The most favourable dopant on the Gd site to produce oxygen vacancies is Ca and its solution energy is lower only by 0.02 eV than that calculated for Sr. Ba2+ ion conduction in this material is slow with the migration energy barrier being higher than 2 eV.

Defect and dopant properties in CaMnO3

CaMnO3-based ceramics have been the subject of considerable research due to their potential application in solid oxide fuel cells, thermoelectric generators, and catalysis. The computational modeling technique based on the classical pair-wise potentials has allowed atomic-scale insights into the defect chemistry, diffusion of Ca2+ and O2− ions, and solution of various dopants in this material. The Ca/Mn anti-site was found to be the most favorable intrinsic defect suggesting disorder, which would be sensitive to synthesis conditions. The second most favorable disorder in CaMnO3 involves loss of CaO, resulting in calcium and oxygen vacancies, which in turn can promote vacancy mediated self-diffusion. The activation energy for oxygen migration (1.25 eV) is much lower than that for calcium (4.42 eV). Favorable isovalent dopants on the Ca and Mn sites were found to be Fe2+ and Ge4+, respectively. The formation of O vacancies can be facilitated by doping of single dopants Fe2+ and Al3+ on the Mn site. Dual dopants Ni–Fe and Al–Ga on the Mn site can also facilitate the introduction of oxygen vacancies required for the vacancy assisted oxygen diffusion.

Effect of oxygen storage/transport capacity of nano-Ce1-xZrxO2 on far-infrared emission property of natural tourmaline

Dispersed nano-Ce1-xZrxO2 (x = 0, 0.25, 0.5, 0.75) successfully grew on the surface of natural tourmaline powders by precipitation method (x = 0) and coprecipitation method (x = 0.25, 0.5, 0.75), respectively. The results of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) about the change in the ratio of Fe3+/Fe inside the tourmaline showed that Ce0.75Zr0.25O2 nanoparticles with a higher oxygen storage/transport capacity (OSC/OTC) could apparently enhance the far-infrared emission property of tourmaline in relation to CeO2, Ce0.5Zr0.5O2 and Ce0.25Zr0.75O2 nanoparticles. This is the first report regarding the effect of the OSC/OTC of nano-Ce1-xZrxO2 on the far-infrared emission property of natural tourmaline. The results of the characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD) and XPS showed that Ce0.75Zr0.25O2 with cubic structure has a higher oxygen evolution capacity (the lowest activation energy for oxygen migration) than CeO2 with cubic structure, Ce0.5Zr0.5O2 and Ce0.25Zr0.75O2 with tetragonal structure, which is beneficial to the unit cell volume shrinkage of tourmaline, thus increasing its far-infrared emissivity.

Cations size mismatch versus bonding characteristics: synthesis, structure and oxygen ion conducting properties of pyrochlore-type lanthanide hafnates

This work describes the synthesis, structural characterization and electrical properties of solid solutions with the general formula Gd2Hf2−xBxO7, where B = Ti4+, Sn4+ and Zr4+. All samples were successfully prepared in ~ 30 h, via a mechanochemical reaction in a planetary ball mill, using the corresponding elemental oxides as starting chemicals. The XRD and Raman spectroscopy analysis of the title samples revealed that on firing at 1500 °C Hf4+ substitution by Sn4+ and Ti4+ produces better ordered pyrochlore structures and decreases the electrical conductivity of Gd2Hf2O7 by more than two orders of magnitude (from 2.7 × 10−4 at 700 °C to 8.71 × 10−7 and 1.12 × 10−6 Sm cm−1, for Gd2Sn2O7 and Gd2Ti2O7, respectively). By contrast, the Gd2Hf2−xZrxO7 system remains disordered with conductivity increasing by almost an order of magnitude and reaching a value for Gd2Zr2O7 of 1.55 × 10−3 Sm cm−1 at 700 °C, whereas the activation energy for oxygen migration decreases in both, the Sn- and Ti-containing systems, and increases slightly in the Zr-containing solid solution. These changes cannot be only explained when taking into account the cations size ratio criteria; the covalency of the metal bond plays also a key role in determining the structural characteristics and electrical properties of the title three systems.

Ionic conductivity of apatite-type rare-earth silicates prepared by mechanical milling

Apatite-type silicates, R E 10 − x ( Si O 4 ) 6 O ′ 2 + y (RE = La, Nd, Gd and Dy), were prepared by mechanical milling (MM) starting from stoichiometric mixtures of the constituent oxides, RE 2O 3 and SiO 2. XRD patterns collected after grinding the starting mixtures in a planetary ball mill for 9 h contain only the characteristic reflections of the target materials. The electrical properties were analyzed on sintered pellets by using impedance spectroscopy and the isothermal conductivity data were successfully fitted to a Jonscher-type empirical expression σ′( ω) ∝ ω n with a decreasing fractional exponent n as the RE 3+ cation size increases. Activation energies for oxygen migration were found to decrease and conductivity to increase as the size of the RE 3+ cation increases with the highest conductivity values obtained for the apatite-type lanthanum silicate. In addition, we also show that the electrical properties of the as-prepared materials are influenced by the silicon source used in their synthesis.

The effect of homovalent A-site substitutions on the ionic conductivity of pyrochlore-type Gd 2Zr 2O 7

We evaluate in this work the effect of Gd substitution on the dc activation energy, E dc, and ionic conductivity of the pyrochlore-type gadolinium zirconate, P-Gd 2Zr 2O 7. Several compositions with the general formulae Gd 2− y Ln y Zr 2O 7 (Ln = Er 3+, Y 3+, Dy 3+, Sm 3+, Nd 3+ and La 3+) were prepared by mechanically milling and firing stoichiometric mixtures of the corresponding elemental oxides and their electrical properties analyzed as a function of frequency and temperature by using impedance spectroscopy. Whereas Gd substitution in P-Gd 2Zr 2O 7 by smaller lanthanides induces a pyrochlore to fluorite phase transition, using larger dopant cations yields partially disordered pyrochlores. Despite of higher structural disorder, ionic conductivity values measured for the fluorite-type materials are lower than those observed for pyrochlores whereas activation energies for oxygen migration in the series decrease monotonically as the average size of the A cation increases.

The Gd2-yLayZr2O7 Solid Solution as a New Electrolyte for High and Intermediate-Temperature SOFC's

Different compositions in the Gd2-yLayZr2O7 solid solution (0 {less than or equal to} y {less than or equal to} 1) were prepared at room-temperature, by mechanically milling stoichiometric mixtures of the corresponding oxides. We found that irrespective of their lanthanum content, as-prepared powder samples consist of single-phase anion deficient fluorite materials, although post-milling thermal treatments at 1200ºC facilitates the long-range ordering of cations and anion vacancies characteristic of pyrochlores for y {greater than or equal to} 0.4. Interestingly, activation energy for oxygen migration in the series decreases from 1.13 {plus minus} 0.02 eV (Gd2Zr2O7) to 0.84 {plus minus} 0.03 eV (GdLaZr2O7) as La-content and the RA/RB ratio increase. These results are explained in terms of weaker ion-ion interactions in better ordered and highlight the importance of structural ordering/disordering in determining the dynamics of mobile oxygen ions.

Lattice effects in cubic La 2Mo 2O 9: Effect of vacuum and correlation with transport properties

This study aims to investigate correlations between lattice effects and transport properties in cubic La 2Mo 2O 9. High temperature neutron diffraction data, recorded in air and under vacuum, are used to follow the evolution with temperature of selected structural parameters, i.e. bond lengths and angles. Results suggest a possible correlation with the experimentally observed decrease of the activation energy for oxygen migration at high temperature. The effect on the structural properties of the low oxygen partial pressure used during the measurements in vacuum is negligible and this represents a valuable information in view of possible applications of the material in solid state devices.

Room-Temperature Synthesis and Electrical Properties of La, Nd and Gd Apatite-type Silicates

AbstractWe show in this paper the possibility of using mechanical milling to prepare apatite-type La, Nd and Gd silicates starting from stoichiometric mixtures of the constituent oxides. XRD patterns collected after grinding the starting mixtures for 9 hours contain only the characteristic reflections of the target materials with no other phase apparently present. Electrical conductivity data were successfully fitted to a Jonscher-type empirical expression with fractional exponent n included in the 0.35-0.75 range. Activation energies for oxygen migration were found to decrease as the size of the rare-earth cation increases. Therefore, the highest conductivity values were found for the apatite-type lanthanum silicate.

Oxygen Diffusion in Yttria‐Stabilized Zirconia: A New Simulation Model

We present a multiscale modeling approach to study oxygen diffusion in cubic yttria‐stabilized zirconia. In this approach, we employ density functional theory methods to calculate activation energies for oxygen migration in different cation environments. These are used in a kinetic Monte Carlo framework to calculate long‐time oxygen diffusivities. Simulation results show that the oxygen diffusivity attains a maximum value at around 0.1 mole fraction yttria. This variation in the oxygen diffusivity with yttria mole fraction and the calculated values for the diffusivity agree well with experiment. The competing effects of increased oxygen vacancy concentration and increasing activation energy and correlation effects for oxygen diffusion with increasing yttria mole fraction are responsible for the observed dopant content dependence of the oxygen diffusivity. We provide a detailed analysis of cation‐dopant‐induced correlation effects in support of the above explanation.

Oxygen migration in A2B2O 7 pyrochlores

Atomic scale computer simulation has been used to predict activation energies for oxygen migration. In total, 54 compounds with the A 2 B 2O 7 pyrochlore structure were simulated. In each case, oxygen migration was assumed to proceed via an oxygen vacancy mechanism with oxygen ions hopping between 48f sites. For some compounds the unoccupied 8a interstitial position played an important role in the migration mechanism. The results were analyzed using a contour map of activation energy versus A cation radius along the ordinate and B cation radius along the abscissa. This identified areas of similar cation radii, which exhibit lower activation energy. Results compare favourably with available experimental data.

Bulk Reduction and Oxygen Migration in the Ceria-Based Oxides

Cubic solid solutions of general formula Ce1-xMxO2 (M = Zr, Th, Hf) have been modeled in the range (0 < x < 1) using atomistic simulation methods. The Ce4+/Ce3+ reduction energy in the bulk materials and the activation energy for oxygen migration have been calculated. The Ce4+/Ce3+ reduction energy decreases with increasing M content for M = Zr, Th, the most remarkable effect being displayed for M = Th. An opposite trend is obtained when M = Hf. The activation energy for oxygen migration decreases with increasing M content. The effect of Th is similar to that of Zr and consists of a substantial decrease of the activation energy at high concentrations. On the other hand, the activation energy decreases only slightly when M = Hf.

Structural and Oxygen Storage/Release Properties of CeO2-Based Solid Solutions

The use of mixed-oxides containing CeO2 as oxygen storage/release components is discussed with special focus on the applications of these materials in auto-exhaust catalytic converters. Ceria easily forms solid solutions with transition-metal/rare-earth oxides over a wide composition range. The incorporation of dopants like Zr4+, Pr3/4+, Tb3/4+ into the cubic fluorite lattice of CeO2 strongly affects the structural and energetic properties of the materials by lowering activation energy for oxygen migration and by increasing reducibility of the cerium cation. This augments both total and kinetic oxygen exchange between the solid and the gas phase under conditions typically encountered in real systems.

Computer Simulation Studies of Bulk Reduction and Oxygen Migration in CeO2−ZrO2 Solid Solutions

Computer simulation techniques have been used to model cubic CeO2−ZrO2 solid solutions in the whole composition range. Aspects related with the oxygen storage capacity of these materials are emphasized. The energetics of the Ce4+/Ce3+ bulk reduction reaction as well as the activation energy for oxygen migration in the lattice are investigated and compared with the corresponding quantities in pure CeO2. It is found that even small additions of ZrO2 decrease the bulk reduction energy of Ce4+ to values comparable to those reported for surface reduction in pure CeO2. Activation energy calculations indicate an almost monotonic increase of oxygen mobility with increasing zirconia content.