C.R.A. Catlow

Dopant ion radius and ionic conductivity in cerium dioxide

Abstract In this paper we examine, using atomistic computer simulations, the effect of dopant ion radius on dopant-vacancy interactions in CeO2. We show that the variation in this interaction is important in determining the ionic conductivity of the material. We compare our theoretical results with those obtained experimentally. We also consider the effect of the dopant ion charge on the magnitude of the interaction energy and we examine the stability of larger clusters in which the oxygen vacancy has up to four surrounding dopant substitutionals.

Transport in doped fluorite oxides

Abstract Theoretical techniques are used to examine the effects of dopant ion radius and dopant concentration on the conductivity of doped fluorite oxides. In the former case we demonstrate a substantial effect; our results are in line with recent data of Gerhardt-Anderson and Nowick. We are also able to suggest an explanation of the maximum in the plot of conductivity versus dopant concentration which is marked feature of these systems. In addition, we comment on the relative stabilities of point defects and microdomains in heavily doped crystals.

Trapping and solution of fission Xe in UO2.: Part 1. Single gas atoms and solution from underpressurized bubbles

Abstract We present results of a new theoretical survey of the energetics of fission Xe in UO 2 , in which improved gas-lattice potentials are employed, and calculations performed for a range of temperatures. We concentrate on single gas atom trapping and migration and their solution from underpressurized bubbles.

A new hybrid scheme of computer simulation based on Hades and Monte Carlo: Application to ionic conductivity in Y3+ doped CeO2

Abstract The computer method of Lattice Statics (HADES/CASCADE) was used to calculate anion migration energies for 30 unique local environments involving the distribution of Y 3+ ions in CeO 2 . The Monte Carlo method was then used to statistically sample the energies in order to calculate the d.c. ionic conductivity. The general trends of the experimental behavior of the ionic conductivity and associated activation energy were followed.

Static lattice simulation of structure and transport in superionic conductors

Abstract We describe the scope, methodology and achievements of static lattice simulation methods in the study of fast-ion conductors. We discuss first the type of conduction mechanism to which simulations may be applied; we contrast the information available from static with that from dynamics simulations. The techniques and potentials used by the calculations are then described. Applications to β-Al2O3, Li3N and fluorite structured compounds are then considered.

Study of Sc2O3-doped ceria by anelastic relaxation

Abstract Anelastic relaxation experiments on Sc 3+ -doped ceria were undertaken in order to better understand the origin of the very high Sc-V 0 association energy obtained from conductivity data (where V 0 = oxygen vacancy). Instead of a simple anelastic peak due to the relaxation of pairs, as found for Y 3+ -doped ceria, a broad peak (made up of at least 2 Debye peaks) was found in the range of activation enthalpy H r = 0.45 to 0.54 eV. Unexpectedly, an additional small peak was observed at lower temperatures, for which H r = 0.21 eV . The dependence of this peak on double doping (i.e. with both Sc 3+ and Y 3+ ) suggests that it is due to Sc Ce defects, which would have to go off-center in order to account for the relaxation. Computer simulation calculations have demonstrated a modified off-center behavior in which nn O 2- ions are displaced asymmetrically to a stable dipolar configuration.

An EXAFS study of the structure of rubidium polyethyleneoxide salt complexes

Abstract EXAFS measurements on the Rb K edge of RbSCN(PEO) x and RbI(PEO) x (x=4,88) for temperatures in the range 80K to 453K have been performed to gain information about the local structural environment of Rb in these complexes. It has been shown that Rb adopts well-defined sites of four co-ordination with ether oxygens, the actual configuration being anion-dependent. For the iodide samples, there are two short and two long Rb-O bonds with the iodide situated outside the first co-ordination shell at 3.7A. The thiocyanate complexes have a single shell of oxygen neighbours and it is suggested that the N atom of the thiocyanate group is also located within the first shell. No anomalous change in thermal motion, nor any change in cation site geometry is observed on heating.

Trapping and solution of fission Xe in UO2: Part 2. Solution from small overpressurized bubbles

Abstract We present results of calculations of formation energies of small aggregates, or bubbles, of Xe atoms in UO2. This is followed by a study of the energetics of resolution of gas atoms from bubbles into the UO2 lattice.

Association energies in Re3+-doped alkaline-earth fluorides studied by computational methods

Abstract Association energies of nearest-neighbour and next-nearest-neighbour associates between substitutional, trivalent rare-earth ions and interstitial fluoride ions CaF 2 , SrF 2 and BaF 2 are obtained by lattice simulation calculations. The dopant ion-fluoride ion interaction is described (i) with a set of potentials obtained with electron gas methods, and (ii) with a set of potentials derived semi-empirically from the host lattice cation-anion interaction potentials. The calculations successfully simulate the experimentally observed variations of the dopant-interstitial binding energies with the radius of the dopant ion, and with the lattice parameter of the host. The better quantitative agreement is obtained with the semi-empirical potentials. The variations are explained by an evaluation of the displacements of the ions constituting the associates.

The defect structure of calcia stabilized zirconia

Abstract Theoretical techniques have been used to examine the two models proposed for the defect structure of stabilized zirconias. The first model is based on simple clusters of dopant ions and the charge compensating oxygen vacancies, while the second suggests that fluorite-related microdomains form within the host lattice. We show that the energies of formation of the clusters and microdomains are almost equal, the microdomain being favoured by 0.09 eV per dopant ion. Thus the calculations suggest that both point defects and microdomains may be present, with point defects predominating at low dopant concentration and higher temperatures.