THE CALCULATION OF HUGONIOTS IN IONIC SOLIDS

The authors demonstrate the quantitative prediction of Hugoniots for an ionic crystal (NaCl) using the shell model with the best available potentials. The calculations show that these shock wave data can be predicted quite accurately by relatively simple use of established computer codes. The Hugoniot results, together with static phase-change data, also provide a very severe test of interatomic potentials. The two sets of empirical potentials ((i) Catlow, Diller and Norgett (1977), (ii) Sangster and Atwood (1978)) both perform much better than electron gas potentials. The same approach can be used to test potentials and to make predictions for the behaviour of solids for extreme conditions of temperature and pressure

CATION DIFFUSION IN ALKALINE-EARTH OXIDES

Absolute jump rates for cation-vacancy interchanges in MgO, CaO, SrO and BaO are calculated from a set of model inter-ionic potentials. Internal energies and vibrational entropies over a wide range of temperatures (i.e. at expansions which within the models correspond to these temperatures in the quasi-harmonic approximation) are evaluated and, from these, migration enthalpies and pre-exponential frequency factors are deduced. Correlations between these two diffusion parameters for the family of oxides are investigated

Atomistic modelling of the metal/oxide interface with image interactions

We calculate the interfacial energy and lowest energy relative position for an Ag (001)/MgO (001) interface. The dominant image terms and short-range repulsions are included in full, and the MgO ions are relaxed to equilibrium using the MIDAS code. An essential new feature is the suppression of charge density fluctuations with wave-vectors greater than a (Fermi wavevector) cutoff. Our results show that the powerful methods based on interatomic potentials, widely used for ionic systems, can be extended to metal/ionic interfaces

Thermodynamic properties of uranium dioxide: Electronic contributions to the specific heat

It has recently been proposed that the anomalous specific heat of uranium dioxide be ascribed to the effect of electronic defects rather than Frenkel disorder on the union sub-lattice. We here present calculations showing that the entropy contribution from electronic defects is large enough to make a major contribution to the specific heat whereas the contribution from Frenkel defects is much smaller

A calculation of the structure and energy of the Nb/Al2O3 interface

We have modelled the (111)(Nb)/(0001)(s)Nb/Al2O3 interface using an atomistic, static lattice simulation technique. The interaction between the metal and the oxide combines the short range interaction between the metal atoms and the oxide ions, the Coulomb interaction between the oxide ions and the induced image charge of the metal, and the energy required to immerse the ionic cores in the metal jellium. The short range interaction between the Al3+ ion and the Nb atom was found to be repulsive, but the O2-/Nb interaction was found to be attractive at separations greater than 0.23 nm. As a result the lowest energy interface was found to terminate on an oxygen plane of the Al2O3; crystal, with the Nb atoms placed over the vacant sites in the Al lattice. The interfacial energy of this interface was calculated to be -3.6 J/m(2). As in previous work the results agree well with LDF calculations. The calculated structure is also in good agreement with the interpretation of the HREM images of Nb films grown on the (0001) face of Al2O3 using Molecular Beam Epitaxy. Copyright (C) 1996 Acta Metallurgica Inc

Vibrational pocket modes: predictions by the embedded crystallite method and their experimental observation

Simulation studies based on the embedded crystallite method are used to predict, with no free parameters, complex dynamical behavior for a simple alkali halide defect system, Na+ in KI. Far infrared spectroscopic measurements, including uniaxial stress, confirm the predicted vibrational properties, indicating that this methodology can readily be used for complex and extended defects in ionic crystals

The dielectric constant of UO2 below the Néel point

We report measurements of the frequency-dependent dielectric constant of UO2 from 4.2 K to above the phase transition at 30 K. The static dielectric constant of 23.6 at 4.2 K is comparable with accepted values at higher temperatures: it is essentially identical in both phases. The effects of undergoing the transition on the dielectric constant are marginal (about 1%) and take place in the temperature range 29 K to 37 K. The displacement of the oxygen sublattice, which occurs at the Ne´el point, should produce only a 0.05% change on the dielectric constant and of the opposite sense to that measured. Hence the structural changes at the transition are not the primary source of the observed small difference between the dielectric constant in the two phases which probably accrues from the influence of the displacements on a defect-related contribution

The pressure dependence of the dielectric constant and electrical conductivity of single crystal uranium dioxide

Complex impedance techniques, within the frequency range 10 Hz to 1 MHz, have been used to make high pressure studies of monocrystalline uranium dioxide at ambient temperature. These techniques have shown that for frequencies below 40 MHz the electrical properties of high pressure samples are dominated by a boundary layer. The impedance methods have enabled us to make the first determination of the pressure dependence of the static dielectric constant of uranium dioxide within the boundary layer. The experimental pressure dependence (−0.03 kbar−1) is in reasonable agreement with that calculated (−0.02 kbar−1) using standard interatomic potentials. We have also measured the conductivity in the boundary layer as a function of pressure (2.5 μS kbar−1). The pressure dependences of the conductivity and the dielectric constant have been used to obtain an estimate of the carrier binding and hopping energies, which have then been compared with values predicted using the shell model

The electrical impedance of single-crystal urania at elevated temperatures

The electrical admittance of single-crystal urania has been measured from 300 to 1500 K, over a frequency range of 10 Hz to 10 MHz, using complex impedance spectroscopy. The data have been analyzed using a simple equivalent circuit of a parallel element comprising a conductance and a capacitance connected in series with a separate capacitance. The simple equivalent circuit also reanalyzes successfully the frequency dependence of the electrical conductivity found by Bates and his co-workers, giving results consistent with the present work. The conductance data show a distinct “kink” at about 1300 K, which is in good agreement with previous work, as are the activation energies: 0.12 eV (T 1300 K). Results are used to estimate the ambipolar contribution to the thermal conductivity above 1500 K

Comparative theoretical study of the Ag-MgO (100) and (110) interfaces

We have calculated the atomic and electronic structures of Ag-MgO(100) and (110) interfaces using a periodic (slab) model and an ab initio Hartree-Fock approach with a posteriori electron correlation corrections. The electronic structure information includes interatomic bond populations, effective charges, and multipole moments of ions. This information is analyzed in conjunction with the interface binding energy and the equilibrium distances for both interfaces for various coverages. There are significant differences between partly covered surfaces and surfaces with several layers of metal, and these can be understood in terms of electrostatics and the electron density changes.For complete monolayer (1:1) coverage of the perfect MgO(100) surface, the most favorable adsorption site energetically for the Ag atom is above the surface oxygen. However, for partial (1:4) coverage of the same surface, the binding energies are very close for all the three likely adsorption positions (Ag over O, Ag over Mg, Ag over a gap position),For a complete (1:1) Ag monolayer coverage of the perfect MgO(110) interface, the preferable Ag adsorption site is over the interatomic gap position, whereas for an Ag bilayer coverage the preferred Ag site is above the subsurface Mg2+ ion (the bridge site between two nearest surface O2- ions). In the case of 1:2 layer coverage, both sites are energetically equivalent. These two adhesion energies for the (110) substrate are by a factor of two to three larger than over other possible adsorption sites on perfect(110) or (100) surfaces.We compare our atomistic calculations for one to three Ag planes with those obtained by the shell model for 10 Ag planes and the Image Interaction Model addressing the case of thick metal layers. Qualitatively, our ab initio results agree well with many features of these models. The main charge redistributions are well in line with those expected from the Image Model. There is also broad agreement in regard to orders of magnitude of energies. (C) 1999 Elsevier Science B.V. All rights reserved