Collective dynamics in superionic CaF2. I. Simulation compared with neutron-scattering experiment

Multiscale modeling of structure formation of C60 on insulating CaF2 substrates

Morphologies of adsorbed molecular films are of interest in a wide range of applications. To study the epitaxial growth of these systems in computer simulations requires access to long time and length scales, and one typically resorts to kinetic Monte Carlo (KMC) simulations. However, KMC simulations require as input transition rates and their dependence on external parameters (such as temperature). Experimental data allow only limited and indirect access to these rates, and models are often oversimplified. Here, we follow a bottom-up approach and aim at systematically constructing all relevant rates for an example system that has shown interesting properties in experiments, buckminsterfullerene on a calcium fluoride substrate. We develop classical force fields (both atomistic and coarse-grained) and perform molecular dynamics simulations of the elementary transitions in order to derive explicit expressions for the transition rates with a minimal number of free parameters.

Optimization of a new interatomic potential to investigate the thermodynamic properties of hypo-stoichiometric mixed oxide fuel U1-yPuyO2-x

The behaviour of stoichiometric U1-yPuyO2compounds used as nuclear fuel is relatively well understood. Conversely, the effects of stoichiometry deviation on fuel performance and fuel stability are intricate and poorly studied. In order to investigate what affect these have on the thermophysical properties of hypo-stoichiometric U1-yPuyO2-xmixed oxide fuel, new interaction parameters based on the many-body CRG (Cooper-Rushton-Grimes) potential formalism were optimized. The new potential has been fitted to match experimental lattice parameters of U0.7Pu0.3O1.99(O/M = 1.99) and U0.7Pu0.3O1.97(O/M = 1.97), where M represents the total amount of metallic cations, through a rigorous procedure combining classical molecular dynamic and classical molecular Monte Carlo simulation methods. This new potential provides an excellent description of the U1-yPuyO2-x system. Concerning lattice parameter, although fitted on only one Pu content (30%) and two stoichiometries (1.99 and 1.97), our potential allows good predictions compared to available experimental results as well as to available recommendations in wide ranges of O/M ratio, Pu content and temperature. For the U0.7Pu0.3O2-xhypo-stoichiometric system (30% Pu content and O/M ratio ranging from 1.94 to 2.00), some direct properties (lattice parameter and enthalpy) and some derivative properties (linear thermal expansion coefficient and specific heat) were systematically investigated from room temperature up to the expected melting temperatures and a good agreement with experiments is found. Moreover, our potential shows a good transferability to the plutonium sesquioxide Pu2O3system.

Low Dimensional String-like Relaxation Underpins Superionic Conduction in Fluorites and Related Structures

Among the superionic conductors that show a Faraday transition – the continuous increase in the ionic conductivity over a range of temperatures – the fluorite structures have enjoyed incisive examinations over the past four decades; yet the fundamental nature of superionicity has remained largely inconclusive. Departing from the traditional quasi-static defect framework, we provide weighty evidence for string-like dynamical structures that govern the fast ion conduction process in fluorites. We show that lower temperatures encourage the growth of longer but slowly relaxing strings and vice-versa – a direct manifestation of heterogeneous dynamics. Remarkably, the ionic conductivity is inversely correlated to the lifetime of the ions that participate in the strings and not explicitly to the ion population. Our analysis methodology, which resolves a long-standing disagreement on defect structures and the mechanism of ionic transport in fcc fluorite structures, is well-positioned to describe the dynamics of low dimensional conduction in a larger class of superionic conductors.

Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials

Mechanocaloric materials experience a change in temperature when a mechanical stress is applied on them adiabatically. Thus, far, only ferroelectrics and superelastic metallic alloys have been considered as potential mechanocaloric compounds to be exploited in solid-state cooling applications. Here we show that giant mechanocaloric effects occur in hitherto overlooked fast ion conductors (FIC), a class of multicomponent materials in which above a critical temperature, Ts, a constituent ionic species undergoes a sudden increase in mobility. Using first-principles and molecular dynamics simulations, we found that the superionic transition in fluorite-structured FIC, which is characterized by a large entropy increase of the order of 10(2) JK(-1) kg(-1), can be externally tuned with hydrostatic, biaxial, or uniaxial stresses. In particular, Ts can be reduced several hundreds of degrees through the application of moderate tensile stresses due to the concomitant drop in the formation energy of Frenkel pair defects. We predict that the adiabatic temperature change in CaF2 and PbF2, two archetypal fluorite-structured FIC, close to their critical points are of the order of 10(2) and 10(1) K, respectively. This work advocates that FIC constitute a new family of mechanocaloric materials showing great promise for prospective solid-state refrigeration applications.

Superionicity and polymorphism in calcium fluoride at high pressure

We present a combined experimental and computational first-principles study of the superionic and structural properties of CaF_{2} at high P-T conditions. We observe an anomalous superionic behavior in the low-P fluorite phase that consists of a decrease of the normal → superionic critical temperature with compression. This unexpected effect can be explained in terms of a P-induced softening of a zone-boundary X phonon that involves exclusively fluorine displacements. Also we find that superionic conductivity is absent in the high-P cotunnite phase. Instead, superionicity develops in a new low-symmetry high-T phase that we identify as monoclinic (space group P2_{1}/c). We discuss the possibility of observing these intriguing phenomena in related isomorphic materials.

Screening and strain in superionic conductors

I calculate the screening length and its relation to elastic constants from mean-field models of superionic conductors. The Debye formula for a continuous fluid's screening length is replaced by one that accounts for the discrete nature of the lattice. Interactions alter the screening length and in some cases give oscillations in the charge structure. The screening lengths derived here are exact not only at low temperature where there are few defects, but also at high temperature where the effective interaction strength is small. The mean-field treatment correctly gives the decrease in an elastic constant observed at the superionic transition for a variety of crystals. This decrease is best explained by a change in the defect creation entropy with density. The effect of doping on the elastic constant also agrees well with experiment. The extension to the mean-field models given here provides a consistent picture of the response of a superionic crystal to electrical, mechanical and thermal stresses, as well as to doping.

A model for the onset of fast-ion conduction in fluorites

The quasielastic neutron scattering from fast-ion conductors with the fluorite structure has been interpreted alternatively as being due to either clusters of defects or to the motion of defects and their associated strain field. In this paper, we examine the hypothesis that the sharp rise in ionic conductivity that occurs in fluorites near the transition temperature ( T c ) from normal to superionic conductivity, is associated with defect clustering. By using lattice simulation techniques it is shown that a wide variety of clusters are stable in SrCl 2 , CaF 2 , SrF 2 , BaF 2 and β-PbF 2 . A cluster consisting of two vacancies, two relaxed lattice ions and three interstitials (which we shall designate 2| 2| 3) is particularly stable. It is shown that the formation of 2| 2| 3 clusters results in an increase in the concentration of vacancies and that this increase can account quantita­tively for the rise in conductivity near T c . Our concern is thus with the possible connection of the onset of superionic conduction with defect clustering and we do not attempt, in this paper, any interpretation of the quasielastic neutron scattering.