Superionic UO₂: A model anharmonic crystalline material

Abstract

Crystalline materials at elevated temperatures and pressures can exhibit properties more reminiscent of simple liquids than ideal crystalline materials. Superionic crystalline materials having a liquid-like conductivity  are particularly interesting for battery, fuel cell, and other energy applications, and we study UO2 as a prototypical superionic material since this material is widely studied given its commercial importance as reactor fuel. Using molecular dynamics, we first investigate basic thermodynamic and structural properties. We then quantify structural relaxation, dynamic heterogeneity, and average ions mobility. We find that the non-Arrhenius diffusion and structural relaxation time of this prototypical superionic material can be quantitatively described in terms of a generalized activated transport model ('string model') in which the activation energy varies in direct proportion to the average string length. Our transport data can also be described equally well by an Adam-Gibbs model in which the excess entropy density of the crystalline material is estimated from specific heat and thermal expansion data, consistent with the average scale of string-like collective motion scaling inversely with the excess entropy of the crystal. Strong differences in the temperature dependence of the interfacial mobility from non-ionic materials are observed, and we suggest that this difference is due to the relatively high cohesive interaction of ionic materials. In summary, the study of superionic UO2 provides insight into the role of cooperative motion in enhancing ion mobility in ionic materials and offers design principles for the development of new superionic materials for use in diverse energy applications. † Corresponding authors: hao.zhang@ualberta.ca; jack.douglas@nist.gov *Official work of the US government-Not subject to copyright in the United States.

Mechanistic Origin of Superionic Lithium Diffusion in Anion-Disordered Li6PS5X Argyrodites

“…The conventional model for ionic diffusion in solid electrolytes assumes that ion transport is effected by a sequence of singleion "hops" between discrete sites [82][83][84], and this model has been assumed in the analysis of diffusion in lithium argyrodites in a number of previous studies [9, 11, 22-25, 28, 41-44, 48, 85]. For many fast-ion solid electrolytes, however, ion transport instead proceeds via collective diffusion processes, whereby multiple ions participate in synchronous cooperative motion [86][87][88][89][90][91][92][93][94][95][96]. Such cooperative motions can be considered a defining characteristic of "superionic" conductivity, in distinction to fast, but conventional, single-particle-hopping [88], and concerted lithium diffusion has recently been proposed to be a contributing factor in the exceptionally high ionic conductivities of Li-excess Li Si, Sn, Ge) argyrodites [48].…”

Anharmonic lattice dynamics of superionic lithium nitride

“…Type-I superionic conductors, such as AgI, exhibit a discontinuous superionic transition, accompanied by a first-order structural phase transition at T s from a poorly-conducting phase to the superionic phase [1]. In contrast, type-II superionic conductors, such as β-PbF 2 , exhibit a continuous superionic transition, accompanied by a second-order phase transition associated with a constant-pressure heat capacity peak at T s [1,8]. For both type-I and type-II superionic conductors, the superionic transition is associated with a marked increase in ionic conductivity as well as a decrease in ionic conductivity activation energy, which is often interpreted as evidence of a qualitative change in diffusion mechanism of the mobile ionic species across the superionic transition.…”

“…Ionic transport in conventional (non-superionic) solids is usually considered as being effected by a sequence of discrete "hops", where individual mobile ions undergo stochastic moves between available crystallographic sites [9,10]. While this model is usually appropriate for ionic solids with low-to-medium ionic conductivities, fast ionic conduction is often associated with highlyconcerted ionic motion, in which groups of ions undergo cooperative near-simultaneous motion [8,11,12,13,14,15]. This observational correlation between fast-ion conduction and cooperative ion-transport mechanisms suggests that ionic motion in superionic conductors might be better described in terms of appropriate collective degrees of freedom, rather than independent single-atom coordinates [16].…”

Measurement of the double-differential neutron cross section of UO2 from room temperature to hot full power conditions

“…The sample is a 4.09(1) cm long cylinder composed of a stack of four depleted UOX pellets of 8.26(1) mm diameter. The total weight of the sample is 23.642(1) g, with a mass-fraction composition in 238 U, 236 U, 235 U, 234 U and 16 O of 87.598(1)%, 0.005(1)% 0.264(1)% 0.002(1)% and 12.131(1)%, respectively. The UO 2 sample was sealed in a glass tube under vaccum and encapsulated in a niobium sample-holder tube.…”

“…It may cause interactions among phonons which are neglected in the present model. Despite anharmonic effects in neutron scattering by crystalline materials were the subject of numerical calculations from the early 1960s [13,14], they still remain an issue to study the resonance line shape of 238 U in UO 2 from room temperature to 1800 K [15], or to explain properties of UO 2 in extreme temperature and pressure conditions [16].…”