Fluorite-structured Materials Research Articles

Enhanced radiation damage tolerance in Zr-doped UO2

This study explores the effect of tetravalent Zr doping on the radiation behaviour of UO2 through a combination of experimental and theoretical approaches. The intrinsic changes that Zr introduces in UO2 were quantified using X-ray diffraction and Raman spectroscopy, which reveal a shrinkage of the lattice volume and the formation of ZrO8-type clusters. Heavy-ion irradiation was carried out on both undoped and doped UO2 under conditions similar to the ballistic regime of fission products in nuclear fuels. Empirical data, together with DTF+U simulations, found that Zr doping modifies the irradiation-induced defect mechanisms by enabling recombination pathways, allowing a rapid recovery of the UO2 lattice. The fundamental mechanisms involving the role of dopant in modifying the radiation damage kinetics are discussed in this paper, as well as the subsequent evolution in fluorite-structured materials relevant to nuclear fuels.

Phonon modal analysis of thermal transport in ThO2 with point defects using equilibrium molecular dynamics

Defects can significantly degrade the thermal conductivity of ThO2, an advanced nuclear fuel material as well as a surrogate for other fluorite-structured materials. We investigate how point defects in ThO2 impact phonon mode-resolved thermal transport. By incorporating phonon modes from lattice dynamics, we decompose the trajectory and heat flux to phonon normal mode space and extract key phonon properties, including phonon relaxation times and their contributions to thermal conductivity. We implement two methods. The first method is based on the Green Kubo formalism to resolve the contribution of each phonon mode to thermal conductivity. The second resolves the lifetime of individual phonon modes and the thermal conductivity is calculated using the Boltzmann transport equation within relaxation time approximation. Notably, a lower contribution of acoustic modes is revealed compared to perturbative approaches considering only three-phonon scattering processes. The effects of four types of point defects are evaluated. The strongest impact on a reduction in thermal conductivity is from Th interstitials, followed by Th vacancies. O interstitials/vacancies have a similar impact, albeit smaller than defects on the thorium sublattice. These observations are consistent with previous studies.

Comparative performance of fluorite-structured materials for nanosupercapacitor applications

Over the last fifteen years, ferroelectric (FE) and antiferroelectric (AFE) ultra-thin films based on fluorite-structured materials have drawn significant attention for a wide variety of applications requiring high integration density. AFE ZrO2, in particular, holds significant promise for nanosupercapacitors, owing to its potential for high energy storage density (ESD) and high efficiency (η). This work assesses the potential of high-performance Hf1−xZrxO2 thin films encapsulated by TiN electrodes that show linear dielectric (LD), FE, and AFE behavior. A wake-up effect is observed for AFE ZrO2, a phenomenon barely reported for pure zirconium oxide and AFE materials in general, correlated with the disappearance of the pinched hysteresis loop commonly observed for Zr-doped HfO2 thin films. ESD and η are compared for FE, AFE, and LD samples at the same electrical field (3.5 MV/cm). As expected, ESD is higher for the FE sample (95 J/cm3), but η is ridiculously small (≈55%) because of the opening of the FE hysteresis curve, inducing high loss. Conversely, LD samples exhibit the highest efficiency (nearly 100%), at the expense of a lower ESD. AFE ZrO2 thin film strikes a balance between FE and LD behavior, showing reduced losses compared to the FE sample but an ESD as high as 52 J/cm3 at 3.5 MV/cm. This value can be further increased up to 84 J/cm3 at a higher electrical field (4.0 MV/cm), with an η of 75%, among the highest values reported for fluorite-structured materials, offering promising perspectives for future optimization.

Switching Dynamics and Energy Storage Properties of Fluorite‐Structured Materials

Fluorite-Structured Materials Recently, ferroelectric and antiferroelectric fluorite-structured materials (HfO2 and ZrO2) have gotten much attention for memory- and energy- related applications because of being suitable for CMOS compatibility and ultra-low deposition thickness. In article 2200403, Faizan Ali, Muhammad Faisal Iqbal, and colleagues discuss the frequency-dependent phase switching and energy storage properties of HfO2-based thin films.

Novel Fluorite-Structured Materials for Solid-State Refrigeration.

Refrigeration based on the electrocaloric effect can offer many advantages over conventional cooling technologies in terms of efficiency, size, weight, and power source. The discovery of ferroelectric and antiferroelectric properties in fluorite-based materials in 2011 has led to diverse applications related to memory (e.g., ferroelectric tunnel junctions, nonvolatile memory, and field-effect transistors) and energy fields (e.g., energy storage and harvesting, electrocaloric refrigeration, and infrared detection). Fluorite-based materials exhibit several properties not shared by most conventional materials (such as in terms of compatibility with complementary metal-oxide semiconductors and 3D nanostructures, deposition thickness at the nanometer scale, and simple composition). Here, the electrocaloric refrigeration properties of fluorite-based ferroelectric/antiferroelectric materials are reviewed by focusing on the advantages of ZrO2 - and HfO2 -based materials (e.g., relative to conventional perovskite- and polymer-based counterparts). Finally, the recent progress made in this research field are also discussed along with its future perspectives.

Investigating finite-size effects in molecular dynamics simulations of ion diffusion, heat transport, and thermal motion in superionic materials.

The effects of the finite size of the simulation box in equilibrium molecular dynamics simulations are investigated for prototypical superionic conductors of different types, namely, the fluorite-structure materials PbF2, CaF2, and UO2 (type II), and the α phase of AgI (type I). Largely validated empirical force-fields are employed to run ns-long simulations and extract general trends for several properties, at increasing size and in a wide temperature range. This work shows that, for the considered type-II superionic conductors, the diffusivity dramatically depends on the system size and that the superionic regime is shifted to larger temperatures in smaller cells. Furthermore, only simulations of several hundred atoms are able to capture the experimentally observed, characteristic change in the activation energy of the diffusion process, occurring at the order-disorder transition to the superionic regime. Finite-size effects on ion diffusion are instead much weaker in α-AgI. The thermal conductivity is found generally smaller for smaller cells, where the temperature-independent (Allen-Feldman) regime is also reached at significantly lower temperatures. The finite-size effects on the thermal motion of the non-mobile ions composing the solid matrix follow the simple law that holds for solids.

Defect chemistry of LaGaO3 doped with divalent cations

Solid oxide fuel cells are highly efficient energy conversion devices and can produce electrical energy from a variety of fuels. One of the main challenges is to decrease its operating temperatures, since the high temperatures currently required for satisfactory oxide-ion conduction at the electrolyte decrease the lifetime of the device. LaGaO3-based materials possess high conductivities in lower temperatures when compared to traditional fluorite-structured materials. In this study, density functional theory calculations were performed to investigate the potential effects of divalent dopants in the ionic conductivity of the perovskite. The most suitable dopants were determined, with steric and electronic effects being found to play a role in how easily these dopants are accepted into the lattice. The distortion of the doped perovskite local structure was observed to be an indicator of the tendency of vacancies to be trapped around dopants, possibly hindering ionic conductivity.

Predicting the Structure of Grain Boundaries in Fluorite-Structured Materials

Interfaces are a type of extended defect which govern the properties of materials. As the nanostructuring of materials becomes more prevalent the impact of interfaces such as grain boundaries (GBs) becomes more important. Computational modelling of GBs is vital to the improvement of our understanding of these defects as it allows us to isolate specific structures and understand resulting properties. The first step to accurately modelling GBs is to generate accurate descriptions of the structures. In this paper, we present low angle mirror tilt GB structures for fluorite structured materials (calcium fluoride and ceria). We compare specific GB structures which are generated computationally to experimentally known structures, wherein we see excellent agreement. The high accuracy of the method which we present for predicting these structures can be used in the future to predict interfaces which have not already been experimentally identified and can also be applied to heterointerfaces.

Energetics of defect production in fluorite-structured CeO2 induced by highly ionizing radiation

Understanding the energetics of radiation-induced defects is critical to the development of next-generation materials for nuclear and other energy systems and for aerospace applications. However, it remains a challenge to experimentally characterize defect morphologies and energies, especially in regard to anion defects in irradiated oxides. Here, using a combination of advanced structural and calorimetric characterization techniques, we show that the energetic response of defects in ${\mathrm{CeO}}_{2}$ is strongly coupled to atomic disordering on the oxygen sublattice induced by energetic heavy ions. Fitting of calorimetric data yields an estimate of stored energy in the form of defects and microstrain. These results provide a means to calculate the efficiency of structural destabilization and aid in a better understanding of defect formation and annealing mechanisms in fluorite-structured materials subject to extreme conditions.

Uranium vacancy mobility at the Σ5 symmetric tilt and Σ5 twist grain boundaries in UO2

Ionic transport at grain boundaries in oxides dictates a number of important phenomena, from ionic conductivity to sintering to creep. For nuclear fuels, it also influences fission gas bubble nucleation and growth. Here, using a combination of atomistic calculations and object kinetic Monte Carlo (okMC) simulations, we examine the kinetic pathways associated with uranium vacancies at two model grain boundaries in UO2. The barriers for vacancy motion were calculated using the nudged elastic band method at all uranium sites at each grain boundary and were used as the basis of the okMC simulations. For both boundaries considered – a simple tilt and a simple twist boundary – the mobility of uranium vacancies is significantly higher than in the bulk. For the tilt boundary, there is clearly preferred migration along the tilt axis as opposed to in the perpendicular direction while, for the twist boundary, migration is essentially isotropic within the boundary plane. These results show that cation defect mobility in fluorite-structured materials is enhanced at certain types of grain boundaries and is dependent on the boundary structure with the tilt boundary exhibiting higher rates of migration than the twist boundary.

TEM study of alpha-damaged plutonium and americium dioxides

Abstract

Dynamical response and instability in ceria under lattice expansion

We present results of density functional theory calculations on the phonon dispersion and elastic constants of bulk ceria (CeO${}_{2}$) as a function of positive and negative isotropic strain, which could be induced thermally or by cationic doping. We find that, as the lattice is expanded, there is a significant softening of the ${B}_{1u}$ mode at the $X$ point. This mode consists of motions of oxygens in the $[001]$ direction. At a strain of $1.6$$%$, corresponding to a temperature of 1600 K, the ${B}_{1u}$ and ${E}_{u}$ modes at the $X$ point cross, with an associated high, narrow peak in the phonon density of states appearing. We infer that this crossing indicates a coupling of the modes, leading to a transition to a superionic phase, where conductivity occurs in the $[001]$ direction, mediated by anion interstitial site occupation. As the lattice is expanded further, the ${B}_{1u}$ mode continues to soften, becoming imaginary at a strain of $3.4$$%$, corresponding to a temperature of 2500 K. Following the imaginary mode would result in a cubic to tetragonal phase transition, similar to those known to occur with reducing temperature in zirconia (ZrO${}_{2}$) and hafnia (HfO${}_{2}$). Our calculated elastic constants, however, indicate that the structure remains mechanically stable, even at this level of expansion. As confirmed by our semiclassical free energy calculations, the cubic phase of ceria remains the most stable, while the imaginary mode indicates a change to a thermally disordered cubic phase, with the majority of disorder occurring on the anion sublattice. Our results explain the high temperature ionic conductivity in ceria and other fluorite-structured materials in terms of the intrinsic lattice dynamics, and give insight to the stability and anionic disorder at elevated temperatures.

Intrinsic electrostatic effects in nanostructured ceramics

Using atomic-level calculations with empirical potentials, we have found that electrostatic dipoles can be created at grain boundaries formed from nonpolar surfaces of fluorite-structured materials. In particular, the $\ensuremath{\Sigma}5(310)/[001]$ symmetric tilt grain boundary reconstructs to break the symmetry in the atomic structure at the boundary, forming the dipole. This dipole results in an abrupt change in electrostatic potential across the boundary. In multilayered ceramics composed of stacks of grain boundaries, the change in electrostatic potential at the boundary results in profound electrostatic effects within the crystalline layers, the nature of which depends on the electrical boundary conditions. For open-circuit boundary conditions, layers with either high or low electrostatic potential are formed. By contrast, for short-circuit boundary conditions, electric fields can be created within each layer, the strength of which then depend on the thickness of the layers. These electrostatic effects have important consequences for the behavior of defects and dopants within these materials.

Radiation tolerance of fluorite-structured oxides subjected to swift heavy ion irradiation

Fluorite-structured materials are known to exhibit an excellent structural stability under irradiation. The radiation stability of urania and yttria-stabilised cubic zirconia single crystals submitted to intense electronic excitations induced by 944-MeV Pb53+ ions was investigated. Various analytical tools (TEM, AFM, RBS/C, XRD) were employed to examine the modifications induced at the surface and in the crystal bulk. At low fluence irradiation leads to the formation of localised ion tracks whose centre is hollowed in the surface region over a depth of ∼100nm and to the formation of nanometer-sized hillocks. Both features are interpreted as resulting from an ejection of matter in the wake of the projectile. Track overlapping at high fluence results in the formation of micrometer-sized domains (∼50nm) in the crystal bulk characterised by a slight disorientation (∼0.2°) with respect to the main crystallographic orientation of the crystal.

Erratum: Photoelastic and elastic properties of the fluorite structure materials, LiF, and Si [Phys. Rev. B 68, 155120 (2003)

Erratum: Photoelastic and elastic properties of the fluorite structure materials, LiF, and Si [Phys. Rev. B 68, 155120 (2003)

Simulation of CaF 2 in the superionic state: comparison of an empirical and realistic potential

The conductivity and heat capacity of CaF 2 in the superionic phase are calculated by computer simulation using an ab initio interaction model which includes polarization effects. The results are compared with those obtained in Gillan's classic study of CaF 2, where an empirical effective pair potential was used. Despite large differences between the potentials at the pair-wise interaction level, the observable properties predicted by the two models are essentially identical in the crystal, and the reasons for the success of the effective pair potential in recapturing the consequences of the polarization effects are discussed. As further aspects of the study, we clarify the simulation predictions for the magnitude of the heat capacity anomaly at the superionic transition and for the number of defects present in the superionic states—matters on which Gillan's results have been taken to be at variance with the body of experimental evidence on other fluorite-structured materials.

Photoelastic and elastic properties of the fluorite structure materials, LiF, and Si

We present computational results of the photoelastic and elastic properties of ${\mathrm{CaF}}_{2},$ ${\mathrm{SrF}}_{2},$ ${\mathrm{BaF}}_{2},$ ${\mathrm{Ca}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{F}}_{2},$ $x=0.25,0.50,0.75,$ LiF, and Si. We also present measurements of the photoelastic properties in the visible through the ultraviolet of ${\mathrm{CaF}}_{2},$ ${\mathrm{SrF}}_{2},$ and ${\mathrm{BaF}}_{2}.$ At least semiquantitative agreement is obtained with most experimental results for all properties, including the low-frequency behavior and dispersion of the photoelastic tensor components.

Dynamics of lithium ions in lithium oxide

Abstract It is known that high ionic conductivity in fluorite-structured materials is associated with thermally created Frenkel-type disorder at temperatures well-below the melting point. Using crystalline Li2O as a model system, we studied the mechanism of formation of Frenkel pairs at temperatures close to the transition to the fast-ionic phase, and ionic motion at temperatures above the transition point. The dynamical behaviour of lithium oxide has been studied using both lattice dynamics within the quasi-harmonic (QH) approximation and molecular dynamics (MD).

Optical properties of Cr3+ in fluorite-structure hosts and in MgF*2

We have examined the optical properties of Cr3+ in MgF2 and in the fluorite-structure hosts: CdF2, CaF2, SrF2, and BaF2. The properties of Cr3+ in MgF2 are similar to those observed for other fluoride crystals that have octahedral substitutional metal sites. Interestingly, Cr3+ is also found to be sixfold coordinated in the fluorite hosts, despite the fact that the metal sites of these crystal lattices are eightfold coordinated. The smaller ionic radius of Cr3+ compared to, say, Ca2+, undoubtedly results in considerable relaxation at the metal site. However, the crystal field stabilization energy present in the 4A2(d3) ground state also provides for the energetic preference of sixfold vs eightfold coordination. The similarity of the observed absorption spectra of Cr3+ in MgF2 and in fluorite give evidence that the ground state is octahedrally coordinated in all of these hosts. The reduction of this electronic stabilization energy in the 4T2(d3) excited state is considered to produce a configurational shift relative to the ground state. This shift may be the reason why Cr3+ luminesces effectively in MgF2 whereas it is largely quenched in the fluorite-structure materials.

Thermal expansion of fluorites at high temperatures

The linear thermal expansions of CaF2, BaF2, SrF2, PbF2 and SrCl2 have been measured from room temperature up to their melting points. The linear coefficient alpha exhibits a Schottky-like bump similar to that observed previously for the heat capacity, Cp, and associated with fast-ion conduction in these fluorite-structure materials. Values for the Gruneisen parameter gamma (T) are included.