Defect Energetics Research Articles

Toward a Consistent Prediction of Defect Chemistry in CeO2.

Polarizable shell-model potentials are widely used for atomic-scale modeling of charged defects in solids using the Mott-Littleton approach and hybrid Quantum Mechanical/Molecular Mechanical (QM/MM) embedded-cluster techniques. However, at the pure MM level of theory, the calculated defect energetics may not satisfy the requirement of quantitative predictions and are limited to only certain charged states. Here, we proposed a novel interatomic potential development scheme that unifies the predictions of all relevant charged defects in CeO2 based on the Mott-Littleton approach and QM/MM electronic-structure calculations. The predicted formation energies of oxygen vacancies accompanied by different excess electron localization patterns at the MM level of theory reach the accuracy of density functional theory (DFT) calculations using hybrid functionals. The new potential also accurately reproduces a wide range of physical properties of CeO2, showing excellent agreement with experimental and other computational studies. These findings provide opportunities for accurate large-scale modeling of the partial reduction and nonstoichiometry in CeO2, as well as a prototype for developing robust interatomic potentials for other defective crystals.

Polaronic defects in monolayer CeO2: Quantum confinement effect and strain engineering.

We uncover the structure, stability, and electronic properties of polaronic defects in monolayer (ML) CeO2 by means of first-principles calculations, with special attention paid to the quantum confinement effect induced by dimensionality reduction. Results show that the polaron can be more stabilized in ML CeO2 than in the bulk, while formation of oxygen vacancy (Vo2+) and polaron-vacancy complexes [(Vo2+-1polaron)1+, (Vo2+-2polaron)0] tends to be more difficult. The polaronic defect states sit deeper in energy within the bandgap of ML CeO2 compared to the bulk case. We further demonstrate that the epitaxial strain in ceria film, as normally exists when grown on metal substrate, plays a crucial role in regulating the defect energetics and electronic structures. In particular, the formation energies of polarons, Vo2+, (Vo2+-1polaron)1+, and (Vo2+-2polaron)0, generally decrease with tensile strain, leading to controllable defect concentration with strain and temperature. This study not only provides physical insights into the polaronic defects in ultrathin oxide films, but also sheds light on their potential technological applications in nanoelectronics, fuel cells, and catalysts.

First-principles approaches and models for crystal defect energetics in metallic alloys

Many physical and mechanical properties of metallic alloys are sensitive to the formation and migration of crystal defects such as vacancies, dislocations, stacking faults, and grain boundaries. Those defect behaviors are intrinsically governed by the energetics associated with the distortion, breaking, and reforming of chemical bonds at the atomistic level. As such, first-principles calculations based on density functional theory, which accurately describe the energies and electronic structures of interatomic bonding, have been widely applied to study the formation and migration energetics of crystal defects in various alloy systems. This review article aims to provide a brief summary of the contemporary approaches and models associated with first-principles calculations for accurate modeling and prediction of energetics of various crystal defects in metallic alloys. The review also highlights the recent progress in the development of physics- or/and data-driven-based surrogate models for more efficient predictions of defect energies with first-principles accuracy in a broad range of alloy systems. Last but not least, some of the remaining challenges and issues associated with the first-principles modeling of crystal defects are discussed, along with recommendations for possible solutions.

A first-principles investigation of point defect structure and energetics in ThO2

The structure and energetics of charged point defects in thorium dioxide (ThO2) have been investigated using the density functional theory (DFT) and phonon simulations. DFT simulations were performed under both zero-pressure and constant volume conditions. Termed as the free volume change of the point defects, the change in volume of the supercell has been computed in the zero-pressure case. Supercell expansion was observed with the increase of the (nominal) charge state of anion (O) interstitials and cation (Th) vacancies from neutral to its maximum. On the contrary, contraction of the supercell has been observed with anion vacancies and cation interstitials as the defect charge increases. The supercell volume change with respect to the charge state has been correlated with the resulting defect energetics. It has been observed that, as the defect charge increased, the internal energy and entropy of defect formation of the cation vacancies and anion interstitials were found to increase, while that of the cation interstitials and anion vacancies decreased. The temperature dependence of internal energy and entropy has also been examined. It was found that, as the temperature increases, the internal energies of the formation of cation vacancies and anion interstitials decrease, while those of the cation interstitials and anion vacancies increase. An opposite observation is seen for the entropies of formation defects when above room temperatures.

Suppressing Charged Cation Antisites via Se Vapor Annealing Enables p-Type Dopability in AgBiSe2 -SnSe Thermoelectrics.

Cation disordering is commonly found in multinary cubic compounds, but its effect on electronic properties has been neglected because of difficulties in determining the ordered structure and defect energetics. An absence of rational understanding of the point defects present has led to poor reproducibility and uncontrolled conduction type. AgBiSe2 is a representative compound that suffers from poor reproducibility of thermoelectric properties, while the origins of its intrinsic n-type conductivity remain speculative. Here, it is demonstrated that cation disordering is facilitated by BiAg charged antisite defects in cubic AgBiSe2 which also act as a principal donor defect that greatly controls the electronic properties. Using density functional theory calculations and in situ Raman spectroscopy, how saturation annealing with selenium vapor can stabilize p-type conductivity in cubic AgBiSe2 alloyed with SnSe at high temperatures is elucidated. With stable and controlled hole concentration, a peak is observed in the weighted mobility and the density-of-states effective mass in AgBiSnSe3 , implying an increased valley degeneracy in this system. These findings corroborate the importance of considering the defect energetics for exploring the dopability of ternary thermoelectric chalcogenides and engineering electronic bands by controlling self-doping.

(Invited) A Coating Strategy for Heterogeneous Photocatalysis Producing Renewable Fuels

Photocatalysts coevolve reductive and oxidative reactions in close proximity. Due to simplified reactor implementation, photocatalysis promises solar fuels production at scale. Despite decades of study, their rates and selectivity were often improved by trial and error, and their solar-to-fuel conversion efficiencies remain much lower than the theoretical limit. I will discuss an emerging coating strategy to stabilize particulate photocatalysts in a photo-reactor that promises solar energy utilization at scale. Those photocatalysts coevolve reductive and oxidative reactions in close proximity, and they potentially overcome the scale-up challenge by photoelectrochemical panels.I will first introduce the Hu-lab invented oxide coatings to protect semiconductors, such as silicon and gallium indium phosphide, and achieve efficient and durable photocatalysis. We elucidate the coupled multi-phase processes, including charge separation, charge transfer, and chemical transport across multiple scales. We will show that the local electrochemical potentials of conduction-band electrons and the branching ratios of local charge transfer kinetics under multiple pathways are mutually dependent, and how charge transfer kinetics and surface energetics sensitively determine the charge separation behavior.[1]Based on the holistic understanding of the photophysical, electrocatalytic, and transport processes coupled at the nanoscale, we employ stabilization coatings to coevolve H2 at a record rate of 48.5 mmol∙h-1∙g-1 or 2.5 mL H2∙h-1∙cm-2 under 1-sun solar illumination in ambient air.[2] Additionally, the discovery of new coatings offers the opportunity to tune the local energetics, kinetics, and reaction environments of supported co-catalysts. Manipulation of the electronic defect energetics enables the semiconductor photoabsorbers of 1.1 – 2.3 eV with sufficient band energetics.Coated photocatalysts can perform H2 evolution, water oxidation, and can further achieve CO2 reduction reactions combining with CO2 capture.[3] Recently, Berlinguette and others showed a CO2 electrolyzer for directly converting dissolved bicarbonates into CO2-reduction products.[4] The analogy in photocatalysis is to locally drive pH swing to release CO2 at the oxidative sites, whereas the nearby reductive sites reduce in-situ generated CO2 into CO2R products. We show that in the presence of quinone redox couples in a bicarbonate solution, CO is produced with a 1-atm CO2-free headspace where the only source of CO2 is the (bi)carbonate anions.[6] We envision the direct solar fuels production from natural resources such as sunlight, bicarbonates from the ocean, or moisture in the air in a durable particle reactor.[5]

Physical properties and radiation tolerance of high-entropy pyrochlores Gd2(Ti0.25Zr0.25Sn0.25Hf0.25)2O7 and individual pyrochlores Gd2X2O7 (X= Ti, Zr, Sn, Hf) from first principles calculations

The physical properties of high-entropy ceramics (HEC) of Gd2(Ti0.25Zr0.25Sn0.25Hf0.25)2O7 and Gd2X2O7 (X = Ti, Zr, Sn, Hf) are investigated using first principles method. The structural properties, anti-site defect energetics, mechanical properties and electronic properties are compared. The lattice constant of HEC is within the range of its constituent pyrochlores, and the local lattice distortion presents in HEC. The cation anti-site defect is much easier to form in HEC than in pyrochlore, which implies that HEC can easily transfer to disordered fluorite structure and exhibit stronger radiation tolerance. The “high entropy” at B site of A2B2O7-type pyrochlore had no obvious effect on mechanical properties. The electronic properties of HEC and pyrochlore are characterized by projected density of states distribution and Bader charge, and the causes of HEC lattice distortion are investigated.

Phenomenological potentials for the refractory metals Cr, Mo and W

Cohesion in the refractory metals Cr, Mo, and W is phenomenologically described in this work via a n-body energy functional with a set of physically motivated parameters that were optimized to reproduce selected experimental properties characteristic of perfect and defective crystals. The functional contains four terms accounting for the hard-core repulsion, the Thomas–Fermi kinetic energy repulsion and for contributions to the binding energy of s and d valence electrons. Lattice dynamics, molecular statics, and molecular dynamics calculations show that this model describes satisfactorily thermodynamic properties of the studied metals whereas, unlike other empirical approaches from the literature, predictions of phonon dispersion relations and of surface and point defect energetics reveal in fair good agreement with experiments. These results suggest that the present model is well adapted to large-scale simulations and whenever total energy calculations of thermodynamic properties are unfeasible.

Energetics of intrinsic point defects and hydrogen in tungsten borides: a first-principles study

To understand the irradiation resistance and hydrogen (H) behavior in tungsten borides (W x B y ) in a burning plasma fusion environment, the energetics of intrinsic point defects and H in six stable ground state W borides, including W2B, WB, WB2, W2B5, WB3 and WB4, have been investigated using first-principles density functional theory calculations. The results show that the formation energies of interstitials and vacancies do not directly depend on the W and B content in W borides. However, the interaction between vacancies of a stoichiometric vacancy (SV) cluster in W x B y is related to the atomic ratio of B to W (y/x). The vacancies of a minimum-size SV cluster in W x B y are energetically repulsive for y/x ⩽ 1, while the vacancies energetically bind together for y/x larger than 1. The formation energy of B Frenkel pairs in each W boride is lower than that of W Frenkel pairs. Among the six evaluated W x B y compositions, WB has the highest and the lowest formation energy of H interstitials and H-vacancy complexes, respectively; however, these two energies in WB2 are in reverse order. The average H binding energies to single vacancies in WB and WB4 are comparable with that in W, while this binding energy in WB3 and WB2 is obviously higher or lower than in W, respectively. The diffusion activation energy of H in W borides is anisotropic. One dimensional (1D) diffusion of H in W2B, as well as 1D/2D diffusion in WB, W2B5, WB3 and WB4 are preferred at relatively low temperatures; however, three-dimensional diffusion of H is predicted in WB2. The diffusion activation energy of H generally increases with B content in W borides due to the increasing local charge deficit caused by strong B–B covalent bonds. This study is useful for evaluating the performance of W borides in a fusion environment.

Defect energetics and stacking fault formation in high-entropy carbide ceramics

Inspired by the concept of entropy stabilization, multicomponent transition metal carbide (MTMC) ceramics have received increasing attention due to their extraordinary performances. However, the role of partial disorder in the transition metal sublattice on the defect properties in MTMCs is still elusive. In this work, we study defect formation and generalized stacking fault energies (GSFEs) in MTMCs. In both cases, we compare the results in MTMCs to binary TMCs. Our results suggest that C-related defects generally exhibit lower formation energies in MTMCs, suggesting that MTMCs are prone to off-stoichiometry with C vacancies. We further show that lower formation energies of C interstitials and higher migration energies of C vacancies account for the experimentally-observed delayed defect evolution. Finally, our calculated GSFEs in different MTMCs are close to the averages from all the constituent binary TMCs, indicating that the rule of mixture (ROM) can be applied to estimate the stacking fault energies for stoichiometric MTMCs.

Towards the dependence of radiation damage on the grain boundary character and grain size in tungsten: A combined study of molecular statics and rate theory

Grain boundary (GB) character and radiation conditions restrict the in-depth understanding of the effectiveness of tailoring grain size in controlling the radiation damage and promoting the radiation resistance of materials. Here, we constructed 46 [0 0 1] W symmetric tilt GBs with tilt angle varying from 6.73° to 82.37° Then, molecular statics (MS) was used to investigate the impact of GB character (tilt angle, GB energy, Σ) on the defect energetics. The present results suggested that it is energetically favorable for Vs/SIAs to segregate into the GB and GBs have the biased absorption of SIAs over Vs. Some high-angle and high-energy GBs could provide larger energetic driving force for the V/SIA to segregate. Multiple Vs/SIAs tended to be bound together inside the GB, with a varying binding strength. Using steady-state rate theory (RT) calculations, we explored the Vs accumulation in the W systems with different grain sizes (50–1000 nm) under the typical radiation conditions of temperature (300–1800 K) and dose rate (10−8–102 dpa/s). In particular, based on the results of MS calculations, the effect of GB character on the Vs accumulation was examined by varying V segregation energies (0.99–2.04 eV). In the process, two extreme values (2.04 and 0.99 eV) of V segregation energy were extracted to investigate its impact, with other values being in this range. The results suggested that nanocrystalline (NC) W would exhibit better radiation tolerance than coarse-grained (CG) W under the conditions of a relatively high temperature and low dose rate. A high V segregation energy led to a low damage accumulation in the grain interior. Some high-angle and high-energy GBs with a large V segregation energy and a small V–V binding energy were expected to improve the radiation resistance of NC.

Structural and energetic properties of vacancy defects in MXene surfaces

Density functional theory calculations were employed to investigate the structure and energetics of vacancy defects on 84 MXene surfaces, composed of different M (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W), X (carbon or nitrogen), T (oxygen of fluorine), and atomic layer stacking (ABC or ABA) combinations. Structurally, we found that, in most cases, the effect of the presence of the vacancy on the surrounding atoms is to push them away from the vacant site due to the missing screening charge and the reinforcement of the remaining interatomic bonds. The reconstruction energy associated to this surface relaxation was found to range from a few tenths of eV to a few eV. On the bare ABC-stacked carbide MXenes, we found M and X vacancy formation energies between 2 and 3 eV. On nitrides, M vacancy formation energies are decreased, while X ones seem to increase. By terminating the MXenes with an O layer, M formation energies are significantly increased, reaching over 9 eV in some cases, in agreement with theoretical and experimental results of the literature, whereas X vacancy formation energies become smaller. The F termination was found to have the same effect as O on X vacancy formation energies with respect to clean MXenes, but to decrease M vacancy formation energies. The F termination allows more easily created M and N vacancies than the O termination. The creation of vacancies on some ABC-stacked ${\mathrm{M}}_{2}{\mathrm{XF}}_{2}$ MXenes led to extreme lattice deformation which suggests that the experimental synthesis of ${\mathrm{M}}_{2}{\mathrm{NF}}_{2}$ with metals from groups 5 and 6 of the Periodic Table, and ${\mathrm{M}}_{2}{\mathrm{CF}}_{2}$ with metals from group 6, through etching using a hydrogen fluoride aqueous solution [HF (aq)] will lead to ABA-stacked MXenes. The formation energies of the MT double vacancies were found to correlate approximately linearly with the sum of the corresponding M and T single-atom vacancy formation energies. All MXenes studied in this work fit in the same correlation line, implying that one can predict the formation energy of double vacancies on MXenes, made of any M, X, and T elements, and with atomic layers stacked in an ABC or ABA fashion, if the formation energies of the corresponding single vacancies are known. The method used by theoretical works, such as this one, to calculate formation energies has two crucial limitations: the values thus obtained depend on the chosen single-atom energy references, and kinetic effects such as energy barriers are not included. Here, we cautiously discuss these limitations, compare formation energies calculated using different energy references, and propose mechanisms for the formation of vacancies that include atomic migration barriers.

Engineering defect energy landscape of CoCrFeNi high-entropy alloys by the introduction of additional dopants

The concept of high-entropy alloys (HEAs) focusing on tuning the overall chemical complexity represents a novel alloy design strategy. In contrast, alloying of a metallic matrix with minor doping elements with limited and localized tunability has been a common practice to improve material performance. Combining the idea of globally engineering defect energy landscape in HEAs and the localized doping strategy in dilute alloys, in this work, we explore doping effects of minor elements in a HEA matrix to further enhance the overall and localized chemical tunability, aiming to improve its irradiation resistance. Specifically, we study the influence of minor Al, Cu, Ti, and Pd substitutional doping elements on defect energetics in a CoCrFeNi model HEA based on density-functional theory (DFT) calculations. The DFT results indicate that the formation and migration energies of vacancies can be strongly influenced when a dopant is introduced at the first nearest neighbor shells around a vacancy. On the other hand, interstitial energetics are only slightly affected. Among the four elements, Ti and Pd generally decrease vacancy formation energies and increase vacancy migration energies more significantly than Al and Cu. The doping effects become more pronounced when the concentration of the substitutional dopants increases. Based on the energy distributions obtained from DFT, we build a kinetic Monte Carlo (kMC) model to assess the impact of dopants on vacancy-mediated diffusivity in the doped HEAs. Our results suggest that Ti and Pd can lower the tracer diffusivity in the considered HEAs and act as trapping sites, whereas Cu may enhance the atomic transport. This work indicates that substitutional doping in HEAs is an effective strategy in metallurgy to further tune the defect and transport properties of complex alloys.

Effects of Defect on Work Function and Energy Alignment of PbI2: Implications for Solar Cell Applications

Two-dimensional (2D) layered lead iodide (PbI2) is an important precursor and common residual species during the synthesis of lead-halide perovskites. There currently exist some debates and uncertainties about the effect of excess PbI2 on the efficiency and stability of the solar cell with respect to its energy alignment and energetics of defects. Herein, by applying the first-principles calculations, we investigate the energetics, changes of work function and the defective levels associated with the iodine vacancy (VI) and interstitial iodine (II) defects of monolayer PbI2 (ML-PbI2). We find that the PbI2 has a very low formation energy of VI of 0.77 and 0.19 eV for dilute and high concentration, respectively, reflecting coalescence tendency of isolated VI, much lower than that of vacancies in other 2D materials like phosphorene. Similar to VI, a low formation energy of II of 0.65 eV is found, implying a high population of such defects. Both defects generate in-gap defective levels which are mainly due to the unsaturated chemical bonds of p-orbitals of exposed Pb or inserted I. Such rich defective levels allow the VI and II as the reservoir or sinks of electron/hole carriers in PbI2. Our results suggest that the remnant PbI2 in perovskite MAPbI3 (or FAPbI3) would play dual opposite roles in affecting the efficiency of the perovskite: (1) Forming Schottky-type interface with MAPbI3 (or FAPbI3) in which the built-in potential would facilitate the electron-hole separation and prolong the carrier lifetime; (2) Acting as the recombination centers due to the deep defective levels. To promote the efficiency by the Schottky effect, our work reveals that the II defect is favored, and to reduce the recombination centers the VI defect should be suppressed. Our results shall be beneficial in improving strategies for the related optoelectronics applications.

Computational design of thermoelectric alloys through optimization of transport and dopability.

Alloying is a common technique to optimize the functional properties of materials for thermoelectrics, photovoltaics, energy storage etc. Designing thermoelectric (TE) alloys is especially challenging because it is a multi-property optimization problem, where the properties that contribute to high TE performance are interdependent. In this work, we develop a computational framework that combines first-principles calculations with alloy and point defect modeling to identify alloy compositions that optimize the electronic, thermal, and defect properties. We apply this framework to design n-type Ba2(1-x)Sr2xCdP2 Zintl thermoelectric alloys. Our predictions of the crystallographic properties such as lattice parameters and site disorder are validated with experiments. To optimize the conduction band electronic structure, we perform band unfolding to sketch the effective band structures of alloys and find a range of compositions that facilitate band convergence and minimize alloy scattering of electrons. We assess the n-type dopability of the alloys by extending the standard approach for computing point defect energetics in ordered structures. Through the application of this framework, we identify an optimal alloy composition range with the desired electronic and thermal transport properties, and n-type dopability. Such a computational framework can also be used to design alloys for other functional applications beyond TE.

Thermal stability of water polyhedra with square faces.

The ability to form numerous crystalline modifications of ice and gas hydrate frameworks is a characteristic feature of water. In fact, this structural variety is much wider due to the proton disorder. Configurations with different arrangements of hydrogen atoms (protons) in hydrogen bonds are not equivalent in their properties. Polyhedral water clusters are convenient objects for studying the effect of proton disorder on the properties of ice-like systems. It was previously established that the stability of water polyhedra is determined by the competition of two factors. The geometric factor gives preference to tetrahedrally coordinated structures with a large number of pentagonal faces. The topological factor takes into account the number of energetically most favorable types of H-bonds. This number increases with the number of square faces. It was found that tetrahedrally coordinated structures are not the most stable. However, these calculations were performed without taking thermal effects into account (Kirov M. V., J Phys Chem A, 124:4463 - 4470, 2020). The purpose of the present article is to study the structural stability of various water polyhedra at different temperatures. In the course of modeling, using the Amoeba force field, the advantage of configurations with a large number of square faces is demonstrated. The structure and energetics of surface defects are studied. Several very stable structures of unusual shape were found, including polyhedra which contain 4-coordinated molecules and polyhedra whose O-H groups are directed to the cluster center. The comparative analysis of cluster stability includes the temperature intervals of melting-like transitions.

Strontium stannate as an alternative anode for Na- and K-Ion batteries: A theoretical study

Theoretical predictions on structural, electronic and transport properties of pristine and alkali-doped strontium stannate (SrSnO3) were made using density functional theory and force field methods. Results of electronic structure computations show that a doping of alkali-ion (Li+, Na+ and K+) into SrSnO3 induces the apparition of extra energy bands on the valence and conduction bands and small translation of the valence and conduction band limits. This is more accentuated in K+-ion doped samples with lower energy gap. Defect energetics computations reveal a low energetic cost associated the alkali incorporation mechanism proposed in this work. Such a mechanism provides us with an intermediate first discharge reaction to describe the delithiation process. Results on alkali ion transport properties in interstitially doped nanocrystalline SrSnO3 samples reveal lower diffusion activation energies of 0.25, 0.28 and 0.44 eV and diffusion coefficient at 25 °C of 9.6, 2.9 × 10−11 and 4.8 × 10−13 cm2s−1 for Li-, Na- and K-doped samples, respectively. These predicted properties bring in new evidence to stimulate a consideration of strontium stannate for use as an alternative anode, in particular for both Na- and K-ion batteries.

Energetics of cracks and defects in soft materials: The role of surface stress

In classical fracture theory, the resistance to fracture is governed by the fracture toughness Gc, which is defined as the energy required to create new crack surfaces. To propagate a crack, the energy released to the crack tip (i.e. the energy release rate J) must overcome the fracture toughness Gc. Previous studies have demonstrated that surface stress can significantly modify the crack opening displacement and the stress field in soft solids and hence alter the energy release rate J, assuming small deformation. In this paper, we present a theoretical work on how surface stress affects fracture of soft solids under large deformation. We provide an expression of a modified J-integral for the energy release rate that accounts for surface stress. The expression is verified using large-deformation finite element analysis (FEA) with surface stress elements. Our results show that when the elastocapillary length, defined as the ratio of surface stress σ to the Young’s modulus of the solid E, is comparable to the relevant material size, surface stress noticeably blunts the crack opening profile and reduces the energy release rate.

Using first-principles calculations to predict the mechanical properties of transmuting tungsten under first wall fusion power-plant conditions

Tungsten and tungsten alloys are being considered as leading candidates for structural and functional materials in future fusion energy devices. The most attractive properties of tungsten for the design of magnetic and inertial fusion energy reactors are its high melting point, high thermal conductivity, low sputtering yield and low long-term disposal radioactive footprint. Yet, despite these relevant features, tungsten also presents a very low fracture toughness, mostly associated with inter-granular failure and bulk plasticity, that limits its applications. Significant neutron-induced transmutation happens in these tungsten components during nuclear fusion reactions, creating transmutant elements including Re, Os and Ta. Density functional theory (DFT) calculations that allow the calculation of defect and solute energetics are critical to better understand the behavior and evolution of tungsten-based materials in a fusion energy environment. In this study, we present a novel computational approach to perform DFT calculations on transmuting materials. In particular, we predict elastic and plastic mechanical properties (such as bulk modulus, shear modulus, ductility parameter, etc) on a variety of W–X compositions that result when pure tungsten is exposed to the EU-DEMO fusion first wall conditions for ten years.

Defect Properties and Lithium Incorporation in Li2ZrO3

Lithium zirconate is a candidate material in the design of electrochemical devices and tritium breeding blankets. Here we employ an atomistic simulation based on the classical pair-wise potentials to examine the defect energetics, diffusion of Li-ions, and solution of dopants. The Li-Frenkel is the lowest defect energy process. The Li-Zr anti-site defect cluster energy is slightly higher than the Li-Frenkel. The Li-ion diffuses along the c axis with an activation energy of 0.55 eV agreeing with experimental values. The most favorable isovalent dopants on the Li and Zr sites were Na and Ti respectively. The formation of additional Li in this material can be processed by doping of Ga on the Zr site. Incorporation of Li was studied using density functional theory simulation. Li incorporation is exoergic with respect to isolated gas phase Li. Furthermore, the semiconducting nature of LZO turns metallic upon Li incorporation.