Frenkel Pairs Research Articles

Low-Temperature Cation Ordering in High Voltage Spinel Cathode Material

The spinel cathode LiNi0.5Mn1.5O4 (LNMO) is a promising material for battery applications; however, issues regarding its cycling stability need to be addressed to fully utilize its potential. Specifically, LNMO suffers from rapid capacity loss following prolonged electrochemical cycling where the capacity drop occurs at an earlier stage for its transition metal ordered form. Further, the disordered form has been found to partially order during cycling, leading to the question if the failing of the disordered form is driven by Ni and Mn ordering in the structure. However, ordering is usually initiated at temperatures above 700 °C in the fully lithiated state, sparking the question if the energy barrier for the ordering process is lowered at the reduced Li content. In the work presented here, in situ neutron diffraction was used to further elucidate the ordering temperature and thermal stability of LixNi0.5Mn1.5O4 (0.000 (10) ≤ x ≤ 0.675 (10)). The temperature for Ni and Mn ordering was found to be dramatically lowered to 310–320 °C for LixNi0.5Mn1.5O4 compositions of 0.222 (8) ≤ x ≤ 0.675 (10), explained by a lower energy barrier for formation of intermediate Frenkel defect states. Li ordering and a lowering of symmetry to P213, together with formation of both a NiMn2O4-type spinel phase and a NiMnO3-type phase, could also be observed. The formation of NiMn2O4- and NiMnO3-type phases could be linked to reorganization of transition metals (TM) within the spinel structure and were found to be in competition with Ni and Mn ordering. At higher Li contents, transition metal diffusion tended to favor Ni and Mn ordering, while NiMn2O4- and NiMnO3-type phases were formed to a greater extent at lower Li contents. These results highlight the importance of suppressing transition metal reorganization in LiNi0.5Mn1.5O4, via increased TM diffusion energy barriers, as a key strategy for minimizing structural rearrangements and ultimately improving its electrochemical cyclability.

Dynamic reactions of defects in ion-implanted 4H-SiC upon high temperature annealing

Single-photon emitters based on intrinsic defects in silicon carbide (SiC) are promising as solid-state qubits for the quantum information storage, whereas defect engineering in a controllable manner still remains challenging. Herein, the thermally-driven defect dynamic reaction in the ion implanted 4H-SiC has been exploited through the optical emission spectra of defects. For the heavy-ion (Si or Ar) implanted samples with abundant Frenkel pairs, the silicon vacancies (VSi) are energetically converted into the carbon antisite-vacancy pair (CSi-VC) upon annealing till 1300 °C for 30 min, accompanied with the gradual lattice recovery and local strain relaxation. The further temperature elevation dissociates the metastable CSi-VC into carbon antisite (CSi) and carbon vacancy (VC), as supported by the consequent quenching of the (CSi-VC)-related emission at 700 nm. Thus, the whole defect reaction is probed as the vacancy interconversion from VSi to VC with the byproduct of stacking faults. In contrast, the intermediate CSi-VC complexes are not energetically favorable during the annealing of the H-implanted sample, which results from the negligible generation of Frenkel pairs, as supported by the x-ray diffraction patterns and Raman scattering analysis. These findings provide guidance for defect engineering in SiC toward the creation of reliable single photon emitters.

Photoluminescence evidence for silicon Frenkel defects in electron irradiated 4H SiC

The nature of defects in 4H SiC was studied by means of low temperature photoluminescence before and after energy-controlled electron irradiation. Analysis of experimental data from irradiation at energies above and below the Si displacement energy together with subsequent annealing leads to the conclusion that Si Frenkel defects have been detected experimentally in this material. Reasons why these are formed in some cases rather than carbon vacancy carbon antisite pairs are explored.

Displacement cascades database from molecular dynamics simulations in tungsten

In this work, we have carried out a series of displacement cascade simulations in tungsten (W), including seven different primary-knock-on atom (PKA) directions, and PKA energy covers 1 to 300 keV at 363 K; 10, 50, 100, and 200 keV cascades are considered at 600 K, and 10 and 50 keV cascades are simulated at 300 K. Then, a systematic database of W cascade simulations has been established, containing more than 2000 cascades. The number of Frenkel pairs, the type of defect, and its spatial distribution were all determined. The effects of PKA direction, PKA energy and temperature have been discussed in terms of defect generation, defect clustering, and dislocation loops. It is found that, the PKA direction may have an effect on the configuration of defects, such as the formation probability of 〈100〉 interstitial loops, C15 clusters and the sub-cascades, and also has an effect on the number of Frankel pairs when PKA energy larger than 50 keV. Furthermore, from 363 K to 600 K, the number of Frankel pairs is reduced by ∼10%, and high temperature can promote the clustering of interstitial. We further found that temperature may affect the distribution of dislocation loops, and the number of 〈100〉 interstitial dislocation loops is more at 363 K than at 600 K. These results can provide a database of key input parameters for the subsequent larger-scale simulations, and give an important reference for the prediction of defect behaviour, mechanical and thermal properties of W-based materials under fusion neutron irradiation.

Strongly reduced hysteresis scintillation luminescence in lanthanide doped fluoride nanoscintillators

X-ray excited optical luminescence (XEOL) imaging shows great potential applications in security inspection and computed tomography. Lanthanide doped fluoride nanoscintillators (NSs) do not only exhibit advantages of simple synthetic procedures, tunable emissions and high environmental stability, but also show many new properties through constructing core/shell architectures. However, the strong hysteresis scintillation luminescence greatly restrict their practical applications. Herein, we verify that this hysteresis can be fully restricted by employing core/shell structure to inhibit the generation of Frenkel defects and incorporating Gd3+ ions to introduce extra energy migration process. As a result, upon continuous X-ray irradiation, the XEOL intensity of the LiYF4: 15Tb/25Gd@LiYF4 core/shell NSs almost keeps unchanged. Our results provide important information for designing new NSs with greatly reduced hysteresis scintillation luminescence, which show promising applications in the field of stable X-ray imaging.

First-principles study of the native defects with charge states in ZrSe[formula omitted

Study of the native defects in transition metal dichalcogenides (TMDCs) is of fundamental interest and potential technological importance. The atomic and electronic structures of native defects with charge states in ZrSe2 are systematically investigated for the first time based on first-principles calculations with the optB86R-vdW and HSE06 functionals. We identify the configurations of relatively stable defects including Zr interstitial (Zrint), Se vacancy (VSe) and Zr antisite (ZrSe). The positive charged Zr interstitial defect (Zrint2+) has the lowest formation energy and thus is most commonly seen in ZrSe2, which is in good agreement with experimental observation. The ZrSe defect is found to form a DX-like configuration. Moreover, the non-interacting Zr Frenkel pairs are the most favorable form of intrinsic defects. The electronic structure analyses of these defects reveal that the extra Zr atoms adjacent to the defects act as donors, which are responsible for the native n-type conductivity of as-grown ZrSe2. Our study can provide an insight into understanding the properties of native defects in ZrSe2.

Ion kinetic energy- and ion flux-dependent mechanical properties and thermal stability of (Ti,Al)N thin films

Ion-irradiation-induced changes in structure, elastic properties, and thermal stability of metastable c-(Ti,Al)N thin films synthesized by high-power pulsed magnetron sputtering (HPPMS) and cathodic arc deposition (CAD) are systematically investigated by experiments and density functional theory (DFT) simulations. While films deposited by HPPMS show a random orientation at ion kinetic energies (Ek)>105 eV, an evolution towards (111) orientation is observed in CAD films for Ek>144 eV. The measured ion energy flux at the growing film surface is 3.3 times larger for CAD compared to HPPMS. Hence, it is inferred that formation of the strong (111) texture in CAD films is caused by the ion flux- and ion energy-induced strain energy minimization in defective c-(Ti,Al)N. The ion energy-dependent elastic modulus can be rationalized by considering the ion energy- and orientation-dependent formation of point defects from DFT predictions: The balancing effects of bombardment-induced Frenkel defects formation and the concurrent evolution of compressive intrinsic stress result in the apparent independence of the elastic modulus from Ek for HPPMS films without preferential orientation. However, an ion energy-dependent elastic modulus reduction of ∼18% for the CAD films can be understood by considering the 34% higher Frenkel pair concentration formed at Ek=182 eV upon irradiation of the experimentally observed (111)-oriented (Ti,Al)N in comparison to the (200)-configuration at similar Ek. Moreover, the effect of Frenkel pair concentration on the thermal stability of metastable c-(Ti,Al)N is investigated by differential scanning calorimetry: Ion-irradiation-induced increase in Frenkel pairs concentration retards the wurtzite formation temperature by up to 206 °C.

Structural dynamics of Schottky and Frenkel defects in CeO2: a density-functional theory study

Cerium dioxide CeO2 (ceria) is an important material in catalysis and energy applications. The intrinsic Frenkel and Schottky defects can impact a wide range of material properties including the oxygen storage capacity, the redox cycle, and the ionic and thermal transport. Here, we study the impact of Frenkel and Schottky defects on the structural dynamics and thermal properties of ceria using density functional theory. The phonon contributions to the free energy are found to reduce the defect formation free energies at elevated temperature. The phonon dispersions of defective CeO2 show significant broadening of the main branches compared to stoichiometric ceria. Phonon modes associated with the defects are identifiable in the infrared spectra through characteristic shoulders on the main features of the stoichiometric fluorite structure. Finally, the presence of Frenkel and Schottky defects are also found to reduce the thermal conductivity by up to 88% compared to stoichiometric CeO2.

On the confluence of ultrafast high‐temperature sintering and flash sintering phenomena

AbstractUltrafast high‐temperature sintering (UHS) and flash sintering are novel methods for rapid sintering of ceramics, often completed in just a few seconds. Here, we show that both also share two additional features: an abrupt rise in electrical conductivity, which is electronic, and electroluminescence. More fundamentally, both are related to phonon physics where MD calculations have shown that proliferation of phonons at the edge of the Brillouin zone can induce Frenkel pairs without the application of electrical fields. Here, we show that, indeed, heating without the application of electric field, can also induce flash: Rapid heating processes of thin films of an oxide‐salt deposited on silk fibers, with a propane torch, are shown to induce electronic conductivity, electroluminescence, and rapid sintering of the oxide. The discussion in this article harkens back to two inventions, more than a century ago, which can now be related to flash and UHS: (i) the Nernst glow lamp circa 1900, made from zirconia, and (ii) the Welsbach mantle, constituted from ceria doped thorium oxide, in the late nineteenth century. Thus, the confluence between high heating rate and electric field induced flash phenomena links the past to the new. The emerging question is how injection of phonons that has been shown to create Frenkels can further induce high electronic conductivity and electroluminescence in oxides. Both electronic conductivity and luminescence are likely related to the generation of electron–hole pairs.

Atomistic simulations of defect accumulation and evolution in heavily irradiated titanium for nuclear-powered spacecraft

Titanium alloys are expected to become one of the candidate materials for nuclear-powered spacecraft due to their excellent overall performance. Nevertheless, atomistic mechanisms of the defect accumulation and evolution of the materials due to long-term exposure to irradiation remain scarcely understood by far. Here we investigate the heavy irradiation damage in α-titanium with a dose as high as 4.0 canonical displacements per atom (cDPA) using atomistic simulations of Frenkel pair accumulation. Results show that the content of surviving defects increases sharply before 0.04 cDPA and then decreases slowly to stabilize, exhibiting a strong correlation with the system energy. Under the current simulation conditions, the defect clustering fraction may be not directly dependent on the irradiation dose. Compared to vacancies, interstitials are more likely to form clusters, which may further cause the formation of 1/3<1¯210> interstitial-type dislocation loops extended along the (¯1010) plane. This study provides an important insight into the understanding of the irradiation damage behaviors for titanium.

Atomistic simulations of defect clustering evolution in heavily irradiated Ti35 alloy

Ti35 alloy (Ti-6wt.%Ta) has greatly promising applications in nuclear industries due to its excellent overall performance. Nevertheless, atomistic mechanisms of its defect clustering evolution due to long-term exposure to irradiation remain scarcely understood by far. Here we investigate the heavy irradiation damage in Ti35 alloy with a dose of up to 4.0 canonical displacement per atom (cDPA) using atomistic simulations of Frenkel pair accumulation. Results show that defect clustering becomes remarkable before 0.04 cDPA and thereafter tends to be relatively stable, and the fraction is not directly dependent on the irradiation dose. Interstitials exhibit a stronger ability than vacancies to form clusters and especially the Nint>5 clusters may cause the formation of 1/3<1‾210> dislocation loops. Compared to the matrix, Nint≤5 interstitial-type defects show a depletion of Ta atoms, while Nint>5 clusters are Ta-rich. This study provides an important insight into the understanding of the irradiation damage behaviors for Ti35 alloy.

Molecular Dynamics Studies of Primary Irradiation Damage in α‐Type Ti35 Alloy

α‐type Ti35 alloy (Ti‐6 wt%Ta) has been recommended as one of the candidate materials for advanced nuclear reactors due to its excellent overall performance. Nonetheless, irradiation effects on the alloy remain scarcely understood by far. Herein, the primary irradiation damage of Ti35 alloy is investigated from multiple perspectives using atomistic simulations. Results show that with the increasing energy of the primary knock‐on atom (2 keV ≤ E PKA ≤ 10 keV), the number of Frenkel pairs (N FP) gradually increases independent of the incident direction of PKA (IDPKA) and the ambient temperature (100 K ≤ T ≤ 700 K), while it exhibits a downtrend at each IDPKA as the T elevates. The magnitude of E PKA can affect the anisotropy of IDPKA on irradiation damage. Unrelated to E PKA, IDPKA, and T, di‐interstitial clusters and N vacancy &gt; 3 clusters respectively account for the largest proportion of their categories, highlighting their stability. In any case, vacancies are more prone to cluster than interstitials. The increase of E PKA has a greater effect on vacancy clustering than interstitial clustering but can promote the increase in both the clustering fractions. An important insight into the understanding of the irradiation damage behaviors for the alloy has been provided.

Molecular dynamics simulation of effects of Al on the evolution of displacement cascades in AlxCoCrFeNi high entropy alloys

This work studied the evolution of displacement cascades in AlxCoCrFeNi high entropy alloys (HEAs) with different Al contents by molecular dynamics (MD) simulation to reveal the effects of Al and its-induced microstructural variations on the primary structure damage of HEAs exposed to energetic particle irradiations. Both the amount of peak Frenkel pairs and recombination efficiency of Frenkel pairs in the displacement cascades are increased with Al content, which are induced by the decrease of atomic displacement energies and prolonged thermal spike, respectively. The amount of surviving defects depends on the competition between the two effects. When the PKA energies are 5 and 10 keV, both effects are comparable, resulting in that the amount of surviving defects first decreases and then increases with Al content. The minimum amount of surviving defects is present for Al content of 5 at. %. When the PKA energy is further increased to 20 keV, the increase of peak Frenkel pairs becomes dominant, resulting in the amount of surviving defects increases monotonically with Al content. Moreover, the vacancy clustering is restrained but the interstitial clustering is promoted with increasing Al content. The current results indicate that the addition of oversized alloying elements could introduce two opposite effects in the primary damage stage of HEAs, which should be considered in the design of novel HEAs with higher irradiation resistances.

Simulation of the Irradiation Cascade Effect of 6H-SiC Based on Molecular Dynamics Principles.

When semiconductor materials are exposed to radiation fields, cascade collision effects may form between the radiation particles in the radiation field and the lattice atoms in the target material, creating irradiation defects that can lead to degradation or failure of the performance of the device. In fact, 6H-SiC is one of the typical materials for third-generation broadband semiconductors and has been widely used in many areas of intense radiation, such as deep space exploration. In this paper, the irradiation cascade effect between irradiated particles of different energies in the radiation and lattice atoms in 6H-SiC target materials is simulated based on the molecular dynamics analysis method, and images of the microscopic trajectory evolution of PKA and SKA are obtained. The recombination rates of the Frenkel pairs were calculated at PKA energies of 1 keV, 2 keV, 5 keV, and 10 keV. The relationship between the number of defects, the spatial distribution pattern of defects, and the clustering of defects in the irradiation cascade effect of 6H-SiC materials with time and the energy of PKA are investigated. The results show that the clusters are dominated by vacant clusters and are mainly distributed near the trajectories of the SKA. The number and size of vacant clusters, the number of Frenkel pairs, and the intensity of cascade collisions of SKAs are positively correlated with the magnitude of the energy of the PKA. The recombination rate of Frenkel pairs is negatively correlated with the magnitude of the energy of PKA.

Deuterium trapping behavior in tungsten surface due to low-energy ion irradiation

Deuterium (D) irradiation at 421–597 K was performed on polycrystalline tungsten with 100 eV D ions up to a fluence of 4.64 × 1023 m−2. A supersaturated layer in the ion stopping range (< 20 nm) with the concentration of 1.2 at.% under the present surface resolution of ∼ 12 nm, a shallow minimum D concentration area of > 100 nm and a subsequent significant increase in the sub-surface were revealed by combining D (12C, D) 12C elastic recoil detection and D (3He, p) 4He nuclear reaction analysis. The first-principle calculations indicated that the trapping of vacancies for hydrogen (H) atoms can greatly reduce the recombination probability of Frenkel pairs and then act to stabilize the Frenkel pair. The stabilizing effect can be enhanced with increasing number of H atoms. Furthermore, the vacancies from Frenkel pairs can trap more surrounding H atoms, resulting in further H enrichment which could account for the formation of supersaturated layers. The typical location of the blister cavities coincides rather well with the subsequent significant increase in the sub-surface area. D-induced damage in the supersaturated layer and the sub-surface layers was examined by combining scanning electron microscope, transmission electron microscope and positron annihilation measurements.

Thermoluminescence characterization of natural and synthetic irradiated Ce-monazites

The thermoluminescence (TL) emission of synthetic and natural Ce-monazites was characterized here to determine the potential application in the identification of microscopic defects from a qualitative point of view trying to link each TL peak to a chemical-physical process. The kinetic parameters that lead the luminescence processes were calculated by means of variable heating rate and computing glow curve deconvolution methods and allow identifying three groups of components at ∼90, 130 and 290 °C (for the mineral sample) and ∼90, 170, 220, 270 and 320 °C (for the synthetic CePO4:Nd0.20,La0.25). The main differences appreciated in these complex TL curves are mainly due to (i) the content of impurities (natural sample contains lanthanides as well as U 0.60% and Th 5.22%) and (ii) the degree of crystallinity of the samples which is directly related to the type of impurities (synthetic monazite relies only on Nd and La). The behavior of the dose response in the range of 1–8 Gy is similar for both samples; the TL intensity increases linearly as the dose increases without changes in the position of the maxima, denoting first-order kinetic luminescence mechanism. Each peak could be mostly associated with structural defects (i.e., phase transitions), chemical reactions (i.e., Ce3+⇆Ce4+ redox reaction, dehydration or dehydroxylation processes) or intrinsic defects (i.e., Frenkel defects, ODCs or NBOHCs).

Local chemical ordering and its impact on radiation damage behavior of multi-principal element alloys

• LCOs promotes the nucleation of dislocations but inhibits their growth in the CrFeNi MPEA under irradiation. • LCOs reduces the number of stair-rod dislocation associated with SFTs, which may help to suppress irradiation swelling. • LCOs is conducive to enhance radiation damage tolerance of MPEAs. Multi-principal element alloys (MPEAs) have attracted much attention as future nuclear materials due to their extraordinary radiation resistances. In this work, we have elucidated the development of local chemical orderings (LCOs) and their influences on radiation damage behavior in the typical CrFeNi MPEA by hybrid-molecular dynamics and Monte Carlo simulations. It was found that considerable LCOs consisting of the Cr-Cr and Ni-Fe short-range orders existed in the ordered configuration with optimized system energy. Through modeling the accumulation cascades up to 1000 recoils, we revealed that the size of defect clusters and dislocation loops is smaller in the ordered configuration than those in the random one, although the former formed more Frenkel pairs (i.e., self-interstitials and vacancies). In addition, the distribution of dislocation loops is relatively more dispersed in the ordered configuration, and the stair-rod dislocations related to irradiation swelling are also smaller, implying that the existence of LCOs is conducive to enhancing radiation damage tolerance. To understand the underlying mechanism, the effects of LCOs on the formation and evolution of defects and radiation resistance were discussed from the aspects of atomic bonding, migration path, and energy of defect diffusion, which provides theoretical guidance for the design of MPEAs with enhanced radiation resistance.

Quantum well formation in AgBr for holes in the region of volumetric charge of intrinsic defects

The paper shows that in the area of the volumetric charge of the Frenkel defects, due to the large bending of the zones and the small Debye length, a quantum well for holes is formed. It is shown that the potential near the surface behaves linearly. The energies of the quantum energy levels and the hole localization regions in the triangular potential well approximation for the {200} and {111} surfaces are calculated. The energies of the levels increase with increasing temperature, and the localization regions of holes decrease due to decreasing Debye length.

Gamma-Ray Irradiation Stability of Zero-Dimensional Cs3Cu2I5 Metal Halide Scintillator Single Crystals

Zero-dimensional Cs3Cu2I5 is one of the most promising metal halide scintillators due to its large Stokes shift, photoluminescence quantum yields, freedom from toxic elements, and excellent energy spectrum resolution. To unlock the full potential of Cs3Cu2I5 as an effective alternative to traditional scintillators for gamma-ray detection, the irradiation stability of Cs3Cu2I5 single crystals under 60Co gamma rays with a maximum accumulated dose of 800 krad was explored. Although the luminescence mechanism remained unchanged after irradiation, the optical properties of Cs3Cu2I5 single crystals demonstrated a dose-dependent change at low accumulated doses (<600 krad). However, a further increase in the accumulated dose did not lead to more severe degradation and even slight performance recovery occurred. Electron paramagnetic resonance and theoretical calculation results revealed that the irradiation-induced Cs+-related Frenkel defects contribute to performance degradation. These results shed light on the microscopic mechanism of gamma-ray irradiation damage of Cs3Cu2I5 single crystal and provide guidance to their real application.

Oxygen diffusion in the fluorite-type oxides CeO2, ThO2, UO2, PuO2, and (U, Pu)O2

This study evaluates the self-diffusion and chemical diffusion coefficients of oxygen in the fluorite-type oxides CeO2, ThO2, UO2, PuO2, and (U, Pu)O2 using point defect chemistry (oxygen vacancies and interstitials). The self-diffusion coefficient changed in proportion to the 1/n power of oxygen partial pressure, similar to the defect concentration. All parameters used to represent the diffusion coefficients were determined, and the experimental data were accurately stated. The defect formation and migration energies of the oxides were compared, and the change in Frenkel defect concentration was found to affect the high-temperature heat capacities of CeO2 and ThO2. The oxygen chemical diffusion was evaluated in the oxides, excluding the line compound ThO2, and the coefficients increased dramatically around the stoichiometric composition, i.e., the chemical diffusion coefficient was much higher at stoichiometric composition, with the oxygen-to-metal ratio equal to 2.00, than in low oxygen-to-metal oxides. This difference altered the mechanism of the reduction and oxidation processes. In the reduction process, the chemical diffusion control rate was dominant and a new phase with the oxygen-to-metal ratio equal to 2.00 was formed, which then expanded from the surface in the oxidation process from a low oxygen-to-metal ratio to the stoichiometric composition.