Defect Formation Energies Research Articles

Phase transformation and radiation resistance of B-site high entropy pyrochlores

Two high entropy pyrochlores (HEPs), Gd2(Ti0.2Zr0.2Sn0.2Hf0.2Ta0.2)2O7 and Gd2(Ti0.2Zr0.2Sn0.2Hf0.2Nb0.2)2O7 have been synthesized and irradiated with 800 keV Kr2+ ions and examined by in situ transmission electron microscopy (TEM). The irradiation-induced order-to-disorder phase transformation from pyrochlore to the fluorite structure in these two HEPs and Gd2Sn2O7 was observed by using selected area electron diffraction (SAED), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive spectroscopy (EDS) mapping. The cation antisite defect (CAD) formation energy and the electron localization function (ELF) were calculated using density functional theory (DFT). Both the experimental results and calculated results suggest that the HEPs exhibit a higher resistance to phase transformation and amorphization than Gd2Sn2O7.

Thermodynamics of native defects in V2O5 crystal: A first-principles method

The defect formation energy and its stability of native point defects in V2O5 are systematically investigated utilizing the DFT + U method and thermodynamic calculations over a range of temperatures, oxygen partial pressures and stoichiometries. The variation of the formation energy for different charged point defects with Fermi level is also discussed in great detail. The findings demonstrate that donors are deep-level defects and provide minimal contributions to an n-type conductivity. Acceptor exhibits shallow-level transition that is easily compensated from the donors and makes intrinsic p-type doping challenging. Furthermore, all the native point defects in V2O5 are extremely sensitive to temperature, oxygen partial pressure and Fermi level. The distribution of the most stable point defects that may exist in crystals in the three-dimensional space of temperature, oxygen partial pressure, and Fermi level has been obtained. The calculated results offer a clear picture for analyzing the different types of point defects that may appear in crystals under different conditions and serves as a guide for regulating the formation of crystal point defects.

VTAnDeM: A python toolkit for simultaneously visualizing phase stability, defect energetics, and carrier concentrations of materials

Phase stability, defect formation energies, and carrier concentrations are closely interrelated features of semiconductors. Due to their joint dependence on the multidimensional chemical potential space, it is challenging to quantitatively establish patterns between these quantities in a given semiconductor, especially when the semiconductor is comprised of multiple elements. To enable synchronous visualization and analysis of these complementary material properties and their interdependence, we developed the Visualization Toolkit for Analyzing Defects in Materials (VTAnDeM). This python-based toolkit allows users to interactively explore how defect formation energies and carrier concentrations vary across the composition and chemical potential spaces of multicomponent semiconductors. Here, we illustrate the computational workflow that employs VTAnDeM as a post-processing tool for first-principles calculations and describe the data organization and theory underlying the visualization scheme. We believe that this software will serve as a useful tool for simultaneously visualizing the often complex and non-intuitive chemical potential – defect – carrier concentration phase space of semiconductors. Program summaryProgram Title: VTAnDeM – Visualization Toolkit for Analyzing Defects in MaterialsCPC Library link to program files:https://doi.org/10.17632/hz7dyc489v.1Developer's repository link:https://github.com/ertekin-research-group/VTAnDeMLicensing provisions: MIT LicenseProgramming language: PythonNature of problem: Defect thermodynamics are often studied from the perspective of phase stability and defect formation energetics using first-principles calculations. The results are comparable to experimentally-measurable carrier concentrations. However, visualizing all properties simultaneously by exploring the multidimensional chemical phase space is not trivial.Solution method: VTAnDeM offers a graphical interface that allows the user to interact directly with the chemical phase space of a given material and to visualize the defect formation energetics and ensuing carrier concentrations. The computational methods derive from standard defect theory within the supercell approach. The synchronous visualization scheme provides a streamlined approach to analyzing defect-related properties in semiconductors and insulators, all in real time.Additional comments including restrictions and unusual features: Required packages, installation, and tutorials can be found on the Github page.

Analysis of the electronic structure of β-SiO2 intrinsic defects based on Density Functional Theory

In order to characterize the effect of β-SiO2 intrinsic defects on the properties of the perfect structure, the electronic properties of the perfect β-SiO2 and its four typical intrinsic defects under the generalized gradient approximation (GGA) based on Density Functional Theory (DFT) were performed, and the defect formation energy, energy band, density of states (DOS) and differential charge density were calculated. The defect formation energy calculation results showed that interstitial defects were easier to form but less stable than vacancy defects. Meanwhile, the analysis of energy band and DOS indicated that the perfect and defective structures of β-SiO2 were indirect band gap insulators. Furthermore, the appearance of DOS at the Fermi level in the silicon interstitial structure and the energy band passing through the Fermi level indicated that the conductivity of the structure was improved. Moreover, the study of differential charge density illustrated charge transferred from silicon atoms to oxygen atoms, and Si-O covalent bonds with strong polarity were formed between silicon and oxygen atoms. Our studies could provide a theoretical basis for the study of the evolution of intrinsic defects and damages in ultra-precision machining such as ion beam figuring.

Band gap engineering of anatase TiO2 by ambipolar doping: A first principles study

Pure Anatase TiO2 is limited to photocatalytic activity only in the UV region of solar energy, (i.e Eg = 3.2 eV and λ = < 388 nm), which utilizes only 5% of the solar spectrum. Adding external impurities to Anatase TiO2 facilitates absorption of the visible spectrum by reducing the band gap of TiO2. Hence, we investigated the effect of Sc and V mono-doping and co-doping on the pure anatase TiO2 by Density Functional Theory (DFT). To elucidate the effect of doping on the electronic structure, the accurate band structure and density of states are derived using the hybrid functional methodology (HSE06). The defect formation energy of the dopants in their different charge states is calculated using the gradient-corrected density functionals using the PBE method. The bonding characteristics were analyzed using the charge density and electron-localization function (ELF) plots, which indicate that the impurities cause a local lattice distortion, which further influences the band features of TiO2. It is observed that the formation of additional energy states by the external impurities below the conduction band minimum, reduces the band gap of the material significantly. The optical absorption analysis shows that Sc and V co-doping not only improves the absorption in the UV region, but also reaches redshift absorption. Hence the ambipolar co-doping by Sc and V to the pure anatase TiO2 makes it a better for visible light active photocatalyst.

Iron behavior during the continuous phase transition of iron-doped titanium dioxide determined via high-temperature in-situ X-ray diffraction, rietveld refinement, and density functional theory studies

Titanium dioxide (TiO2) is widely used in various fields, and doped TiO2 exhibits different characteristics in different fields. Although numerous studies of Fe-doped TiO2 have been reported, they mainly analyze the influence of Fe on TiO2. The behavior of Fe during the phase transition of TiO2 has been rarely studied. To explore the continuous behavior of Fe during the phase transition of Fe-doped TiO2, the real-time phase transitions of Fe-doped and pure TiO2 at 300–1000 °C were characterized using high-temperature in-situ X-ray diffraction (XRD). Based on the experimental results, models of Fe-doped TiO2 were established using density functional theory (DFT). By comparing the calculated and refined values of the in-situ XRD data of Fe-doped TiO2 and an analysis of the defect formation energies, the evolution of Fe was as follows: Fe is located at a lattice interstice of anatase with an oxygen vacancy → Fe at a Ti site in anatase with an oxygen vacancy → Fe at a Ti site in rutile with an oxygen vacancy → Fe at a Ti site in rutile without an oxygen vacancy. This study reveals the deep connection between the microscopic properties of Fe and the phase transition of Fe-doped TiO2 and clarifies the states of Fe. It provides a theoretical basis for determining the states of doped atoms during the phase transition of TiO2 and theoretical guidance for research regarding the phase transitions of doped TiO2 systems.

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.

Influence of Sulfur Defects on the Electronic and Optical Properties of Torsional NbS2: First‐Principles Calculation

For single‐layer NbS2, the effect of torsional deformation on the electronic structure of defect NbS2 is studied by first principles. The electronic structure changes caused by two defect states are recorded: energy band, bond length, defect formation energy, electronic state density, and differential charge density. Single‐air sulfur defect VS and double‐air sulfur defect V2S are studied. Based on the two states, the defect system is twisted. The results show that the indirect bandgap of NbS2 is successfully induced by twisting the defect system, and the single‐layer NbS2 is transited into a diluted semiconductor. At the same time, the electronic characteristics can be controlled regularly. On the other hand, the hybrid system can significantly improve the absorption coefficient and transmittance of single‐layer NbS2 in the visible light region, promote the reflection of ultraviolet light, and effectively improve the optical response and photocatalytic activity in the low‐energy region. The results herein lay a foundation for the flexible application of 1T‐NbS2 according to its electronic and optical properties.

Passivation effect of theophylline on the surface defects of MAPbI3 perovskite

Employing molecular passivators to reduce defect density of perovskite surface is an effective way to improve the stability and photovoltaic performance of perovskite solar cells. Herein, the passivation effect and mechanism of theophylline molecule on the defects of MAPbI3 (001) surface were investigated by first-principles calculations based on density functional theory. Our results reveal that the theophylline can stably absorb on the MAPbI3 surface. Two passivation modes: defect-inhibiting mode and defect-healing mode, are proposed to describe the passivation mechanism. In the defect-inhibiting mode, the theophylline adsorbed on the MAPbI3 surface leads to the increase of formation energies of most defects, suppressing the generation of defects. In the defect-healing mode, the adsorption of theophylline on the defect site of MAPbI3 surface can effectively eliminate or weaken the defect impurity states in the bandgap, consequently healing the defects. Hence, incorporating theophylline molecule into MAPbI3 perovskite film is an effective strategy to passivate surface defects and improve the stability and performance of perovskite solar cells.

White Light Emission from Single-Component Cs7Cd3Br13:Pb2+,Mn2+ Crystals with High Quantum Efficiency and Enhanced Thermodynamic Stability

Single-component white light emitters are particularly attractive for fabricating economical solid-state devices for display and lighting. Herein, an efficient white light with photoluminescence quantum yields approaching 98% has been obtained in Mn2+ and Pb2+ co-doped Cs7Cd3Br13 crystals. The quantified bonding and nonbonding characters calculated by the crystal orbital Hamilton population analysis demonstrate that the Mn2+ and Pb2+ occupy preferentially in the [CdBr4]2– tetrahedrons rather than the [CdBr6]4– octahedrons in Cs7Cd3Br13. According to the PL spectral characteristic and the theoretical results, the broadband white light emission from Cs7Cd3Br13:Pb2+,Mn2+ crystals is ascribed to the multiple PL origins, i.e., the first self-trapped exciton (STE1) from the intrinsic trapping states of host lattice Cs7Cd3Br13, the d–d transition of Mn2+, and the second self-trapped exciton (STE2) induced by the introduction of Pb2+, respectively. The results calculated by density functional theory reveal that the incorporation of Mn2+ and Pb2+ has significantly improved the thermodynamic stability of the Cs7Cd3Br13 structure with lower defect formation energies, which is verified by the experimental observation in the temperature-dependent PL spectra of the emitting crystals. The findings here provide a new perspective for a single-component white light via creating multiple PL centers in a single matrix as well as help in the elucidation of the preferential site occupancy mechanism of the dopants in different symmetrical units.

Improved Defect Tolerance and Charge Carrier Lifetime in Tin-Lead Mixed Perovskites: Ab Initio Quantum Dynamics.

Simulations by nonadiabatic (NA) molecular dynamics demonstrate that mixing tin with lead in CH3NH3PbI3 can passivate the midgap state created by an interstitial iodine (Ii) via the imposed compressive strain and upshifted valence band maximum, reduce NA coupling by decreasing electron-hole wave functions overlap, and shortens pure-dephasing time by introducing high-frequency phonon modes. Thus, the charge carrier lifetime extends to 3.6 ns due to the significantly reduced nonradiative electron-hole recombination, which is an order of magnitude longer than the Ii-containing CH3NH3PbI3, over 2.5 times longer than the pristine CH3NH3PbI3 (1.4 ns), and even 1.7 times longer than the tin-lead mixed perovskite without the Ii defects (2.1 ns). Tin-lead alloying simultaneously increases the Ii defect formation energy to 0.402 eV from 0.179 eV in CH3NH3PbI3, which effectively enhances defect tolerance by reducing the defect concentration. The study reveals the factors controlling the enhanced performance of tin-lead mixed perovskite solar cells.

Intrinsic defects on α, γ and δ-CsPbI3 (001) surfaces and implications for the α/γ to δ phase transition.

Compared with traditional organic-inorganic hybrid perovskites, CsPbI3 is considered to be a better solar photovoltaic absorption material. However, under environmental conditions, it will undergo the phase transition from the α phase to the γ phase and finally to the non-perovskite phase (δ), especially in a humid environment. Considering the important role of surface intrinsic defects in the phase transition process, we investigated the intrinsic defects on α, γ and δ-CsPbI3 (001) surfaces by first-principles calculations based on density functional theory (DFT). The formation energy of most defects on the surface is similar to that in the bulk in all three phases except for VPb and VI. The formation energy of VPb and VI on the α-CsPbI3 (001) surface is significantly increased, and the formation energy of VPb on the γ-CsPbI3 (001) surface is also increased, due to the relaxation and distortion of the surface Cs and the Pb-I octahedron. The formation energy of interstitial defects on the α-CsPbI3 (001) surface is the lowest due to the remaining large dodecahedral void, even though the Pb-I octahedron distortion has largely enhanced the stability of the α-CsPbI3 (001) surface. The formation energy of VCs is the lowest in all three phases, indicating that Cs ions in CsPbI3 are indeed flexible in CsPbI3. The results are expected to provide a theoretical basis and guidance for the stability improvement of all-inorganic halide perovskites especially in a humid environment.

Semi-supervised generative approach to chemical disorder: application to point-defect formation in uranium-plutonium mixed oxides.

Chemical disorder has a major impact on the characterization of the atomic-scale properties of highly complex chemical compounds, such as the properties of point defects. Due to the vast amount of possible atomic configurations, the study of such properties becomes intractable if treated with direct sampling. In this work, we propose an alternative approach, in which samples are selected based on the local atomic composition around the defect, and the defect formation energy is obtained as a function of this local composition with a reduced computational cost. We apply this approach to (U, Pu)O2 nuclear fuels. The formation-energy distribution is computed using machine-learning generative methods, and used to investigate the impact of chemical disorder and the range of influence of local composition on the defect properties. The predicted distributions are then used to calculate the concentration of thermal defects. This approach allows for the first time for the computation of the latter property with a physically meaningful exploration of the configuration space, and opens the way to a more efficient determination of physico-chemical properties in other chemically-disordered compounds such as high-entropy alloys.

First-principles study of influence of Si on &lt;i&gt;γ&lt;/i&gt; phase in Inconel 718 alloy

Inconel 718 (IN 718) is the most widely used nickel-based high-temperature alloy today. It is widely adopted in important fields such as aerospace, energy and chemicals, and is also one of the few high-temperature alloys, of which some can be fabricated by using additive manufacturing. There is a lack of research on the effect of Si on the structure and properties of IN 718 alloy on a microscopic scale. In this paper, the effect of Si doping on the γ phase in IN 718 alloy is investigated by first-principles calculations through using the CASTEP package. The lattice constants, total energy, defect formation energy, formation enthalpy, cohesive energy, density of states, and electron density difference of the γ phase are calculated before and after Si doping, and population analysis is performed. The calculation of the lattice constant reveals that the doping of Si atoms expands the cell volume of the γ phase supercell, which contributes to a certain solution strengthening effect, and is conducive to the improvement of the hardness of the alloy. The energy and electronic structure calculations show that the Si atoms prefer to occupy the Ni atomic positions in the γ phase. The number of valence electrons between the atoms, the distribution of the charge density, and the strength of the bonds between the atoms also change with Si doping, thus modifying the interaction of the atoms within the γ phase, reducing the stability of the γ phase, and favouring the precipitation of the second phase. Besides, uniform and dense IN 718 coatings with low-coat Si doping are successfully fabricated by using plasma cladding. The experimental results demonstrate that Si doping has no significant effect on the type of matrix structure of IN 718 coatings, but causes a slight expansion of the lattice of the alloy, which is consistent with the calculation result. The addition of Si can result in a transformation of the alloy coating from columnar crystal to equiaxed crystal, refining the grain size of the alloy, while reducing the volume fraction of the γ phase and increasing the volume fraction of the second phase. Moreover, the addition of Si exacerbates the segregation of Nb and Cr elements in the IN 718 coatings.

Dimension-dependent intrinsic point defect characteristics of binary photovoltaic materials

Because the absorption layer materials in solar cells have different dimensions, they have different material properties. Defect formation energy can increase with the increase of material dimensions.

In situ dipole formation to achieve high open-circuit voltage in inverted perovskite solar cells via fluorinated pseudohalide engineering.

Many studies have shown that the severe photoluminescence quantum yield (PLQY) loss at the interface between the perovskite and electron transport layer (ETL) is the main cause of voltage loss in inverted perovskite solar cells (p-i-n PSCs). However, currently there are no effective in situ passivation techniques to minimize this nonradiative recombination. Here, the fluorinated pseudohalide ionic liquid (FPH-IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) is introduced into the perovskite precursor formulation. EMIMTFSI can change the dielectric environment and energy-level arrangement of the perovskite by accumulating on the top surface and spontaneously forming dipoles. As a result, the excitonic binding energy (Eb) and nonradiative recombination loss are significantly reduced. At the same time, TFSI- reduces the formation energy of vacancy defects and stabilizes the perovskite phase by forming N-H⋯F hydrogen bonds between FA+ and the C-F bond in EMIMTFSI. Finally, the EMIMTFSI-modified p-i-n PSCs achieve an excellent efficiency of 24.81% with an impressive open-circuit voltage of 1.191 V for a 1.57 eV low-bandgap perovskite. In addition, the modified devices can maintain more than 95% PCE after continuous thermal aging at 85 °C for 500 h or illumination at the maximum power point for 800 h. This work provides a new idea for minimizing the non-radiative recombination losses in p-i-n PSCs.

Preparation and thermoelectric properties of Sc-doped Ti&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;NiSb half-Heusler alloys

The nominal composition TiNiSb with 19 valence electrons is demonstrated to be composed of off-stoichiometric half-Heusler phase and impurities. In this work, the Ti&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;NiSb (&lt;i&gt;x&lt;/i&gt; = 0, 0.10, 0.15, 0.20, 0.25) samples are prepared by ball milling and spark plasma sintering. The single-phase Ti&lt;sub&gt;0.9&lt;/sub&gt;NiSb sample, deviating from the theoretical composition Ti&lt;sub&gt;0.75&lt;/sub&gt;NiSb base on 18-electron rule, is obtained, which might be ascribed to the small defect formation energy of Ti filling the vacancy as well as our ball-milling preparation method. With the single-phase Ti&lt;sub&gt;0.9&lt;/sub&gt;NiSb sample used as the base material, a small amount of Sc is used to partially replace Ti in order to further reduce the carrier concentration. Thus, the Ti&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;–&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;Sc&lt;sub&gt;&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;NiSb (&lt;i&gt;x&lt;/i&gt; = 0.10, 0.15; &lt;i&gt;y&lt;/i&gt; = 0.03, 0.05) samples are designed to investigate the effect of Sc doping on the thermoelectric properties. The X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) analysis confirm the single-phase nature of the Ti&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;–&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;Sc&lt;sub&gt;&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;NiSb samples. Energy-dispersive X-ray spectroscopy (EDS) results indicate that the actual compositions of the Ti&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;–&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;Sc&lt;sub&gt;&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt;NiSb samples are consistent well with their nominal compositions, and all elements are distributed uniformly in the sample. Moreover, the doping of Sc can increase the content of Ti vacancy while maintaining the single-phase structure, which could be attributed to the higher binding energy between Sc and Sb because the electronegativity of Sc is less than that of Ti. Both the substitution of Sc for Ti and the increase of the Ti vacancies significantly reduce the carrier concentration, which decreases from ~13.6 × 10&lt;sup&gt;21&lt;/sup&gt; cm&lt;sup&gt;–3&lt;/sup&gt; for Ti&lt;sub&gt;0.9&lt;/sub&gt;NiSb to ~3.4 × 10&lt;sup&gt;21&lt;/sup&gt; cm&lt;sup&gt;–3&lt;/sup&gt; for Ti&lt;sub&gt;0.8&lt;/sub&gt;Sc&lt;sub&gt;0.05&lt;/sub&gt;NiSb. The reduced carrier concentration results in greatly increased Seebeck coefficient, therefore the Ti&lt;sub&gt;0.8&lt;/sub&gt;Sc&lt;sub&gt;0.05&lt;/sub&gt;NiSb sample achieves a power factor as high as 17.7 μW·cm&lt;sup&gt;-1&lt;/sup&gt;·K&lt;sup&gt;-2&lt;/sup&gt; at 973 K. Although the lattice thermal conductivity of Sc-doped sample increases slightly due to the reduction of electron–phonon scattering and the enhancement of chemical bonds, the total thermal conductivity decreases dramatically due to the electronic thermal conductivity decreasing greatly. Finally, the Ti&lt;sub&gt;0.8&lt;/sub&gt;Sc&lt;sub&gt;0.05&lt;/sub&gt;NiSb sample reaches a &lt;i&gt;ZT&lt;/i&gt; value of ~0.42 at 973 K, which is 180% higher than that of Ti&lt;sub&gt;0.9&lt;/sub&gt;NiSb sample. Despite the fact that the thermoelectric performance of our sample is still inferior to those of the state-of-the-art off-stoichiometric 19-electron half-Heusler alloys, this work demonstrates that the thermoelectric performance of Ti&lt;sub&gt;1–&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;NiSb can be further improved by non-isoelectronic doping.

Radiation response of Y3Al5O12 and Nd3+-Y3Al5O12 to Swift heavy ions: insight into structural damage and defect dynamics.

Understanding the behavior of a material under irradiation is paramount to its application in the nuclear industry. The present work explores the radiation response of garnet Y3Al5O12 (YAG) and Nd3+-substituted Y3Al5O12 (Nd-YAG) under a 100 MeV Iodine beam at varying fluences to mimic the effect of fission fragments. This is relevant to the potential application of garnet as a host for minor actinide (MA) transmutation (Nd3+: surrogate for long-lived MA (Am3+, Np3+, Cm3+)). The un-irradiated and irradiated YAG and Nd-YAG samples were investigated by X-ray diffraction and Raman spectroscopy. Positron annihilation spectroscopy, thermal spike modelling and theoretical studies have been employed to understand the role of substitution and defect energetics in influencing this radiation response. Although both materials were not completely amorphized under the present irradiation conditions, a tremendous loss in crystallinity could be observed with increase in fluence, the damage being much more in Nd-YAG. Ion track radii of 2.17 nm and 2.91 nm were estimated for YAG and Nd-YAG respectively. Thermal-spike calculations show an increase in radiation-induced transient temperatures upon Nd-substitution that causes greater radiation damage in Nd-YAG. The enhancement in radiation-induced damage with increasing ion-fluence manifests in broadening and weakening of the Raman modes and XRD peaks. An increase in the average positron annihilation lifetime indicated the creation of oxygen vacancies. The defect formation energies of Y3Al5O12 have been theoretically estimated via density functional theory (DFT) and unfavorable energies required for creating cation pair anti-sites have been proposed as one of the possible reasons for the relatively poorer radiation response of YAG. The irradiation behavior of Y3Al5O12 has been compared with disordered fluorite (YSZ) and zirconate pyrochlores, which are well-researched ceramics for MA transmutation.

Enhanced stability of triple-halide perovskites CsPbI3-x-yBrxCly (x and y = 0-0.024): understanding the role of Cl doping from ab initio calculations.

Doping of chloride in mixed iodide-bromide perovskites has been shown experimentally to suppress the photo-induced halide-ion segregation and enhance the stability of triple-halide perovskites (THP). However, a fundamental understanding of the effects of Cl doping is yet to be achieved especially when the doping concentration is low. Here we report the results of a state-of-the-art ab initio study of the atomic structure of THP by considering small doping concentrations of Br and Cl in CsPbI3. We find a reduction in the Pb-I bond lengths and tilting of PbI6 octahedra with Cl doping which lead to exothermic heat of mixing and therefore higher stability of THP. Moreover, using quasi-chemical approximation, our results show that there is a very small contribution of configurational entropy to Gibbs free energy at such low doping concentrations and at the operational temperature of 50 °C. This suggests that the favorable heat of mixing value is more important for the stability at low doping concentrations of Cl while a higher concentration of Cl increases the risk of halide segregation. Further calculations on Frenkel defect formation energy of I or Br-interstitial shows that the doping of Cl in I/Br mixed binary-compounds hinders the formation of Frenkel defects. These results support experiments and help to understand the role of chloride in suppressing the halide ion mobility with only a slight increase in the band gap. Accordingly, the THPs manifest a promising pathway for developing single-phase perovskites for solar cells and light-emitting diodes with improved performance and enhanced stability.

Probing site-selective doping and charge compensating defects in KMgF3: insights from a hybrid DFT study.

Design of optoelectronic materials with tunable properties using activators and defect clusters has become one of the prime interests of current research. In this study, detailed Density Functional Theory based calculations have been presented to investigate the geometries and electronic structures of various possible defect clusters using Eu-KMgF3 as a probe which has numerous technological and industrial applications. Using a more reliable hybrid density functional, we have calculated defect formation energies and thermodynamic transition levels to get knowledge about the site selectivity of Eu. It has been observed that the electronic structure of Eu-KMgF3 is not only dependent on the site of doping but also on the oxidation state of Eu (2+/3+). The present study also investigates the relative stability of different kinds of defects and defect clusters under various synthetic growth conditions. The ultimate aim is to find out the microscopic origin of the fundamental optical properties of Eu-KMgF3 and provide an unambiguous explanation of available experimental results. Thus, it has been revealed that doping with Eu results in the spontaneous formation of intrinsic defects, which contribute to the observed optical behaviour. We have also extended our study to investigate the role of codoping with Li in determining the geometry and electronic structure of Eu-KMgF3 aiming to explain its impact on the optical properties. Thus, a complete presentation of the influence of the activator in the absence and presence of lattice defects on the optical properties of KMgF3 has been accomplished in the current study. We strongly believe that the present study will be helpful in designing tunable phosphor materials by a defect-controlled synthesis strategy.