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At least 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

A machine learning approach to quantify degradation of nuclear fuels and the effects of fission products

Nuclear fuel performance is critically dependent on understanding the evolution of fuel properties under operational conditions, a complex challenge driven by chemical changes and substantial radiation damage during fission. Traditionally, property evolution has been determined via empirical data collected following irradiation. However, these empirical correlations are limited in their applicability beyond the specific conditions in which they were obtained. This study explores a novel approach to address this challenge by applying materials informatics to develop a machine learning random forest (ML-RF) model that captures the effects of fission products on fuel compounds. The model predicts formation enthalpy (ΔH f ) by leveraging extensive quantum materials property data and correlating it with material descriptors such as composition, atomic and site features, and crystal lattice properties. This ML-RF model enables rapid interpolation across the compositional and structural spaces covered by the training data, thus supporting high-throughput screening and energetic ranking of candidate phases. The model demonstrates the ability to predict ΔH f with a mean absolute error (MAE) of approximately 0.1 to 0.2 eV/atom across a wide range of compounds, including key nuclear fuel systems (U-O, U-N, U-C, U-Si, and U-Mo). For example, it was used to assess shifts in stoichiometry for UO 2 (O/M) and UN (N/M) fuels, revealing their distinct tendencies in chemical potential variation and enabling preliminary convex hull analyses. Furthermore, the model provides insights into how individual fission products affect fuel properties. Results indicate that larger fission products (e.g., Nd, Pu, Ce) have a more pronounced impact on UO 2 , while lighter ones (e.g., Zr) strongly influence UN. Here, the model developed in this work can be used to support the Accelerated Fuel Qualification approach by facilitating preliminary evaluations prior to extensive materials modeling and experimentation. To this end, the trained model has been made available to the fuel community to support ongoing fuel development efforts.

Accelerated fuel qualification↗
DOE OSTI2025

Ab-initio informed cluster dynamics simulation of self- and Xe diffusivity in uranium mononitride under irradiation

Uranium mononitride (UN) is one of the ceramic nuclear fuel alternatives to oxide fuels considered for light water reactor and advanced reactor designs, as it presents significant advantages such as high uranium density (better economics) and high thermal conductivity and melting point (increased safety). Self- and fission gas diffusivities need to be better understood, given that they influence key fuel performance phenomena like swelling and fission gas release. Recently, radiation enhanced diffusivity was investigated in UN by means of cluster dynamics simulations relying on empirical potential-based parameterizations, the reliability of which highly depends on the interatomic potential accuracy. Here, in this work, we refine this approach by determining, using ab-initio calculations, the properties of defect clusters containing vacancies, self-interstitials and Xe impurities. We also consider larger clusters than previous studies. The obtained dataset (formation enthalpies, entropies, and migration barriers) is used to parameterize a cluster dynamics model of mobile clusters, and to calculate the defect cluster concentrations under irradiation. This gives us access to the radiation enhanced self- and fission gas diffusivities. Although the resulting diffusivities are close to the values reported in the literature, we find important qualitative differences in the diffusion mechanisms. Capturing the correct mechanisms is crucial to properly describe the chemistry and fission rate dependence of the model.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Thermodynamic properties of Ca–Pb electrodes determined by electromotive force measurements

Thermodynamic properties of Ca–Pb alloys are investigated by electromotive force (emf) measurements to determine equilibrium cell potentials and phase properties for their application in energy storage systems such as liquid metal batteries. Using the electrochemical cell Ca(s) | CaF 2 (s) | Ca(in Pb) at 700–1060 K, cell emf is measured for thirteen Ca–Pb alloys at mole fractions, x Ca = 0.06–0.80. At 873 K, the equilibrium potentials of liquid Ca–Pb alloys are 0.57–0.62 V versus Ca and the activity values are as low as a Ca = 6.2 × 10 -8 at x Ca = 0.06. In addition, the emf values as a function of temperature provide partial molar quantities (entropy and enthalpy) as well as phase transitions which are corroborated by determining transition temperatures and phase constituents using differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD). This study establishes the fundamental data necessary for the design of Pb-containing liquid metal electrodes through the integration of electrochemical, thermal, and structural properties of Ca–Pb electrodes.

25 ENERGY STORAGE↗
DOE OSTI2021

Potentiometric entropy and operando calorimetric measurements reveal fast charging mechanisms in PNb 9 O 25

The Wadsley–Roth compound PNb 9 O 25 is a promising fast charging lithium ion battery anode material with high operating voltage to prevent solid electrolyte interface formation. Here, in this work, we present potentiometric entropy measurements featuring signatures of semiconductor-to-metal transition and intralayer ordering upon lithiation in the anode material PNb 9 O 25 that could not be observed with in situ X-ray diffraction. In addition, the instantaneous heat generation rates at the PNb 9 O 25 working electrode and at the lithium metal counter electrode during galvanostatic cycling were measured individually for the first time by operando isothermal calorimetry. The heat generation rate decreased at the PNb 9 O 25 electrode upon lithiation due to the decrease in electrical resistivity caused by the semiconductor-to-metal transition observed in potentiometric entropy measurements. Furthermore, the heat generation rate at the lithium metal electrode was positive during delithiation due to the exothermic plating of Li + ions on the lithium metal counter electrode associated with dendrite formation. Furthermore, calorimetric measurements established that the entropy change dominated the reversible heat generation rate at each electrode. Finally, the contribution of enthalpy of mixing was relatively small even at high C-rates thanks to the high Li + ion mobility in PNb 9 O 25 confirming its promises as a fast charging anode material.

25 ENERGY STORAGE↗
DOE OSTI2026

Gas-driven short disconnection mitigates thermal runaway in Li-ion batteries under mechanical abuse

Mechanical abuse poses a critical safety risk to Li-ion batteries by inducing internal short circuits that initiate thermal runaway. Remarkably, voltage recovery frequently emerges during thermal runaway initiation, a phenomenon that conventional theories fail to explain. Here, our study develops a new multiphysics mechanism to explain voltage recovery, termed gas-driven short disconnection, whereby internal gas pressure mechanically disengages short-circuit contacts and causes the cell voltage to rebound. This mechanism incorporates gas generation and its structural impact on the short circuit. Real-time optical and thermal imaging and X-ray computed tomography reveal fluid-structure interaction between internal gas flow and adjacent shorting contacts. We establish a mechanistic framework linking gas-driven short disconnection to cell-level voltage and temperature responses, elucidating the extension–truncation pattern of voltage recovery. Furthermore, thermal regime maps show that a voltage recovery duration exceeding 5 s correlates with limited temperature rise below 150 °C, indicating that sustained short-circuit disconnection suppresses Joule heating. Additionally, a dimensionless criterion is deduced from scaling analysis for physical plausibility of gas-driven short disconnection in mechanically abused cells. This finding inspires smart venting control, which regulates gas release to maintain the internal pressure while dissipating gas enthalpy, thereby providing a device-level strategy for thermal runaway mitigation.

Gas-driven short disconnection↗
DOE OSTI2015

Oxygen potentials and phase equilibria in the system Ca–Co–O and thermodynamic properties of Ca{sub 3}Co{sub 2}O{sub 6} and Ca{sub 3}Co{sub 4}O{sub 9.163}

Oxygen potentials established by the equilibrium between three condensed phases, CaO{sub ss}+CoO{sub ss}+Ca{sub 3}Co{sub 2}O{sub 6} and CoO{sub ss}+Ca{sub 3}Co{sub 2}O{sub 6}+Ca{sub 3}Co{sub 3.93+α}O{sub 9.36−δ}, are measured as a function of temperature using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte and pure oxygen as the reference electrode. Cation non-stoichiometry and oxygen non-stoichiometry in Ca{sub 3}Co{sub 3.93+α}O{sub 9.36−δ} are determined using different techniques under defined conditions. Decomposition temperatures and thermodynamic properties of Ca{sub 3}Co{sub 2}O{sub 6} and Ca{sub 3}Co{sub 4}O{sub 9.163} are calculated from the results. The standard entropy and enthalpy of formation of Ca{sub 3}Co{sub 2}O{sub 6} at 298.15 K are evaluated. Using thermodynamic data from this study and auxiliary information from the literature, phase diagram for the ternary system Ca–Co–O is computed. Isothermal sections at representative temperatures are displayed to demonstrate the evolution of phase relations with temperature. - Graphical abstract: Isothermal section of the phase diagram of the system Ca–Co–O at 1250 K. - Highlights: • Improved definition of cation and oxygen nonstoichiometry of Ca{sub 3}Co{sub 3.93+α}O{sub 9.36−δ}. • Measurement of Δμ{sub O{sub 2}} associated with two 3-phase fields as a function of temperature. • Use of solid-state electrochemical cells for accurate measurement of Δμ{sub O{sub 2}}. • Decomposition temperatures and thermodynamic properties for ternary oxides. • Characterization of ternary phase diagram of the system Ca–Co–O.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Exploring phase stability, electronic and mechanical properties of Ce–Pb intermetallic compounds using first-principles calculations

The phase stability, electronic and mechanical properties of Ce–Pb intermetallics have been investigated by using first-principles calculations. Five stable and four metastable phases of Ce–Pb intermetallics were verified. Among them, CePb{sub 2} has been confirmed as HfGa{sub 2}-type structure. For Ce{sub 5}Pb{sub 3}, the high pressure phase transformation from D8{sub m} to D8{sub 8} with trivalent Ce has been predicted to occur at P=1.2 GPa and a high temperature phase transformation has been predicted from D8{sub m} to D8{sub 8} with tetravalent Ce at 531.5 K. The calculated lattice constants of the five stable phases are in good agreement with experimental values. The electronic density of states, charge density and electron localization function of Ce{sub 3}Pb have been calculated, which indicated that the Ce and Pb show ionic behavior. The polycrystalline bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are also estimated from the calculated single crystalline elastic constants. All of the calculated elastic constants satisfy mechanical stability criteria. The microhardness and mechanical anisotropy are predicted. The anisotropic nature of the Ce–Pb intermetallic compounds are demonstrated by the three-dimensional orientation dependent surfaces of Young's moduli and linear compressibility are also demonstrated. The longitudinal, transverse and average sound velocities and the Debye temperatures are also obtained in this work. The Ce{sub 3}Pb has the largest Debye temperature of 192.6 K, which means the Ce{sub 3}Pb has a highest melting point and high thermal conductivity than other compounds. - Graphical abstract: The convex hull plots of the enthalpies of formation for Ce–Pb binary systems calculated at 0 K. - Highlights: • The five stable and four metastable phases in the Ce–Pb binary system were predicted. • The crystal structure of CePb{sub 2} has been confirmed as HfGa{sub 2}-type.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

The role of deep acceptor centers in the oxidation of acceptor-doped wide-band-gap perovskites ABO{sub 3}

The impact of deep acceptor centers on defect thermodynamics and oxidation of wide-band-gap acceptor-doped perovskites without mixed-valence cations is studied. These deep centers are formed by the acceptor-bound small hole polarons whose stabilization energy can be high enough (significantly higher than the hole-acceptor Coulomb interaction energy). It is shown that the oxidation enthalpy ΔH{sub ox} of oxide is determined by the energy ε{sub A} of acceptor-bound states along with the formation energy E{sub V} of oxygen vacancies. The oxidation reaction is demonstrated to be either endothermic or exothermic, and the regions of ε{sub A} and E{sub V} values corresponding to the positive or negative ΔH{sub ox} are determined. The contribution of acceptor-bound holes to the defect thermodynamics strongly depends on the acceptor states depth ε{sub A}: it becomes negligible at ε{sub A} less than a certain value (at which the acceptor levels are still deep). With increasing ε{sub A}, the concentration of acceptor-bound small hole polarons can reach the values comparable to the dopant content. The results are illustrated with the acceptor-doped BaZrO{sub 3} as an example. It is shown that the experimental data on the bulk hole conductivity of barium zirconate can be described both in the band transport model and in the model of hopping small polarons localized on oxygen ions away from the acceptor centers. Depending on the ε{sub A} magnitude, the oxidation reaction can be either endothermic or exothermic for both mobility mechanisms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Hierarchical porous silver metal using Pluronic F-127 and graphene oxide as reinforcing agents for the reduction of o-nitroaniline to 1, 2-benzenediamine

An elegant method is used to prepare silver monoliths with Pluronic F-127(F-127) as sacrificial template by modified sol-gel method. Si nanoparticles (SiNPs) and graphene oxide (GO) are added in situ to Ag/F-127 hydrogel for the reduction of ο-nitroaniline (ο-NA) to 1, 2-benzenediamine. Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Thermogravimetric analysis (TGA), Raman Spectroscopy, Powder X-Ray Diffraction (PXRD) analysis and Brunauer-Emmett-Teller (BET) Nitrogen adsorption techniques were used for characterization of monoliths. An epoch-making catalytic activity of Ag/F-127/GO monoliths is observed in the reduction of ο-NA to 1, 2-benzenediamine in presence of NaBH{sub 4} in aqueous media. The catalyst Ag/F-127/GO took only 2 min which is the minimum time reported so far with significant rate constant claimed itself a leading catalyst for the reduction of ο-NA to 1,2-benzenediamine. Pseudo first order rate constant (k) and Turn over frequency (TOF) values are 0.231 min{sup −1} and 30.053×10{sup 19} molecules min{sup −1} respectively suggest that the catalyst has industrial importance. Recyclability and stability of Ag/F-127/GO catalyst are studied successfully up to 10 cycles. Energy of activation (E{sub a}), and thermodynamic parameters viz. activation enthalpy (ΔH{sup ≠}), activation Gibbs free energy (ΔG{sup ≠}), and entropy of activation (ΔS{sup ≠}) were also ascertained. Catalytic activities of Ag/F-127, Ag/F-127/Dextran, Ag/F-127/Trimethylbenzene (TMB), Ag/F-127/SiNPs, and Ag/F-127/Si/GO monoliths were also studied. - Graphical abstract: Significant catalytic activities of silver monoliths against the reduction of ο-NA to 1,2 benzenediamine. - Highlights: • A new catalyst synthesized Ag/F-127/GO for the reduction of ο- NA to 1, 2- benzenediamine took only 2 min. • Turn over frequency of as synthesized catalyst was 30.053×10{sup 19} molecules min{sup −1} claimed itself a leading catalyst. • Recyclability of the catalyst was up to 10 cycles. • The synthesis is non toxic, economically viable and environmentally benign.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

The native point defects of ternary C14 Laves phase Mg{sub 2}Cu{sub 3}Si: Ab initio investigation

Nine possible native point defects in ternary C14 Laves phase Mg{sub 2}Cu{sub 3}Si are investigated from ab initio calculation based on density function theory. The two-dimensional phase diagram of chemical potentials is determined, and then defect formation energy is calculated. The energetic results show that Cu{sub Si} and Cu{sub Mg} are more favorable over a broad range of chemical potentials. The formation enthalpies for defective Mg{sub 2}Cu{sub 3}Si also demonstrate the same stability. Local distortions around point defects increase from Si{sub Cu} to Si{sub Mg}, showing that the polyhedral symmetry and coordination number for constituent atoms will affect the structure of defects. The electronic structure shows the strong (Cu, Si)–Si covalent bonding, also indicates that the Cu{sub Mg} is favorable because the weak Mg-Cu bonding is replaced by the strong Cu–Cu bonding, whereas Mg{sub Cu} and Mg{sub Si} are unfavorable because the strong Cu–Si bonding is replaced by the weak Mg–(Cu, Si) bonding. - Graphical abstract: Defects formation energy of τ phase as a function of the chemical potential over the stable scopes A-B-C-O-D-A. Display Omitted - Highlights: • The two-dimensional phase diagram of chemical potentials is determined. • The defect Cu{sub Si} and Cu{sub Mg} is more favorable than other defects in Mg{sub 2}Cu{sub 3}Si. • Local distortions from Cu{sub Si} to Mg{sub Si} are similar to the trend of energetic results. • The Mg{sub Cu} and Mg{sub Si} are unfavorable because Cu (Si)–Mg bonding is weaker than Cu-Si. • Mg{sub 2}Cu{sub 3}Si phase has the strong Cu-Si and Si-Si covalent character.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Electronic structural, optical and phonon lattice dynamical properties of pure- and La-doped SrTiO 3 : An ab initio thermodynamics study

To better understand the electronic, optical, and thermodynamic behaviors of lanthanum doped strontium titanate (La-STO) with different doping levels at high temperature, ab initio thermodynamics have been employed to investigate their electronic structures and thermodynamic evolutions versus temperatures. Here, the results show that with increasing La-doping level, the corresponding structures of La-STO become less stable with an increasing number of phonon soft modes. The calculated thermodynamic properties show that with increasing La-doping levels, the free energy and enthalpy changes of La-STOs are decreased at given temperature. For all La-STOs with different La-doping levels investigated, we found that in three cases doping with 25%, 37.5% and 75% La over Sr, the diagonal elements of their dielectric constant matrix are not all equal, which means these crystal structures are uniaxial and anisotropic. Our results indicate that doping La into STO could alter the optical properties of pure STO to be birefringent.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2018

Synthesis, reversibility and mechanism of the dehydration of monoclinic CaZn{sub 2}(PO{sub 4}){sub 2}·2H{sub 2}O

Highlights: • Reversibility of dehydration of synthetic monoclinic CaZn{sub 2}(PO{sub 4}){sub 2}·2H{sub 2}O to CaZn{sub 2}(PO{sub 4}){sub 2}·H{sub 2}O is proven. • Morphological changes upon dehydration to mono- and anhydrate are investigated. • The diffraction pattern of the monohydrate is presented and indexed. • Lattice constants up to the monohydrate are derived from in-situ XRD measurements. • A topotactic dehydration mechanism is deduced. - Abstract: The reversibility of the dehydration of monoclinic CaZn{sub 2}(PO{sub 4}){sub 2}·2H{sub 2}O to the corresponding monohydrate is proven and the dehydration enthalpy determined by combination of thermal analysis and X-ray diffraction. The mechanism of the monohydrate formation is investigated by scanning electron microscopy (SEM) and temperature dependent in situ X-ray powder diffraction. Mechanistic impact on the powder bulk properties are deduced. As for the orthorhombic dimorph, a crystalline monohydrate species is identified and firstly indexed. Thermal induced changes of the lattice parameters up to the formation of the monohydrate are analyzed. A linear elongation of the lattice parallel to the directions [100] and [001] is found. The corresponding elongation coefficients and the volumetric expansion of the unit cell are determined. With regard to the option of synthetic production of the compound, a possible application of the material for thermochemical energy storage and conversion is conceivable. The relevance of fundamental mechanistic features on the bulk behaviour upon cyclic de- and rehydration is discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Unraveling the formation mechanism of NaCoPO 4 polymorphs

Phosphate compounds are one of the attractive classes of cathodes for their potential application in rechargeable Na-ion batteries (NIBs) due to their high insertion voltages and stabilities. Unlike olivine-LiMPO 4 (M = Fe, Mn, Co, Ni) cathodes, maricite-NaMPO 4 phases are electrochemically inactive due to lack of sodium diffusion channels. Interestingly, cobalt bearing compounds exhibit rich polymorphism (α-, β-, ABW- and Red-NaCoPO 4 ). However, the preparation of NaCoPO 4 single phases is challenging due to their closer enthalpies of formation. In this study, we have attempted to understand the formation mechanism of NaCoPO 4 polymorphs during solid state reaction using two different cobalt precursors (Co(OH) 2 and CoCO 3 ). The temperature dependent in-situ powder X-ray Diffraction (PXRD) studies showed the formation of β-NaCoPO 4 at a much lower temperature compared to the one reported in the literature, whilst we have discovered a new high temperature polymorph gamma-NaCoPO 4 . The choice of precursors was also found to be important for the preparation of ABW-NaCoPO 4 , i.e., the rapid decomposition of Co(OH) 2 has resulted in its formation whilst the high stability of CoCO 3 prohibited the same. The order of the formation of the NaCoPO 4 polymorphs during heating was found to be ABW→β→α →$\gamma$. During normal cooling, gamma-NaCoPO 4 transformed primarily into alpha-polymorph whilst quenching in liquid N-2 resulted in the formation of β-NaCoPO 4 The as-synthesized beta- and ABW-NaCoPO 4 phases exhibited high sodium (de)-intercalation voltages (similar to 4.2 and 4.5 V vs. Na + /Na 0 , respectively), albeit with limited reversible capacities. This study is expected to open new avenues to harvest metastable polymorphs based on other transition metals (Fe, Mn and Ni).

25 ENERGY STORAGE↗
DOE OSTI2024

From phase decomposition to evaporation: A multi-modal evaluation of thermally degraded model lightweight high-entropy alloy

Lightweight high-entropy alloys (LHEAs) have the potential to replace conventional lightweight materials due to their superior mechanical properties and thermal stability. However, the thermal degradation pattern of LHEAs from phase decomposition to evaporation is not clear. We develop a new Al-based dual phase (FCC + HCP) LHEA—AlTi 0.45 CuZn, and further investigate its thermal degradation behavior for potential high-temperature structural applications. Using multimodal advanced characterization techniques such as differential scanning calorimetry/thermogravimetric analysis, scanning/transmission electron microscopy, and synchrotron X-ray diffraction/pair distribution function (XRD/PDF), a sequence of thermal degradation events beyond the thermal phase stability limit—between 250 and 360 °C—is observed. These include phase decomposition at ~360 °C, Zn evaporation at ~750 °C, and LHEA melting at 880 °C which results in ~25% cumulative weight loss. The formation of Al-Ti phase off the AlTi 0.45 CuZn matrix is due to the largest negative mixing enthalpy for Al-Ti than other binary pairs. Similarly, Zn evaporation from AlTi 0.45 CuZn LHEA is due to its faster evaporation rate than other constituent elements. The high-resolution synchrotron XRD and PDF results support the aforementioned observations; in addition, they reveal local atomic arrangements, local strain, and sluggish grain growth in the LHEA. Among other LHEAs of close density range (5.55 ≤ ρ ≤ 5.85 g/cc), the investigated LHEA exhibits outstanding nano-indentation hardness values due to the coupled grain size effect and HCP phase strengthening of the FCC matrix. As the search for LHEAs for lightweight applications grows, this study shows the potential use of AlTi 0.45 CuZn LHEA for structural applications even at elevated temperatures.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

A new strategy to design eutectic high-entropy alloys using simple mixture method

Eutectic high entropy alloys (EHEAs) hold promising industrial application potential, but how to design EHEA compositions remains challenging. In the present work, a simple and effective strategy by combining mixing enthalpy and constituent binary eutectic compositions was proposed to design EHEA compositions. This strategy was then applied to a series of (CoCrFeNi)M x (M = Nb, Ta, Zr, Hf) HEAs, leading to the discovery of new EHEAs, namely, CoCrFeNiNb 0.45 , CoCrFeNiTa 0.4 , CoCrFeNiZr 0.55 and CoCrFeNiHf 0.4 . The microstructure of these new EHEAs comprised of FCC and Laves phases in the as-cast state. In conclusion, the experimental result shows that this new alloy design strategy can be used to locate new EHEAs effectively.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural, electronic and thermodynamic properties of britholites Ca{sub 10−x}La{sub x}(PO{sub 4}){sub 6−x}(SiO{sub 4}){sub x}F{sub 2} (0 ≤ x ≤ 6): Experiment and theory

Highlights: • The structural and electronic properties of apatites with the general formula Ca{sub 10−x}La{sub x}(PO{sub 4}){sub 6−x}(SiO{sub 4}){sub x}F{sub 2} (0 ≤ x ≤ 6) have been investigated by means of the density functional theory. • The calculated enthalpy of formation of the compounds increased with the substitution degree. • The decrease in stability is probably due to the disorder induced in the Ca/LaO{sub 6}F polyhedron, following the substitution. - Abstract: The apatite-type compounds Ca{sub 10−x}La{sub x}(PO{sub 4}){sub 6−x}(SiO{sub 4}){sub x}F{sub 2} with 0 ≤ x ≤ 6 were prepared using a high temperature solid state reaction and were characterized by X-ray diffraction. The crystal structure, chemical bonding, electronic structure and formation energy of all relaxed structures were analyzed by density functional theory (DFT). The calculated results show that the predicted geometry can well reproduce the structural parameters. The incorporation of La{sup 3+} into the fluorapatite (FA) structure induced especially at the level of the S(2) sites a certain disorder which is responsible for the weakening in the stability with x. Excellent agreement were obtained between the calculated and experimental results. Moreover, the band structure indicates that despite the reduction of the band gap with x content all materials remain insulating.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Systematic DFT study of the impact of anionic variations on the physical properties of Cd1-xMnxX (XS, Se, Te; x = 6.25%)

Highlights: • The structural and electronic properties are investigated with most accurate mBJ potential. • The FM state stability is illustrated for Cd{sub 0.9375}Mn{sub 0.0625}S/Se/Te DMSs. • The FM semiconducting nature depicts potential applications in magneto-optic applications. • The absorption shifts within the visible range with S to Te variation that is verified by E{sub g}. • Thermoelectric properties indicate dominant hole-carriers and suitability at high temperatures. - Abstract: The structural and electronic properties are theoretically investigated for Cd{sub 1-x}Mn{sub x}X (XS, Se, Te; x = 6.25%) diluted magnetic semiconductors, which are further employed for extracting the magnetic, optical and thermoelectric properties using modified-Becke-Johnson (mBJ) potential. Ferromagnetic state stability is confirmed through the computed enthalpy of formation and the cohesive energies. The electronic properties elucidate Cd{sub 1-x}Mn{sub x}X (XS, Se, Te; x = 6.25%) as ferromagnetic semiconductors because both spin channels have Fermi energy level (E{sub F}) within the band gap, depicting potential applications in spintronic devices. The calculated optical response indicates shifts in the absorption within the visible energy that is consistent with the computed direct band gap values (1.9 eV–3.1 eV). Thermoelectric properties observed within 200 K–800 K show positive Seebeck coefficient indicating holes as majority charge carriers. Although Mn-doping suppresses the power factor, however, it improves with temperature, showing a potential for optimizing the thermoelectric performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗