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All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

First-principles investigation of cerium and neodymium diffusion in BCC chromium and vanadium via vacancy-mediated transport

Lanthanide transport plays a crucial role in the performance and longevity of metallic nuclear fuels. This study examines the diffusion behavior of Ce and Nd—two major fission products—in body-centered cubic (BCC) Cr and V, which are potential liner or coating materials for mitigating fuel-cladding chemical interactions (FCCI). Using density functional theory (DFT) calculations and self-consistent mean-field (SCMF) analysis, the vacancy-mediated diffusion coefficients are evaluated. Our findings reveal that Ce and Nd act as oversized solutes and are strongly bound to vacancies in BCC Cr and V, with diffusivities in Cr significantly lower than in V and in hexagonal closed-packed (HCP) Zr, as investigated in our previous work. The activation energies for Ce and Nd diffusion are 3.39 and 3.32 eV, respectively, in BCC Cr, and 2.56 and 2.33 eV, respectively, in BCC V. Analysis of vacancy drag and partial diffusion coefficient ratios indicates a strong tendency for lanthanide enrichment at vacancy sinks in BCC Cr, and to a lesser extent in BCC V, with this effect persisting up to the melting point in Cr and remaining substantial for Nd in V at high temperatures. Under irradiation, the increase in vacancy concentration is expected to enhance lanthanide transport, potentially accelerating interactions at liner-cladding interfaces. Although BCC Cr exhibits relatively low lanthanide diffusivities under equilibrium conditions, the expected segregation tendencies under irradiation suggest that Zr liners may be a more favorable option. Further investigations using rate theory, cluster dynamics, and phase-field modeling are required to quantitatively assess the performance of these materials in reactor environments.

36 - MATERIALS SCIENCE↗
DOE OSTI2012

Elastic and electronic properties of the Ti{sub 5}X{sub 3} (X=Si, Ge, Sn, Pb) compounds from first-principles calculations

The structural stabilities, elastic and electronic properties of Ti{sub 5}X{sub 3} (X=Si, Ge, Sn, Pb) compounds with D8{sub 8} structure have been systematically investigated within the frame work of density functional theory. With increase of atomic number of the element X, the calculated lattice constants of Ti{sub 5}X{sub 3} compounds are found to increase, while stabilities of these compounds decrease because the formation enthalpies increase. Moreover, with exception of C{sub 12}, the calculated elastic constants C{sub ij} are reduced with increase of atomic number of element X, the bulk, shear, Young's modulus and Debye temperature are also lowered, while Poisson's ratio is larger, and the ductility is slightly improved. The elastic anisotropies were further studied, especially the 3D direction dependence of these elastic modulus was visually described. The density of states and charge density distribution reveal bonding feature of Ti{sub 5}X{sub 3}. Especially, the X-X and Ti-X covalent bonding strength is found to decrease with increase of atomic number of element X. - Graphical abstract: The charge density contour plots on the (0 0 0 1) plane for Ti{sub 5}Si{sub 3} (a), Ti{sub 5}Ge{sub 3} (b), Ti{sub 5}Sn{sub 3} (c) and Ti{sub 5}Pb{sub 3} (d). Highlights: Black-Right-Pointing-Pointer The elastic constants except C{sub 12} were lowered with increase of atomic number of X. Black-Right-Pointing-Pointer The bulk and shear modulus as well as Young's modulus was also decreased. Black-Right-Pointing-Pointer The elastic anisotropies of Ti{sub 5}X{sub 3} were further studied visually. Black-Right-Pointing-Pointer The X-X and Ti-X covalent bonding strength was lowered.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Electronic structure and optical properties of the nonlinear optical crystal Pb{sub 4}O(BO{sub 3}){sub 2} by first-principles calculations

Pb{sub 4}O(BO{sub 3}){sub 2} has a layered-type arrangement with optimally aligned BO{sub 3} triangles. The optical band gap is 3.317 eV obtained via the extrapolation method from the UV-vis-IR optical diffuse reflectance spectrum, consequently the absorption edge is about 374 nm. Density functional calculations using a generalized gradient approximation were utilized to investigate the electronic structures and optical properties of Pb{sub 4}O(BO{sub 3}){sub 2}. The calculated band structures show a direct gap of 2.608 eV, which is in agreement with the experimental optical band gap. A delocalized {pi} bonding of BO{sub 3} triangles and the stereo-effect of the lone pair 6s{sup 2} of lead cations are studied in electron densities. The birefringence is about 0.039-0.061 with the wavelength larger than about 375 nm. The calculated second-order susceptibility d{sub 24}=3.5 d{sub 36} (KDP) which is well consistent with the powder SHG intensity. - Graphical abstract: The density of state (DOS) show that the bottom of the valence bands is mainly derived from of the lone pair 6s{sup 2} of Pb{sup 2+}, and the top of the valence band is attributed to the hybridization orbitals from B-O groups. Calculated electronic structures indicate that the BO{sub 3} group with typical delocalization {pi} orbitals and strongly distorted lead oxygen polyhedra with highly asymmetric lobes on lead cations make a large SHG effect in Pb{sub 4}O(BO{sub 3}){sub 2}. Highlights: Black-Right-Pointing-Pointer Lone pair effect on Pb{sup 2+} and delocalization {pi} orbital in BO{sub 3} group is studied. Black-Right-Pointing-Pointer The combination of PbO{sub n} (n=3,4,5) and BO{sub 3} group makes Pb{sub 4}O(BO{sub 3}){sub 2} a large SHG effect. Black-Right-Pointing-Pointer Pb{sub 4}O(BO{sub 3}){sub 2} is a direct gap material with the gap 2.608 eV by the ab initio method. Black-Right-Pointing-Pointer The calculated birefringence is about 0.039-0.061 with the wavelength of about 375 nm. Black-Right-Pointing-Pointer The second-order susceptibility d{sub 24}=3.5d{sub 36} (KDP) in accordance with the experimental result.

36 MATERIALS SCIENCE↗
DOE OSTI2013

First-principles study of structural, electronic and thermodynamic properties of EuMg{sub 2} and EuMg{sub 2}H{sub 6}

Within framework of the density functional theory, the structural and electronic properties of EuMg{sub 2} and its hydride EuMg{sub 2}H{sub 6} have been investigated. The calculated structural parameters are in good agreement with the experimental results, and the obtained formation enthalpies indicate that the stability of EuMg{sub 2} is lower than that of EuMg{sub 2}H{sub 6}. The calculated electronic structures show that both compounds exhibit a strong spin polarization feature near the Fermi level owing to the localized Eu-f electrons, and the EuMg{sub 2}H{sub 6} demonstrates mainly ionic characteristic. For thermodynamics of hydrogenation/dehydrogenation process of this system, the calculated reaction enthalpies along possible five reaction routes show that the most possible hydrogenation pathway is EuMg{sub 2}+3H{sub 2}{yields}EuMg{sub 2}H{sub 6}, while the favorable dehydrogenation pathway is EuMg{sub 2}H{sub 6}{yields}EuH{sub 2}+2Mg+2H{sub 2}. - The charge difference distribution (e/Bohr{sup 3}) (a) on (1010) plane of EuMg{sub 2}, (b) on (110) and (c) (020) planes of its hydride EuMg{sub 2}H{sub 6}. Highlights: Black-Right-Pointing-Pointer The Eu-f electron has important effects on stability and hydrogen storage behavior. Black-Right-Pointing-Pointer The hydride exhibits primary ionic characteristics. Black-Right-Pointing-Pointer Three types of Mg--H bonding display also obvious ionic feature with some covalence. Black-Right-Pointing-Pointer The dehydrogenation temperature of EuMg{sub 2}H{sub 6} is further determined.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Structural and elastic properties of La{sub 2}Mg{sub 17} from first-principles calculations

Structural and elastic properties of La{sub 2}Mg{sub 17} with layer structure have been investigated within framework of the density functional theory. Different from the general layer-structured materials, the obtained c/a is less than unity. The calculated elastic constants C{sub 33} is larger than C{sub 11}, being novel in comparison with other alloys with layer structure. The calculated bulk, shear and Young’s modulus of La{sub 2}Mg{sub 17} are higher than other Mg–La alloys with higher La content, implying the stronger covalent bonding. Moreover, the elastic isotropies of La{sub 2}Mg{sub 17} are more excellent. The electronic structure within basal plane is highly symmetric, and the electronic interaction within basal plane is slightly weaker than one between basal planes, which reveal the underlying mechanism for the structural and elastic properties of La{sub 2}Mg{sub 17}. - Graphical abstract: The crystal structure (a) and the atomic positions for (b) (0 0 0 2), (c) (0 0 0 4) and (d) (1 2{sup ¯} 1 0) plane of La{sub 2}Mg{sub 17}. Display Omitted - Highlights: • The c/a of La{sub 2}Mg{sub 17} is anomalously less than unity. • It is novel that for La{sub 2}Mg{sub 17} the elastic constants C{sub 33} is larger than C{sub 11}. • The elastic modulus of La{sub 2}Mg{sub 17} is higher than other Mg–La alloys. • The elastic isotropy of La{sub 2}Mg{sub 17} is excellent. • The electronic structure within basal plane is highly symmetric.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Effects of the slab thickness on the crystal and electronic structures of In{sub 2}O{sub 3}(ZnO){sub m} revealed by first-principles calculations

Many conflicting electron microscopy data for In{sub 2}O{sub 3}(ZnO){sub m} indicate that it may have the polymorphous and polytypoid structures. We investigate their stabilities based on four controversial models. The calculated results confirm that the models with the zigzag feature are more stable than the others and it is possible to form different zigzag configurations in the samples as observed in the experiments. The dynamic process of eliminating the dangling bonds and the requirements of maximizing the symmetry and the distances between the In atoms in the slabs can be regarded as the dominant rules to stabilize the system, but the statistical equilibrium processes have the chances to transform it from the ground state structures to the other model structures. The study of the electronic structures based on the plane and zigzag models reveals that their band gaps and effective masses increase monotonically with m. The predicted band gaps are consistent with the experimental results. The anisotropic feature of electron effective mass tensor exhibited in the plane model differs from that of the zigzag one, which is so notable that can be employed to determine which model is more close to the actual structure of a given sample. The calculated results confirm the possibilities of the separation of conduction electrons and defects and the existence of the natural optimized transport channels in the layered structures, which demonstrate its advantage over ZnO to transport electrons and benefit its applications in the optoelectronic devices. - Graphical abstract: The conduction electrons are mainly distributed around the boundaries of the plane or zigzag shape. The optimized transport channels can be formed around the boundaries. - Highlights: • The formation mechanisms for the polytypoid structure of In{sub 2}O{sub 3}(ZnO){sub m} are revealed. • The predicted band gaps are consistent with the experimental results. • The natural optimized transport channels in the layered structures are confirmed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Prediction of novel hard phases of Si{sub 3}N{sub 4}: First-principles calculations

Exploration of novel hard metastable phases of silicon nitride was performed using a recently developed particle-swarm optimization method within the CALYPSO software package. Three potential hard metastable phases of t-Si{sub 3}N{sub 4}, m-Si{sub 3}N{sub 4}, and o-Si{sub 3}N{sub 4} were predicted. These phases are mechanically and dynamically stable at ambient pressure based on their elastic constants and phonon dispersions. t-Si{sub 3}N{sub 4} and m-Si{sub 3}N{sub 4} exhibit lower energies than γ-Si{sub 3}N{sub 4} at pressures below 2.5 GPa and 2.9 GPa, respectively, which promise that the formers could be obtained by quenching from γ-Si{sub 3}N{sub 4}. o-Si{sub 3}N{sub 4} is a better high-pressure metastable phase than CaTi{sub 2}O{sub 4}-type Si{sub 3}N{sub 4} proposed by Tatsumi et al. and it can come from the transition of γ-Si{sub 3}N{sub 4} under 198 GPa. The theoretical band gaps of t-Si{sub 3}N{sub 4}, m-Si{sub 3}N{sub 4}, and o-Si{sub 3}N{sub 4} at ambient pressure were 3.15 eV, 3.90 eV, and 3.36 eV, respectively. At ambient pressure, the Vickers hardness values of t-Si{sub 3}N{sub 4} (32.6 GPa), m-Si{sub 3}N{sub 4} (31.5 GPa), and o-Si{sub 3}N{sub 4} (36.1 GPa) are comparable to β-Si{sub 3}N{sub 4} and γ-Si{sub 3}N{sub 4}. With the pressure increasing, t-Si{sub 3}N{sub 4}, m-Si{sub 3}N{sub 4}, and o-Si{sub 3}N{sub 4} will change from the brittle to ductile state at about 15.7 GPa, 7.3 GPa and 28.9 GPa, respectively. - Graphical abstract: This figure shows the crystal structures of three Si{sub 3}N{sub 4} predicted in this manuscript, and left to right: t-Si{sub 3}N{sub 4}, m-Si{sub 3}N{sub 4} and o-Si{sub 3}N{sub 4}. - Highlights: • We explored three metastable phases of Si{sub 3}N{sub 4} — t-Si{sub 3}N{sub 4}, m-Si{sub 3}N{sub 4}, and o-Si{sub 3}N{sub 4}. • The enthalpies of t and m- are much lower than that of γ at ambient pressure. • ois one further high pressure phase than γ. • o-Si{sub 3}N{sub 4} is the most hardest phase in Si{sub 3}N4 system. • Increasing pressure will change them from brittle to ductile state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Lithium insertion properties of Li{sub x}TiNb{sub 2}O{sub 7} investigated by neutron diffraction and first-principles modelling

TiNb{sub 2}O{sub 7}, a good candidate as anode in lithium batteries, was treated with n-butyllithium to synthesize Li{sub x}TiNb{sub 2}O{sub 7} phases similar to those formed during electrochemical reactions. The Li{sub 2.67}TiNb{sub 2}O{sub 7} and Li{sub 3.33}TiNb{sub 2}O{sub 7} compounds, monoclinic C2/m, were studied by time-of-flight powder neutron diffraction. Their crystal structures, containing 3×3 blocks of (Ti,Nb)O{sub 6} octahedra, were Rietveld refined including Li positions. The Li atoms are distributed in similar amounts over sites with (Li–O) Coordination Numbers 5 and 4, although CN=5 should be preferred for having lower energy. Quantum-mechanical calculations were also performed, determining the average charge–discharge voltages to be 1.415 and 1.571 V for Li{sub 3.33}TiNb{sub 2}O{sub 7} and Li{sub 2.67}TiNb{sub 2}O{sub 7}, respectively, in good agreement with experimental results. An analysis of the theoretical charge distribution shows that, on lithiation, the chemical reduction of Ti and Nb atoms concentrates in the more condensed peripheral octahedra of the 3×3 block. This corresponds to electrons moving into a partly spin-polarized small band which gives rise to semi-metallic conductivity. - Graphical abstract: Structure of monoclinic Li{sub 3.33}TiNb{sub 2}O{sub 7}, and corresponding electron density of states. - Highlights: • Lithium was chemically inserted into the battery anode material TiNb{sub 2}O{sub 7}. • Li atoms are distributed over four- and fivefold oxygen coordinated sites. • On lithiation Nb and Ti atoms are reduced according to a peculiar structural pattern. • Theoretically predicted charge–discharge voltages fit experiments very well.

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

First-principles calculation on electronic structure and optical property of BaSi{sub 2}O{sub 2}N{sub 2}:Eu{sup 2+} phosphor

The crystal structure, electronic structure and optical properties of BaSi{sub 2}O{sub 2}N{sub 2}:Eu{sup 2+} with varying Eu doping concentrations are computed by the density functional theory (DFT) and compared with experimental results. The results show that the lattice parameters of primitive cells of Ba{sub 1−x}Si{sub 2}O{sub 2}N{sub 2}:Eu{sub x} become smaller and Eu–N bond length shortens as Eu concentration increases. The band structure of Ba{sub 1−x}Si{sub 2}O{sub 2}N{sub 2}:Eu{sub x} exhibits a direct optical band gap and it's propitious to luminescence. The energy differences from the lowest Eu 5d state to the lowest Eu 4f state decrease with increasing Eu concentrations. The analysis of simulative absorption spectra indicates that the electron transition from Eu 4f states to 5d states of both Eu and Ba atoms contributes to the absorption of Ba{sub 1−x}Si{sub 2}O{sub 2}N{sub 2}:Eu{sub x}. Under the coupling effect between Eu and Ba, Ba in BaSi{sub 2}O{sub 2}N{sub 2} exhibits longer wavelength absorption and increases absorption efficiency. The emission wavelength is deduced by measuring energy differences from the lowest Eu 5d state to the lowest Eu 4f state, and the result is in good agreement with experimental value within experimental Eu{sup 2+} doping range. - Graphical abstract: The structure and optical property of BaSi{sub 2}O{sub 2}N{sub 2}:Eu{sup 2+} are computed by DFT and its absorption mechanism is analysed. Results show that absorption peak α is from the host lattice absorption. The absorption peaks β, γ and δ are from Eu 4f to Eu 5d and Ba 6s 5d states. The absorption is attributed to the coupling effect of Eu and Ba atom. - Highlights: • The crystal, electronic structure and optical properties of BaSi{sub 2}O{sub 2}N{sub 2}:Eu{sup 2+} are computed by DFT. • The lattice parameters of primitive cells reduces and Eu–N bond length shortens as Eu{sup 2+} increases. • The energy gap from Eu 5d state to Eu 4f state decrease with increasing Eu concentrations. • Both Eu and Ba atoms contributes to the absorption of Ba{sub 1−x}Si{sub 2}O{sub 2}N{sub 2}:Eu{sub x}. • The deduced emission wavelength is in good agreement with experimental value.

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

Electrochemical properties and lithium ion diffusion in Li{sub 4}FeSbO{sub 6} studied by first principle

Due to the high capacity, Li-rich materials Li{sub 2}MO{sub 3} (M = transition metal) have attracted considerable attention as the next generation of Li-ion batteries. Li{sub 4}FeSbO{sub 6} is a new Li-rich layered oxide material with antiferromagnet honeycomb structure. In this work, the electrochemical behavior, charging process and oxygen stability of Li{sub x}FeSbO{sub 6} (0 ≤ x<4) material are investigated by using the spin-polarized density functional theory. Moreover, climbing image nudged elastic band (CI-NEB) method is used to calculate the Li ion migration properties. The results show that Li{sub 4}FeSbO{sub 6} exhibits poor inherent conductivity with the band gap of 2.45 eV. During charging process, the structure presents the excellent volume stability. When Li is extracted, the charge compensation is mainly contributed by the oxygen atoms through analyzing the Bader charges of each element. In addition, oxygen evolution reactions will occur in Li{sub x}FeSbO{sub 6} (x ≤ 1.5), which will decay the capacities during cycling process. Finally, we calculated that the lithium ion can diffuse in a three-dimensional pathway with the activation barriers from 0.36 eV to 0.67 eV. - Graphical abstract: Li{sub 4}FeSbO{sub 6} is a new Li-rich layered oxide material with antiferromagnet honeycomb structure. We calculated that the lithium ion can diffuse in a three-dimensional pathway Display Omitted.

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

First-principles calculation of lattice distortions in four single phase high entropy alloys with experimental validation

Exceptional properties of high entropy alloys (HEAs) are attributed to the disordered random solid solution of multiple alloying elements. Despite numerous studies, the fundamental understanding at the electronic and atomic levels is still missing. We report a comparative study of four fcc HEAs of NiFeCoCr and NiFeCoCrX (X = Mn, Cu, or Pd) based on ab initio calculations using large supercells with 500 atoms in equal composition. After fully optimizing their structures using the VASP package, their electronic structure, interatomic bonding, partial charge distribution, and mechanical properties are calculated and compared, revealing the intricate interdependence among them. A novel parameter based on the quantum mechanical metric for internal cohesion, the total bond order density (TBOD), is used to interpret the calculated properties. The highest TBOD is found in Cantor alloy NiFeCoCrMn but lower in NiFeCoCrPd. The atomic radii vary depending on their local chemical environment. The resulting lattice distortions is validated experimentally in NiFeCoCrMn and NiFeCoCrPd. Moreover, modeling of Cu and Pd clustering in the supercell shows they have lower total energy in agreement with the observation of Cu-enhanced nano-participates in NiFeCoCrCu and Pd-induced concentration wave in NiFeCoCrPd HEAs.

36 MATERIALS SCIENCE↗