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All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Cation Disorder of ${\text{Mg}}_{\mathbf{2}}{\text{SiO}}_{\mathbf{4}}$ in Super‐Earth Mantles

Understanding the mineralogy of exoplanets is essential for unraveling their interior structures, dynamics, and evolution. For large super-Earths, the post-post spinel ${\text{Mg}}_{\mathbf{2}}{\text{SiO}}_{\mathbf{4}}$, one of the major mantle phases, may undergo the order-disorder transition (ODT) at high temperatures. However, the ODT phase boundary of ${\text{Mg}}_{\mathbf{2}}{\text{SiO}}_{\mathbf{4}}$ has not been rigorously constrained. Additionally, fundamental thermodynamic properties of the disordered ${\text{Mg}}_{\mathbf{2}}{\text{SiO}}_{\mathbf{4}}$ remain poorly investigated. Here, we develop a unified machine learning potential (MLP) for ${\text{Mg}}_{\mathbf{2}}{\text{SiO}}_{\mathbf{4}}$ of ab initio accuracy under super-Earth mantle conditions. With the efficient MLP, we extensively calculate the free energy of post-post spinel ${\text{Mg}}_{\mathbf{2}}{\text{SiO}}_{\mathbf{4}}$ via the thermodynamic integration method. The results are used to constrain the ODT phase boundary. Furthermore, we report the P-V-T equation of state and Grüneisen parameters for post-post spinel ${\text{Mg}}_{\mathbf{2}}{\text{SiO}}_{\mathbf{4}}$ across various degrees of disorder. These thermodynamic properties are further applied to update the adiabatic thermal profiles and the mass-radius relation of super-Earths.

36 MATERIALS SCIENCE↗
DOE OSTI2021

High Mg activation in implanted GaN by high temperature and ultrahigh pressure annealing

Not provided.We demonstrate high p-type conductivity and hole concentrations >10 18 cm -3 in Mg-implanted GaN. The implantation was performed at room temperature and by post-implantation annealing at 1 GPa of N 2 and in a temperature range of 1200–1400 °C. The high pressure thermodynamically stabilized the GaN surface without the need of a capping layer. We introduce a “diffusion budget,” related to the diffusion length, as a convenient engineering parameter for comparing samples annealed at different temperatures and for different times. Although damage recovery, as measured by XRD, was achieved at relatively low diffusion budgets, these samples did not show p-type conductivity. Further analyses showed heavy compensation by the implantation-induced defects. Higher diffusion budgets resulted in a low Mg ionization energy (~115 meV) and almost complete Mg activation. For even higher diffusion budgets, we observed significant loss of Mg to the surface and a commensurate reduction in the hole conductivity. High compensation at low diffusion budgets and loss of Mg at high diffusion budgets present a unique challenge for shallow implants. A direct control of the formation of compensating defects arising from the implantation damage may be necessary to achieve both hole conductivity and low Mg diffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2022

β -Ga 2 O 3 MESFETs with insulating Mg-doped buffer grown by plasma-assisted molecular beam epitaxy

In this work, we develop in situ Mg doping techniques in plasma-assisted molecular beam epitaxy (PAMBE) of β -Ga 2 O 3 to compensate Si dopants at the substrate epilayer growth interface and eliminate parasitic leakage paths. Both abrupt and uniform Mg doping profiles over a wide range of concentrations were achieved in β -Ga 2 O 3 epilayers grown by PAMBE. Capacitance–voltage characteristics of Si and Mg co-doped samples confirmed the compensating effect of the Mg dopants. Mg delta-doping was then integrated into a β -Ga 2 O 3 metal-semiconductor field effect transistor structure and shown to be effective in eliminating source leakage. In conclusion, the results presented here show that Mg doping is a promising way to engineer insulating buffer layers for β -Ga 2 O 3 lateral devices grown by PAMBE.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Dissolution of Mg-enriched defects in implanted GaN and increased p-type dopant activation

Annealing Mg-implanted homoepitaxial GaN at temperatures above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects, in the form of inversion domains, contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N 2 overpressure), which results in a low dopant activation efficiency. Triple-axis x-ray data reveal that implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that strain-inducing point defects formed during implantation have reconfigured and inversion domains are formed. However, annealing at temperatures of 1400–1500 °C (one GPa N 2 overpressure) eliminates the presence of the inversion domains. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop density is observed after annealing at the higher temperatures and longer times. Additionally, once inversion domains are formed and the samples are cooled to room temperature, they are shown to dissolve with subsequent annealing above 1400 °C. While such defects have been observed before, the important finding that such defects can be dissolved with a short, higher temperature step is key. Earlier work [Breckenridge et al., J. Appl. Phys. Lett. 118, 022101 (2021)] addressing electrical measurements of these types of samples showed that annealing at 1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. In conclusion, this work complements earlier work by identifying the inversion domains, which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (>1400 °C) annealing cycles to activate Mg in GaN, even if the Mg-containing inversion domains had been formed during lower temperature annealing.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Mg and native defects in cubic silicon carbide from first principles

The diffusion of Mg defects in 3C-SiC is studied using the density functional theory. Mg has the highest burn-in rate as a transmutant in 3C-SiC when it is placed in high-energy neutron irradiation environment of a fusion reactor. The presence and evolution of transmutant defects impact thermal and mechanical properties of this important structural material. This study is focused on understanding the structure, stability, and evolution of Mg defects and the interaction of Mg with native defects in 3C-SiC. Our calculations of diffusion coefficients for different Mg defects suggest that Mg is likely to diffuse faster in pristine 3C-SiC than in the damaged one, in agreement with earlier experimental observations.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Host galaxies of ultrastrong Mg ii absorbers at z ∼ 0.5

ABSTRACT From a sample of 109 candidate ultrastrong Mg ii (USMg ii; having rest equivalent width of Mg ii, W2796 > 3.0 Å) systems at z = 0.4–0.6, we confirm 27 and identify host galaxies of 20 systems based on associated nebular line emission from our SALT observations or from Sloan Digital Sky Survey (SDSS) fiber spectra. The measured impact parameter, [O ii] luminosity, star formation rate, B-band luminosity, and stellar mass are in the ranges 7.3 ≤ D[kpc] ≤ 79, $0.2\le L_{[\mathrm{ O}\,~\small {\rm II}]}[ 10^{41}~\mathrm{ erg} \mathrm{ s}^{-1}]\le 4.5$, 2.59 ≤ SFR[M⊙yr−1] ≤ 33.51, $0.15L_B^{*}\le L_B\le 1.63L_B^{*}$, and 10.21 ≤ log[M*/ M ⊙] ≤ 11.62, respectively. The impact parameters found are larger than that predicted by the W2796 versus D relationship of the general population of Mg ii absorbers. At a given D, USMg ii host galaxies are more luminous and massive compared to typical Mg ii absorbers. However, the measured SFRs are slightly lower than that of main-sequence galaxies with same M⋆ at z ∼ 0.5. We report a correlation between $L_{[\mathrm{ O}\,\small {\rm II}]}$ and W2796 for the full population of Mg ii absorbers, driven mainly by the host galaxies of weak Mg ii absorbers that tend to have low $L_{[\mathrm{ O}\,\small {\rm II}]}$ and large impact parameters. We find at least ∼33 per cent of the USMg ii host galaxies (with a limiting magnitude of mr < 23.6) are isolated and the large W2796 in these cases may originate from gas flows (infall/outflow) in single haloes of massive but not starburst galaxies. We also find galaxy interactions could be responsible for large velocity widths in at least ∼17 per cent cases.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Fusion of 12 C+ 24 Mg far below the barrier: Evidence for the hindrance effect

Background: The phenomenon of fusion hindrance may have important consequences on the nuclear processes occurring in astrophysical scenarios, if it is a general behavior of heavy-ion fusion at extreme subbarrier energies, including reactions involving lighter systems, e.g., reactions in the carbon and oxygen burning stages of heavy stars. The hindrance is generally identified by the observation of a maximum of the S-factor vs energy. Whether there is an S-factor maximum at very low energies for systems with a positive fusion Q value is an experimentally challenging question. Purpose: Our aim has been to search for evidence of fusion hindrance in 12 C+ 24 Mg which is a medium-light m system with positive Q value for fusion, besides the heavier cases where hindrance is recognized to be a general phenomenon. 12 C+ 24 Mg is very close to the 16 O + 16 O and 12 C + 12 C systems that are important for the late evolution of heavy stars. Methods: The experiment has been performed in inverse kinematics using the 24 Mg beam from the XTU Tandem accelerator of LNL in the energy range 26-52 MeV with an intensity of 4-8 pnA. The targets were 12 C evaporations 50 mu g/cm 2 thick, isotopically enriched to 99.9%. The fusion-evaporation residues were detected at small angles by a E-Delta E-ToF detector telescope following an electrostatic beam deflector. Results: Previous measurements of fusion cross section for 12 C+ 24 Mg were limited to above-barrier energies. In the present experiment the excitation function has been extended down to similar or equal to 15 mu b and it appears that the S factor develops a clear maximum vs energy, indicating the presence of hindrance. This is the first convincing evidence of an S factor maximum in a medium-light system with a positive fusion Q value. These results have been fitted following a recently suggested method and a detailed analysis within the coupled-channels model that has been performed using a Woods-Saxon potential and including the ground state rotational band of 24 Mg. The coupled-channels calculations give a good account of the data near and above the barrier but overpredict the cross sections at very low energies. Conclusions: The hindrance phenomenon is clearly observed in 12 C+ 24 Mg, and its energy threshold is in reasonable agreement with the systematics observed for several medium-light systems. The fusion cross sections at the hindrance threshold show that the highest value (sigma(s) = 1.6 mb) is indeed found for this system. Therefore it may even be possible to extend the measurements further down in energy to better establish the position of the S-factor maximum.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Relative population of states in 21 Mg from few-nucleon removal reactions

Nuclear reactions with intermediate-energy beams in which three, four, and five nucleons are removed are expected to proceed through a combination of nondissipative and statistical processes. In an experiment at the National Superconducting Cyclotron Laboratory, in-beam γ-ray spectroscopy was utilized to study few-nucleon removal reactions from incoming beams of 24 Mg, 25 Al, and 26 Si projectiles to the same reaction product, 21 Mg. Here, new γ-ray transitions and γ-γ coincidences in 21 Mg were established using the CsI(Na) array CAESAR and the inclusive cross section for three-neutron removal from 24 Mg was measured. Significant differences in the relative population of states in 21 Mg from 25 Al compared to 26 Si were observed and found to be correlated with the spins of the 21 Mg states. As a result, this intermediate regime between direct and statistical nucleon removal may have potential as a tool to deliver unique patterns of level populations in fast-beam experiments and alter isomer to ground state ratios in the production of exotic beams.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2026

Low-energy measurement of the 25 Mg ⁢(𝛼,𝑛)⁢ 28 Si reaction via neutron spectroscopy

During core helium and carbon burning in massive stars, neutrons are produced mainly by the 22 Ne⁢ (𝛼,𝑛) ⁢25 Mg reaction. Some of these released neutrons are captured by heavy seed nuclei from previous nucleosynthesis events, resulting in the slow production of many of the elements between masses 60 ≤ 𝐴 ≤ 90 via the weak 𝑠 process. Determining the overall neutron flux available in this environment is one of the main challenges in modeling its contributions to heavy element production. Not only must the reaction rate be well defined for the primary neutron source, but the rates of neutron poison and recycling reactions must also be well determined. One critical reaction in the simulation network is 25 Mg ⁢(𝛼,𝑛) ⁢ 28 Si. This reaction, together with 25 Mg ⁢(𝑛,𝛾)⁢ 26 Mg and 25 Mg⁢ (𝛼,𝛾) ⁢ 29 Si, determine how 25 Mg nuclei affect the available neutron flux. Past attempts to measure the 25 Mg ⁢(𝛼,𝑛)⁢ 28 Si cross section using neutron moderator counters have been greatly hindered by lower-𝑍 background reactions. Here, in the present work, neutron spectroscopy with deuterated liquid scintillator detectors has been used. The experimental spectra have been analyzed by applying spectrum unfolding techniques to achieve improved background discrimination for the 25 Mg⁢ (𝛼,𝑛)⁢ 28 Si reaction at low energies, down to 𝐸 𝛼 = 1.75 MeV. The separation of the different background contributions gives further insight into the results of previous moderator counter measurements and the measurements lead to a revised and more reliable determination of the reaction rate.

hydrostatic stellar nucleosynthesis↗
DOE OSTI2023

Mitigating Corrosion in Mg Sheet in Conjunction with a Sheet-Joining Method that Satisfies Structural Requirements within Sub-assemblies

This work was undertaken as a LightMAT project funded by the DOE-Vehicles Technology Office. The goal of this work was to develop corrosion protection strategies that simultaneously mitigate corrosion and achieve Class-A surface finish for Mg components in automotive applications. While automotive metals such as steel and aluminum are protected against corrosion through a variety of coating schemes/packages, the efficacy of these existing coating schemes for Mg and Mg- joints is not clear and needs to be determined. Therefore, five commercially available coating schemes and two joining techniques (riveting and Arplas resistance spot welding) were evaluated. The corresponding individual Mg sheet coupons or Mg/Mg joint test coupons were provided by Magna that were then corrosion tested at PNNL using ASTM B117 procedure. The microstructures and mechanical properties of the coupons were analyzed to determine the effectiveness of the joining and corrosion mitigation strategies. Of the coating schemes evaluated, Henkel Bonderite MgC 2.0 pre-treatment + E-coat showed the best corrosion protection and surface finish for individual Mg coupons and Arplas resistance spot welded coupons. However, the strength of Mg/Mg welded joint was reduced after corrosion testing due to some corrosion at the weld nugget. Mg/Mg rivet joints in conjunction with Chemetall oxisilan pre-treatment + polyurethane coating showed good corrosion resistance and some discoloration on the surface finish. Coating schemes comprising pre-treatment with Alodine 5200 + E-coat or Bonderite 1455 + polyurethane coating, in conjunction with Al rivet joints, showed significant corrosion and extensive discoloration of the surface. We anticipate that the results from this work will provide useful guidance to the automotive industry in selecting the appropriate combinations of corrosion protection coatings and joining techniques to fabricate light-weight Mg-based automotive components.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Improving Interface Stability of Si Anodes by Mg Coating in Li-Ion Batteries

Silicon (Si) is a promising anode material for high-energy-density lithium-ion batteries (LIBs), but its short calendar life and poor cycling performance prevent its large-scale adoption. Introducing magnesium (Mg) salt into the electrolyte has been recently shown to form a ternary Li–Mg–Si Zintl phase upon lithiation of Si and improve the cycling performance. However, the ternary Zintl phase formation mechanism and its impact on the solid electrolyte interphase (SEI) are not yet well understood. In this work, we demonstrate the formation of a ternary Li–Mg–Si Zintl phase by Mg coating of the Si anode, where Mg diffuses into the Si film upon deposition and intermixes further during the lithiation process. The presence of the Zintl phase improves the interface stability, alters the nature of the SEI, and enhances the cycling performance of the Si anode. This study provides insights into the formation mechanism of the ternary Zintl phase and guidelines for the future design of Si anodes.

25 ENERGY STORAGE↗
DOE OSTI2024

Quantifying the Correlation between Coordination Chemistry, Interfacial Formation, and Electrochemical Performances for Mg Battery Electrolytes

Here, the rise of magnesium batteries as promising post-Li-ion energy storage technologies has sparked considerable attention toward understanding the fundamental aspects of coordination chemistry concerning Mg cations in multivalent electrolytes. This exploration includes investigating how coordination influences crucial electrolyte properties like solubility, electroreduction stability, and the formation of the interphase, all of which are pivotal for practical battery applications. Despite recent progress in developing a few functional electrolytes, a comprehensive understanding of the solvation structure that can facilitate efficient Mg deposition performance and the formulation of general design rules based on the solvation structure is still lacking. In our study, we endeavor to establish a connection between solvent and anion interactions with Mg 2+ , interface formation, and cycling performance through a series of organic ether solvents (tetrahydrofuran, glyme, diglyme, and triglyme) and amine solvents (dimethylamine, 3-methoxypropylamine, and dimethoxyethylamine). Our findings reveal a distinct coordination trend for solvent/Mg 2+ and (Mg-TFSI):solvent across various solvents, which dictates the extent of ion pairing for TFSI salts with increasing solvent molecule size and denticity. The solvated species in the bulk electrolyte across different solvents lead to diverse interfacial chemistries with varying decomposition components. We also explore the cycling efficiency as well as Mg deposition overpotentials for different solvents. A correlation analysis was conducted to assess the interplay between the structure and performance. Lastly, we apply the insights gained from these results to tailor the relative anion/Mg 2+ coordination structures using cosolvent systems, aiming for improved cell performance.

25 ENERGY STORAGE↗
DOE OSTI2020

Materials Data on Mg(NO3)2 by Materials Project

Mg(NO3)2 crystallizes in the trigonal R-3 space group. The structure is three-dimensional. there are four inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded in an octahedral geometry to six O2- atoms. There are a spread of Mg–O bond distances ranging from 2.06–2.16 Å. In the second Mg2+ site, Mg2+ is bonded in an octahedral geometry to six equivalent O2- atoms. All Mg–O bond lengths are 2.10 Å. In the third Mg2+ site, Mg2+ is bonded in an octahedral geometry to six O2- atoms. There are a spread of Mg–O bond distances ranging from 2.10–2.15 Å. In the fourth Mg2+ site, Mg2+ is bonded in an octahedral geometry to six O2- atoms. There are three shorter (2.08 Å) and three longer (2.15 Å) Mg–O bond lengths. There are six inequivalent N5+ sites. In the first N5+ site, N5+ is bonded in a trigonal planar geometry to three equivalent O2- atoms. All N–O bond lengths are 1.26 Å. In the second N5+ site, N5+ is bonded in a trigonal planar geometry to three O2- atoms. All N–O bond lengths are 1.26 Å. In the third N5+ site, N5+ is bonded in a trigonal planar geometry to three equivalent O2- atoms. All N–O bond lengths are 1.26 Å. In the fourth N5+ site, N5+ is bonded in a trigonal planar geometry to three equivalent O2- atoms. All N–O bond lengths are 1.26 Å. In the fifth N5+ site, N5+ is bonded in a trigonal planar geometry to three O2- atoms. All N–O bond lengths are 1.26 Å. In the sixth N5+ site, N5+ is bonded in a trigonal planar geometry to three O2- atoms. There is two shorter (1.26 Å) and one longer (1.27 Å) N–O bond length. There are twelve inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Mg2+ and one N5+ atom. In the second O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Mg2+ and one N5+ atom. In the third O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Mg2+ and one N5+ atom. In the fourth O2- site, O2- is bonded in a bent 150 degrees geometry to one Mg2+ and one N5+ atom. In the fifth O2- site, O2- is bonded in a bent 120 degrees geometry to one Mg2+ and one N5+ atom. In the sixth O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Mg2+ and one N5+ atom. In the seventh O2- site, O2- is bonded in a distorted bent 150 degrees geometry to one Mg2+ and one N5+ atom. In the eighth O2- site, O2- is bonded in a bent 120 degrees geometry to one Mg2+ and one N5+ atom. In the ninth O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Mg2+ and one N5+ atom. In the tenth O2- site, O2- is bonded in a bent 120 degrees geometry to one Mg2+ and one N5+ atom. In the eleventh O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Mg2+ and one N5+ atom. In the twelfth O2- site, O2- is bonded in a bent 120 degrees geometry to one Mg2+ and one N5+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(NO6)2 by Materials Project

MgO6(NO3)2 crystallizes in the monoclinic P2_1/c space group. The structure is zero-dimensional and consists of four nitric acid molecules and two MgO6 clusters. In each MgO6 cluster, Mg is bonded in a 6-coordinate geometry to six O atoms. There are a spread of Mg–O bond distances ranging from 1.93–2.60 Å. There are three inequivalent O sites. In the first O site, O is bonded in an L-shaped geometry to one Mg and one O atom. The O–O bond length is 1.25 Å. In the second O site, O is bonded in a single-bond geometry to one Mg atom. In the third O site, O is bonded in a 1-coordinate geometry to one Mg and one O atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(MnGe)6 by Materials Project

Mg(MnGe)6 crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Mg is bonded to eight Ge atoms to form distorted edge-sharing MgGe8 hexagonal bipyramids. There are two shorter (2.68 Å) and six longer (2.98 Å) Mg–Ge bond lengths. Mn is bonded in a 12-coordinate geometry to six Ge atoms. There are a spread of Mn–Ge bond distances ranging from 2.50–2.67 Å. There are three inequivalent Ge sites. In the first Ge site, Ge is bonded in a 8-coordinate geometry to one Mg, six equivalent Mn, and one Ge atom. The Ge–Ge bond length is 2.68 Å. In the second Ge site, Ge is bonded in a 6-coordinate geometry to six equivalent Mn atoms. In the third Ge site, Ge is bonded in a 9-coordinate geometry to three equivalent Mg and six equivalent Mn atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(BH3)2 by Materials Project

Mg(BH3)2 crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are four inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded in a 5-coordinate geometry to seven H+0.67+ atoms. There are a spread of Mg–H bond distances ranging from 2.00–2.30 Å. In the second Mg2+ site, Mg2+ is bonded in a 6-coordinate geometry to six H+0.67+ atoms. There are a spread of Mg–H bond distances ranging from 1.95–2.17 Å. In the third Mg2+ site, Mg2+ is bonded in a 6-coordinate geometry to six H+0.67+ atoms. There are a spread of Mg–H bond distances ranging from 1.98–2.04 Å. In the fourth Mg2+ site, Mg2+ is bonded in a 6-coordinate geometry to six H+0.67+ atoms. There are a spread of Mg–H bond distances ranging from 2.00–2.08 Å. There are eight inequivalent B3- sites. In the first B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H+0.67+ atoms. There are a spread of B–H bond distances ranging from 1.22–1.25 Å. In the second B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H+0.67+ atoms. There is one shorter (1.23 Å) and two longer (1.24 Å) B–H bond length. In the third B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H+0.67+ atoms. There are a spread of B–H bond distances ranging from 1.22–1.25 Å. In the fourth B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H+0.67+ atoms. There are a spread of B–H bond distances ranging from 1.23–1.25 Å. In the fifth B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H+0.67+ atoms. There are a spread of B–H bond distances ranging from 1.23–1.25 Å. In the sixth B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H+0.67+ atoms. There are a spread of B–H bond distances ranging from 1.22–1.25 Å. In the seventh B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H+0.67+ atoms. All B–H bond lengths are 1.24 Å. In the eighth B3- site, B3- is bonded in a distorted trigonal non-coplanar geometry to three H+0.67+ atoms. There are a spread of B–H bond distances ranging from 1.22–1.25 Å. There are twenty-four inequivalent H+0.67+ sites. In the first H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the second H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the third H+0.67+ site, H+0.67+ is bonded in a distorted L-shaped geometry to one Mg2+ and one B3- atom. In the fourth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the fifth H+0.67+ site, H+0.67+ is bonded in a 2-coordinate geometry to two Mg2+ and one B3- atom. In the sixth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the seventh H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the eighth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the ninth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the tenth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the eleventh H+0.67+ site, H+0.67+ is bonded in a distorted L-shaped geometry to one Mg2+ and one B3- atom. In the twelfth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the thirteenth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the fourteenth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the fifteenth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the sixteenth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the seventeenth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the eighteenth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the nineteenth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the twentieth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the twenty-first H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the twenty-second H+0.67+ site, H+0.67+ is bonded in a distorted single-bond geometry to one Mg2+ and one B3- atom. In the twenty-third H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom. In the twenty-fourth H+0.67+ site, H+0.67+ is bonded in an L-shaped geometry to one Mg2+ and one B3- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Solvation Structure and Dynamics of Mg(TFSI) 2 Aqueous Electrolyte

Using ab initio molecular dynamics (AIMD) simulations, classical molecular dynamics (CMD) simulations, small-angle X-ray scattering (SAXS), and pulsed-field gradient nuclear magnetic resonance (PFG-NMR), the solvation structure and ion dynamics of magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI) 2 ) aqueous electrolyte at 1, 2, and 3 m concentrations are investigated. From AIMD and CMD simulations, the first solvation shell of an Mg 2+ ion is found to be composed of six water molecules in an octahedral configuration and the solvation shell is rather rigid. The TFSI - ions prefer to stay in the second solvation shell and beyond. Meanwhile, the comparable diffusion coefficients of positive and negative ions in Mg(TFSI) 2 aqueous electrolytes have been observed, which is mainly due to the formation of the stable [Mg(H 2 O) 6 ] 2+ complex, and, as a result, the increased effective Mg ion size. Finally, the calculated correlated transference numbers are lower than the uncorrelated ones even at the low concentration of 2 and 3 m, suggesting the enhanced correlations between ions in the multivalent electrolytes. This work provides a molecular-level understanding of how the solvation structure and multivalency of the ion affect the dynamics and transport properties of the multivalent electrolyte, providing insight for rational designs of electrolytes for improved ion transport properties.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Atomistic simulations and machine learning of solute grain boundary segregation in Mg alloys at finite temperatures

Understanding solute segregation thermodynamics is the first step in investigating grain boundary (GB) properties, such as strong yttrium (Y) effects on grain growth and texture evolution in micro-scale polycrystalline magnesium (Mg) alloys. To estimate the average GB segregation behavior in low-solute-concentration Mg alloys (e.g., 2 at.% Y), a state-of-the-art spectral approach is applied based on a per-site segregation energy spectrum for Y solute atoms at zero K obtained from molecular statistics (MS) simulations of ~10 4 GB sites in Mg symmetric tilt GBs (STGBs). Although selected MS simulation results are consistent with verification by density functional theory (DFT) calculations, estimates of average segregation tendency based on the zero-K energy spectrum deviate from experimental observations. To resolve this problem, thermodynamic integration (TI) methods based on molecular dynamics (MD) simulations are used to determine the per-site segregation free energies of Y at representative GB sites, which show contributions beyond harmonic approximations can be important for certain GB sites at high temperatures. A surrogate model of per-site segregation free energy is constructed from a small set of TI data points using stacking cross-validation regressors and physics-informed descriptors. This model is applied to predict the Y segregation free energy spectra for thousands of GB sites in Mg STGBs with uncertainty quantification. Finally, the average segregation tendency predicted by the spectral approach based on the free energy spectra agrees well (within the uncertainty range) with experimental observations for micro-scale polycrystalline Mg alloys at typical thermomechanical processing temperatures (500 ~ 800 K), where thermodynamic equilibrium states are likely to be achieved due to fast diffusion.

Atomistic simulations↗