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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Evidence for oxygenation of Fe-Mg oxides at mid-mantle conditions and the rise of deep oxygen

Abstract As the reaction product of subducted water and the iron core, FeO2 with more oxygen than hematite (Fe2O3) has been recently recognized as an important component in the D” layer just above the Earth's core-mantle boundary. Here, we report a new oxygen-excess phase (Mg, Fe)2O3+δ (0 < δ < 1, denoted as ‘OE-phase’). It forms at pressures greater than 40 gigapascal when (Mg, Fe)-bearing hydrous materials are heated over 1500 kelvin. The OE-phase is fully recoverable to ambient conditions for ex situ investigation using transmission electron microscopy, which indicates that the OE-phase contains ferric iron (Fe3+) as in Fe2O3 but holds excess oxygen through interactions between oxygen atoms. The new OE-phase provides strong evidence that H2O has extraordinary oxidation power at high pressure. Unlike the formation of pyrite-type FeO2Hx which usually requires saturated water, the OE-phase can be formed with under-saturated water at mid-mantle conditions, and is expected to be more ubiquitous at depths greater than 1000 km in the Earth's mantle. The emergence of oxygen-excess reservoirs out of primordial or subducted (Mg, Fe)-bearing hydrous materials may revise our view on the deep-mantle redox chemistry.

58 GEOSCIENCES↗
DOE OSTI2025

Indirect Measurement of the 23 Na(p, γ) 24 Mg Direct Capture Reaction Rate via ( 3 He,d) Spectroscopy

The cross section of the $^{23}\text{Na}(p,γ)^{24}\text{Mg}$ reaction is dominated by direct capture at low energies relevant for stellar burning. Such cross sections can be constrained using spectroscopic factors($C^2S$) or asymptotic normalization coefficients(ANCs) from transfer reactions. In this work, the $^{23}\text{Na}(^3\text{He},d)^{24}\text{Mg}$ reaction was measured at $E_{lab}=21$ MeV to extract spectroscopic factors for $^{24}\text{Mg}$ states with excitation energies in $E_x=7 - 12$ MeV using the Enge split-pole spectrograph at the Triangle Universities Nuclear Laboratory. A new non-resonant astrophysical S factor and the direct capture reaction rate for the $^{23}\text{Na}(p,γ)$ reaction are calculated and presented based on this measurement. The new rate at $T<0.04$ GK is 43$\%$ smaller than in previous studies. Rigorous treatments of uncertainties are presented using a Bayesian Markov Chain Monte Carlo (MCMC) method. Finally, sources of uncertainties for computing the direct capture cross section are also discussed in detail.

electromagnetic moments↗
DOE OSTI2021

Proton decays in 16 Ne and 18 Mg and isospin-symmetry breaking in carbon isotopes and isotones

We report that proton-rich nuclei possess unique properties in the nuclear chart. Due to the presence of both continuum coupling and Coulomb interaction, phenomena such as halos, Thomas-Ehrman shift, and proton emissions can occur. Relevant experimental data are difficult to obtain, so that theoretical calculations are needed to understand nuclei at drip lines and to guide experimentalists. In particular, the 16 Ne and 18 Mg isotopes are supposed to be one-proton and/or two-proton emitting nuclei, but associated experimental data are either incomplete or even unavailable. Consequently, we performed Gamow shell model calculations of carbon isotones bearing A = 15 - 18 . Isospin-symmetry breaking occurring in carbon isotones and isotopes is also discussed. It is hereby shown that the mixed effects of continuum coupling and Coulomb interaction at drip lines generate complex patterns in isospin multiplets. Added to that, it is possible to determine the one-proton and two-proton widths of 16 Ne and 18 Mg . Obtained decay patterns are in agreement with those obtained in previous experimental and theoretical works. Moreover, to our knowledge, this is the first theoretical calculation of binding energy and partial decay widths of 18 Mg in a configuration interaction picture.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Investigation of the isoscalar response of 24 Mg to 6 Li scattering

24 Mg is a strongly deformed nucleus in the ground state. Deformation effects can be observed in the structure of the isoscalar giant monopole and quadrupole resonances. 24 Mg is also a nucleus that is well known to present different types of cluster-oscillation modes. Both giant resonances and cluster states are strongly populated by isoscalar transitions. To extract the E0, E1, and E2 transition strengths via 6 Li scattering. The 6 Li probe is a powerful tool for investigating the isoscalar nuclear response with a very favorable ratio of resonance-to-continuum background. Double-differential cross sections of 6 Li inelastic scattering, at the beam energy of 100 MeV/u, were measured in the excitation-energy range 10 – 40 MeV and scattering angles 0 - 3°. A multipole-decomposition analysis was performed for extracting the isoscalar E0 , E1, and E2 strength distributions. Results: The extracted multipole strengths were compared with predictions from consistent quasiparticle random phase approximation calculations. The theoretical predictions are in fair agreement with the experimental data. The E0 strength was also compared with results from antisymmetrized molecular dynamics calculations found in the literature. A few peaks in the experimental data might be associated with clustering in 24 Mg. Ground-state deformation effects were observed in the isoscalar giant monopole resonance (ISGMR) and isoscalar giant quadrupole resonance (ISGQR) distributions. The ISGMR strength is split in two peaks around 19 and 28 MeV. The ISGQR exhibits a pronounced peak at 20 MeV with a broadening at the low-energy region, similar to predictions from microscopic calculations. Signatures of excitation of cluster states were observed in the E0 response. Further studies including particle-decay measurements will be required to confirm the nature of the observed peaks.

20 ≤ A ≤ 38↗
DOE OSTI2021

Lifetime measurements probing collectivity in the ground-state band of 32 Mg

The signatures of inversion between normal and intruder configurations of particle-hole excitations across the N = 20 shell gap in the neutron-rich isotope 32 Mg have long been of keen interest. Electromagnetic transition rates in the ground-state band are key quantities that provide insights into collective properties associated with the contributions of the 2p2h and 4p4h intruder configurations. The combination of TRIPLEX, GRETINA, and the S800 spectrograph enables model-independent lifetime measurements to determine electromagnetic transition rates in rare isotopes. The reduced E2 transition rates in 32 Mg between the 2$^{+}_{1}$ and 0$^{+}_{1}$ states and between the 4$^{+}_{1}$ and 2$^{+}_{1}$ states have been measured, the latter representing the first experimental B(E2) value for this transition. Here, the B(E2) strengths indicate large collectivity and strong contributions from the 2p2h and 4p4h intruder configurations that may change with spin in the ground-state band of 32 Mg.

20 ≤ A ≤ 38↗
DOE OSTI2020

Corrosion of Commercial Mg–Al Alloys: AZ31B, AM60, and AZ91D

Aqueous corrosion, atmospheric corrosion, and ionic liquid dissolution studies were performed on commercial alloys AZ31B, AM60, and AZ91D and compared with previously reported results for single-phase binary Mg–Al alloys containing similar Al concentrations (αMg-2 at% Al, αMg-5 at% Al, and Mg-8 at% Al). Polarization studies in 0.6 M NaCl were used to characterize the aqueous free corrosion behavior during 20-h free immersion. Accelerated corrosion testing was performed using a rotating-disk electrode which revealed the evolution of an Al-rich mud-cracking and platelet morphology. Atmospheric droplet testing showed rapid pH increases that depended on the Al concentration in the alloy. Time-dependent contact angle measurements showed that the degree of droplet wetting increased during free corrosion by ~50° over 20 h in 0.6 M NaCl. Ionic liquid dissolution studies in 1:2 M choline chloride:urea deep eutectic solvent were performed in order to examine the current-voltage behavior of these alloys in the absence of water and hydrogen reduction. The results of these studies revealed the formation of nanowire corrosion morphologies within a honeycomb lattice structure which we attribute to selective dissolution of Mg via a two-dimensional stepflow process

25 ENERGY STORAGE↗
DOE OSTI2025

Conformational heterogeneity in the dGsw purine riboswitch: role of Mg²⁺ and 2’-dG in aptamer folding

Recent advancements in RNA structural biology have focused on unraveling the complexities of non-coding mRNA elements like riboswitches. These cis-acting regulatory regions undergo structural changes in response to specific cellular metabolites, leading to up or downregulation of downstream genes. The purine riboswitch family regulates many prokaryotic genes involved in purine degradation and biosynthesis. They feature an aptamer domain organized around a 3-way helical junction, where ligand encapsulation occurs at the junctional core. In our study, we chemically probed the aptamer domain of the 2’-dG-sensing purine riboswitch from Mesoplasma florum (dGsw) under various solution conditions to understand how Mg²⁺ and 2’-dG influence riboswitch folding. Here, we find that efficient 2’-dG binding strongly depends on Mg²⁺, indicating that Mg²⁺ is essential for priming dGsw for ligand interactions. We identified a previously undescribed sequence in the 5’ tail of dGsw that is complementary to a conserved helix. The inclusion of this region in a construct led to intramolecular competition between the alternate helix, Palt, and P1. Mutational analysis confirmed that 5’ flanking end of the aptamer domain forms an alternate helix in the absence of ligand. Molecular dynamics simulations revealed that this alternative conformation is stable. This helix may, therefore, facilitate the formation of an anti-terminator helix by opening the 3-way junction surrounding the 2’-dG binding site. Our study further establishes the importance of a closed terminal P1 helix conformation for metabolite binding and suggests that the delicate interplay between P1 and Palt may fine-tune downstream gene regulation. These insights offer a new perspective on riboswitch structure and enhance our understanding of the role that a conformational ensemble plays in riboswitch activity and regulation.

Biochemistry & Molecular Biology↗
DOE OSTI2024

High-pressure phase transition of olivine-type Mg 2 GeO 4 to a metastable forsterite-III type structure and their equations of state

Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg 2 GeO 4 using diamond-anvil cell experiments combined with synchrotron X-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-type Mg 2 GeO 4 remains stable up to 30 GPa. At higher pressures, a phase transition to a forsterite-III type (Cmc2 1 ) structure was observed, which remained stable to the peak pressure of 105 GPa. Using a third-order Birch Murnaghan fit to the experimental data, we obtained V 0 = 305.1(3) Å3, K 0 = 124.6(14) GPa, and $K'_0$ = 3.86 (fixed) for forsterite-type Mg 2 GeO 4 and V 0 = 263.5(15) Å 3 , K 0 = 175(7) GPa, and $K'_0$ = 4.2 (fixed) for the forsterite-III type phase. The forsterite-III type structure was found to be metastable when compared to the stable assemblage of perovskite/post-perovskite + MgO, as observed during laser-heating experiments. Understanding the phase relations and physical properties of metastable phases is crucial for studying the mineralogy of impact sites, understanding metastable wedges in subducting slabs, and interpreting the results of shock compression experiments.

58 GEOSCIENCES↗
DOE OSTI2020

The Elastic Properties of β-Mg 2 SiO 4 Containing 0.73 wt.% of H 2 O to 10 GPa and 600 K by Ultrasonic Interferometry with Synchrotron X-Radiation

Here, we measured the elastic velocities of a synthetic polycrystalline β-Mg 2 SiO 4 containing 0.73 wt.% H 2 O to 10 GPa and 600 K using ultrasonic interferometry combined with synchrotron X-radiation. Third-order Eulerian finite strain analysis of the high P and T data set yielded K so = 161.5(2) GPa, G o = 101.6(1) GPa, and (∂K s /∂P) T = 4.84(4), (∂G/∂P) T = 1.68(2) indistinguishable from K so = 161.1(3) GPa, G o = 101.4(1) GPa, and (∂K s /∂P) T = 4.93(4), (∂G/∂P) T = 1.73(2) from the linear fit. The hydration of the wadsleyite by 0.73 wt.% decreases K s and G moduli by 5.3% and 8.6%, respectively, but no measurable effect was noted for (∂K s /∂P) T and (∂G/∂P) T . The temperature derivatives of the K s and G moduli from the finite strain analysis (∂K S /∂T) P = –0.013(2) GPaK –1 , (∂G/∂T) P = –0.015(0.4) GPaK –1 , and the linear fit (∂K S /∂T) P = –0.015(1) GPaK –1 , (∂G/∂T) P = –0.016(1) GPaK –1 are in agreement, and both data sets indicating the |(∂G/∂T) P | to be greater than |(∂K S /∂T) P |. Calculations yield ΔV p(α-β) = 9.88% and ΔV S(α-β) = 8.70% for the hydrous β-Mg 2 SiO 4 and hydrous α-Mg 2 SiO 4 , implying 46–52% olivine volume content in the Earth’s mantle to satisfy the seismic velocity contrast ΔV s = ΔV P = 4.6% at the 410 km depth.

58 GEOSCIENCES↗
DOE OSTI2020

Materials Data on Mg(C2N3)2 by Materials Project

Mg(C2N3)2 crystallizes in the orthorhombic Pnnm space group. The structure is three-dimensional. Mg2+ is bonded in an octahedral geometry to six N3- atoms. There are four shorter (2.13 Å) and two longer (2.31 Å) Mg–N bond lengths. C4+ is bonded in a linear geometry to two N3- atoms. There is one shorter (1.17 Å) and one longer (1.31 Å) C–N bond length. There are two inequivalent N3- sites. In the first N3- site, N3- is bonded in a trigonal planar geometry to one Mg2+ and two equivalent C4+ atoms. In the second N3- site, N3- is bonded in a linear geometry to one Mg2+ and one C4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(BeN)2 by Materials Project

Mg(BeN)2 crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. Mg2+ is bonded to six equivalent N3- atoms to form MgN6 octahedra that share corners with twelve equivalent BeN4 tetrahedra, edges with six equivalent MgN6 octahedra, and edges with six equivalent BeN4 tetrahedra. All Mg–N bond lengths are 2.22 Å. Be2+ is bonded to four equivalent N3- atoms to form BeN4 tetrahedra that share corners with six equivalent MgN6 octahedra, corners with six equivalent BeN4 tetrahedra, edges with three equivalent MgN6 octahedra, and edges with three equivalent BeN4 tetrahedra. The corner-sharing octahedra tilt angles range from 24–50°. There is three shorter (1.77 Å) and one longer (1.78 Å) Be–N bond length. N3- is bonded in a 7-coordinate geometry to three equivalent Mg2+ and four equivalent Be2+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(ScS2)2 by Materials Project

Mg(ScS2)2 is Spinel structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Mg2+ is bonded to four equivalent S2- atoms to form MgS4 tetrahedra that share corners with twelve equivalent ScS6 octahedra. The corner-sharing octahedral tilt angles are 57°. All Mg–S bond lengths are 2.46 Å. Sc3+ is bonded to six equivalent S2- atoms to form ScS6 octahedra that share corners with six equivalent MgS4 tetrahedra and edges with six equivalent ScS6 octahedra. All Sc–S bond lengths are 2.59 Å. S2- is bonded to one Mg2+ and three equivalent Sc3+ atoms to form a mixture of distorted corner and edge-sharing SMgSc3 trigonal pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(InTe2)2 by Materials Project

Mg(InTe2)2 crystallizes in the monoclinic Cm space group. The structure is three-dimensional. Mg2+ is bonded to four Te2- atoms to form MgTe4 tetrahedra that share corners with eight equivalent InTe4 tetrahedra. There are three shorter (2.83 Å) and one longer (2.84 Å) Mg–Te bond lengths. In3+ is bonded to four Te2- atoms to form InTe4 tetrahedra that share corners with four equivalent MgTe4 tetrahedra and corners with four equivalent InTe4 tetrahedra. All In–Te bond lengths are 2.85 Å. There are three inequivalent Te2- sites. In the first Te2- site, Te2- is bonded in a distorted trigonal non-coplanar geometry to one Mg2+ and two equivalent In3+ atoms. In the second Te2- site, Te2- is bonded in a distorted trigonal non-coplanar geometry to one Mg2+ and two equivalent In3+ atoms. In the third Te2- site, Te2- is bonded in a distorted trigonal non-coplanar geometry to one Mg2+ and two equivalent In3+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(BC)2 by Materials Project

MgB2C2 crystallizes in the orthorhombic Cmce space group. The structure is three-dimensional. there are two inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded in a distorted hexagonal planar geometry to six C4- atoms. There are a spread of Mg–C bond distances ranging from 2.43–2.50 Å. In the second Mg2+ site, Mg2+ is bonded in a 6-coordinate geometry to six C4- atoms. There are a spread of Mg–C bond distances ranging from 2.26–2.51 Å. There are three inequivalent B3+ sites. In the first B3+ site, B3+ is bonded in a trigonal planar geometry to three C4- atoms. There are a spread of B–C bond distances ranging from 1.56–1.60 Å. In the second B3+ site, B3+ is bonded in a trigonal planar geometry to three C4- atoms. All B–C bond lengths are 1.58 Å. In the third B3+ site, B3+ is bonded in a trigonal planar geometry to three C4- atoms. All B–C bond lengths are 1.59 Å. There are three inequivalent C4- sites. In the first C4- site, C4- is bonded in a distorted trigonal planar geometry to four Mg2+ and three B3+ atoms. In the second C4- site, C4- is bonded in a distorted trigonal planar geometry to three Mg2+ and three B3+ atoms. In the third C4- site, C4- is bonded in a distorted trigonal planar geometry to two equivalent Mg2+ and three B3+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(TiS2)8 by Materials Project

Mg(TiS2)8 crystallizes in the orthorhombic Pnnm space group. The structure is three-dimensional. Mg2+ is bonded to six S2- atoms to form MgS6 octahedra that share corners with six TiS6 octahedra and edges with six TiS6 octahedra. The corner-sharing octahedra tilt angles range from 2–4°. There are two shorter (2.52 Å) and four longer (2.53 Å) Mg–S bond lengths. There are four inequivalent Ti+3.75+ sites. In the first Ti+3.75+ site, Ti+3.75+ is bonded to six S2- atoms to form TiS6 octahedra that share an edgeedge with one MgS6 octahedra and edges with six TiS6 octahedra. There are a spread of Ti–S bond distances ranging from 2.39–2.51 Å. In the second Ti+3.75+ site, Ti+3.75+ is bonded to six S2- atoms to form TiS6 octahedra that share corners with two equivalent MgS6 octahedra and edges with six TiS6 octahedra. The corner-sharing octahedral tilt angles are 4°. There are a spread of Ti–S bond distances ranging from 2.38–2.50 Å. In the third Ti+3.75+ site, Ti+3.75+ is bonded to six S2- atoms to form TiS6 octahedra that share edges with two equivalent MgS6 octahedra and edges with six TiS6 octahedra. There are two shorter (2.45 Å) and four longer (2.46 Å) Ti–S bond lengths. In the fourth Ti+3.75+ site, Ti+3.75+ is bonded to six S2- atoms to form TiS6 octahedra that share corners with two equivalent MgS6 octahedra and edges with six TiS6 octahedra. The corner-sharing octahedral tilt angles are 2°. There are two shorter (2.43 Å) and four longer (2.44 Å) Ti–S bond lengths. There are six inequivalent S2- sites. In the first S2- site, S2- is bonded in a distorted T-shaped geometry to three Ti+3.75+ atoms. In the second S2- site, S2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three Ti+3.75+ atoms. In the third S2- site, S2- is bonded in a distorted T-shaped geometry to three Ti+3.75+ atoms. In the fourth S2- site, S2- is bonded in a distorted T-shaped geometry to three Ti+3.75+ atoms. In the fifth S2- site, S2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three Ti+3.75+ atoms. In the sixth S2- site, S2- is bonded in a distorted T-shaped geometry to three Ti+3.75+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(TiS2)4 by Materials Project

Mg(TiS2)4 crystallizes in the trigonal R-3m space group. The structure is three-dimensional. Mg2+ is bonded to six equivalent S2- atoms to form MgS6 octahedra that share corners with six equivalent TiS6 octahedra and edges with six equivalent TiS6 octahedra. The corner-sharing octahedral tilt angles are 1°. All Mg–S bond lengths are 2.53 Å. There are two inequivalent Ti+3.50+ sites. In the first Ti+3.50+ site, Ti+3.50+ is bonded to six equivalent S2- atoms to form TiS6 octahedra that share corners with six equivalent MgS6 octahedra and edges with six equivalent TiS6 octahedra. The corner-sharing octahedral tilt angles are 1°. All Ti–S bond lengths are 2.44 Å. In the second Ti+3.50+ site, Ti+3.50+ is bonded to six S2- atoms to form TiS6 octahedra that share edges with two equivalent MgS6 octahedra and edges with six TiS6 octahedra. There are two shorter (2.45 Å) and four longer (2.47 Å) Ti–S bond lengths. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a rectangular see-saw-like geometry to one Mg2+ and three Ti+3.50+ atoms. In the second S2- site, S2- is bonded in a distorted T-shaped geometry to three equivalent Ti+3.50+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(ScSe2)2 by Materials Project

Mg(ScSe2)2 is Spinel structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Mg2+ is bonded to four equivalent Se2- atoms to form MgSe4 tetrahedra that share corners with twelve equivalent ScSe6 octahedra. The corner-sharing octahedral tilt angles are 57°. All Mg–Se bond lengths are 2.58 Å. Sc3+ is bonded to six equivalent Se2- atoms to form ScSe6 octahedra that share corners with six equivalent MgSe4 tetrahedra and edges with six equivalent ScSe6 octahedra. All Sc–Se bond lengths are 2.72 Å. Se2- is bonded to one Mg2+ and three equivalent Sc3+ atoms to form a mixture of distorted edge and corner-sharing SeMgSc3 trigonal pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mg(InSe2)2 by Materials Project

Mg(InSe2)2 is Spinel structured and crystallizes in the cubic Fd-3m space group. The structure is three-dimensional. Mg2+ is bonded to four equivalent Se2- atoms to form MgSe4 tetrahedra that share corners with twelve equivalent InSe6 octahedra. The corner-sharing octahedral tilt angles are 57°. All Mg–Se bond lengths are 2.62 Å. In3+ is bonded to six equivalent Se2- atoms to form InSe6 octahedra that share corners with six equivalent MgSe4 tetrahedra and edges with six equivalent InSe6 octahedra. All In–Se bond lengths are 2.78 Å. Se2- is bonded in a distorted rectangular see-saw-like geometry to one Mg2+ and three equivalent In3+ atoms.

36 MATERIALS SCIENCE↗