SEARCH · Engineering Papers

Results for “MgO”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5

All sources NASA NTRS DOE OSTI

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO crystallizes in the orthorhombic Ibam space group. The structure is three-dimensional. there are three inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to five O2- atoms to form a mixture of edge and corner-sharing MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 2.02–2.11 Å. In the second Mg2+ site, Mg2+ is bonded to five O2- atoms to form a mixture of edge and corner-sharing MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 2.03–2.11 Å. In the third Mg2+ site, Mg2+ is bonded to five O2- atoms to form a mixture of edge and corner-sharing MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 2.01–2.11 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded to five Mg2+ atoms to form a mixture of edge and corner-sharing OMg5 trigonal bipyramids. In the second O2- site, O2- is bonded to five Mg2+ atoms to form a mixture of edge and corner-sharing OMg5 trigonal bipyramids. In the third O2- site, O2- is bonded to five Mg2+ atoms to form a mixture of edge and corner-sharing OMg5 trigonal bipyramids.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO is Molybdenum Carbide MAX Phase-like structured and crystallizes in the trigonal R-3m space group. The structure is three-dimensional. there are four inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form a mixture of distorted corner and edge-sharing MgO6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 2°. All Mg–O bond lengths are 2.17 Å. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form a mixture of corner and edge-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Mg–O bond lengths are 2.12 Å. In the third Mg2+ site, Mg2+ is bonded to six O2- atoms to form a mixture of corner and edge-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Mg–O bond lengths are 2.12 Å. In the fourth Mg2+ site, Mg2+ is bonded to six equivalent O2- atoms to form a mixture of corner and edge-sharing MgO6 octahedra. All Mg–O bond lengths are 2.12 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded to six equivalent Mg2+ atoms to form a mixture of corner and edge-sharing OMg6 octahedra. The corner-sharing octahedral tilt angles are 0°. In the second O2- site, O2- is bonded to six Mg2+ atoms to form a mixture of corner and edge-sharing OMg6 octahedra. The corner-sharing octahedral tilt angles are 0°. In the third O2- site, O2- is bonded to six Mg2+ atoms to form a mixture of corner, edge, and face-sharing OMg6 octahedra. The corner-sharing octahedra tilt angles range from 0–47°. In the fourth O2- site, O2- is bonded to six Mg2+ atoms to form a mixture of corner, edge, and face-sharing OMg6 octahedra. The corner-sharing octahedra tilt angles range from 0–47°.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO crystallizes in the hexagonal P6/mcc space group. The structure is three-dimensional. Mg2+ is bonded to five equivalent O2- atoms to form a mixture of corner and edge-sharing MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 2.01–2.10 Å. O2- is bonded to five equivalent Mg2+ atoms to form a mixture of corner and edge-sharing OMg5 trigonal bipyramids.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO crystallizes in the monoclinic Cm space group. The structure is three-dimensional. there are twelve inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to four O2- atoms to form distorted MgO4 tetrahedra that share corners with five MgO4 tetrahedra, corners with three MgO5 trigonal bipyramids, corners with two equivalent MgO4 trigonal pyramids, and edges with two equivalent MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 1.96–2.06 Å. In the second Mg2+ site, Mg2+ is bonded to four O2- atoms to form MgO4 trigonal pyramids that share a cornercorner with one MgO4 tetrahedra, corners with three MgO5 trigonal bipyramids, corners with four MgO4 trigonal pyramids, and edges with two equivalent MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 1.96–2.09 Å. In the third Mg2+ site, Mg2+ is bonded to four O2- atoms to form MgO4 tetrahedra that share corners with three MgO4 tetrahedra, corners with four MgO4 trigonal pyramids, and edges with two equivalent MgO5 trigonal bipyramids. There are three shorter (1.98 Å) and one longer (2.02 Å) Mg–O bond lengths. In the fourth Mg2+ site, Mg2+ is bonded in a see-saw-like geometry to four O2- atoms. There are a spread of Mg–O bond distances ranging from 1.96–2.03 Å. In the fifth Mg2+ site, Mg2+ is bonded to five O2- atoms to form MgO5 trigonal bipyramids that share corners with two equivalent MgO4 tetrahedra, corners with four MgO5 trigonal bipyramids, corners with two MgO4 trigonal pyramids, an edgeedge with one MgO4 tetrahedra, and edges with five MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 2.04–2.20 Å. In the sixth Mg2+ site, Mg2+ is bonded to five O2- atoms to form MgO5 trigonal bipyramids that share corners with three MgO4 tetrahedra, corners with five MgO5 trigonal bipyramids, edges with three MgO5 trigonal bipyramids, and edges with two equivalent MgO4 trigonal pyramids. There are a spread of Mg–O bond distances ranging from 2.02–2.18 Å. In the seventh Mg2+ site, Mg2+ is bonded to five O2- atoms to form MgO5 trigonal bipyramids that share corners with two equivalent MgO4 tetrahedra, corners with four MgO5 trigonal bipyramids, corners with three MgO4 trigonal pyramids, and edges with two MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 2.05–2.18 Å. In the eighth Mg2+ site, Mg2+ is bonded to four O2- atoms to form MgO4 trigonal pyramids that share corners with four MgO4 tetrahedra, a cornercorner with one MgO5 trigonal bipyramid, corners with two equivalent MgO4 trigonal pyramids, and edges with three MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 1.95–2.13 Å. In the ninth Mg2+ site, Mg2+ is bonded to four O2- atoms to form MgO4 trigonal pyramids that share corners with two equivalent MgO4 tetrahedra, a cornercorner with one MgO5 trigonal bipyramid, corners with four MgO4 trigonal pyramids, and an edgeedge with one MgO5 trigonal bipyramid. There are a spread of Mg–O bond distances ranging from 1.96–2.09 Å. In the tenth Mg2+ site, Mg2+ is bonded to five O2- atoms to form MgO5 trigonal bipyramids that share a cornercorner with one MgO4 tetrahedra, corners with four MgO5 trigonal bipyramids, edges with four MgO4 tetrahedra, an edgeedge with one MgO5 trigonal bipyramid, and an edgeedge with one MgO4 trigonal pyramid. There are a spread of Mg–O bond distances ranging from 1.97–2.24 Å. In the eleventh Mg2+ site, Mg2+ is bonded to five O2- atoms to form MgO5 trigonal bipyramids that share corners with two equivalent MgO4 tetrahedra, corners with three MgO5 trigonal bipyramids, edges with three MgO5 trigonal bipyramids, and edges with three MgO4 trigonal pyramids. There are a spread of Mg–O bond distances ranging from 2.00–2.21 Å. In the twelfth Mg2+ site, Mg2+ is bonded to four O2- atoms to form distorted MgO4 tetrahedra that share corners with four MgO4 tetrahedra, corners with seven MgO5 trigonal bipyramids, a cornercorner with one MgO4 trigonal pyramid, and an edgeedge with one MgO5 trigonal bipyramid. There are a spread of Mg–O bond distances ranging from 1.99–2.05 Å. There are twelve inequivalent O2- sites. In the first O2- site, O2- is bonded to five Mg2+ atoms to form OMg5 trigonal bipyramids that share corners with two equivalent OMg4 tetrahedra, corners with four OMg5 trigonal bipyramids, corners with three OMg4 trigonal pyramids, and edges with two OMg5 trigonal bipyramids. In the second O2- site, O2- is bonded to four Mg2+ atoms to form OMg4 trigonal pyramids that share corners with seven OMg4 trigonal pyramids and edges with two equivalent OMg5 trigonal bipyramids. In the third O2- site, O2- is bonded to five Mg2+ atoms to form OMg5 trigonal bipyramids that share corners with two equivalent OMg4 tetrahedra, corners with five OMg5 trigonal bipyramids, a cornercorner with one OMg4 trigonal pyramid, edges with three OMg5 trigonal bipyramids, and edges with two equivalent OMg4 trigonal pyramids. In the fourth O2- site, O2- is bonded in a see-saw-like geometry to four Mg2+ atoms. In the fifth O2- site, O2- is bonded to four Mg2+ atoms to form OMg4 trigonal pyramids that share a cornercorner with one OMg4 tetrahedra, corners with three OMg5 trigonal bipyramids, corners with four OMg4 trigonal pyramids, and edges with two equivalent OMg5 trigonal bipyramids. In the sixth O2- site, O2- is bonded to four Mg2+ atoms to form distorted OMg4 trigonal pyramids that share corners with two equivalent OMg4 tetrahedra, corners with three OMg5 trigonal bipyramids, corners with five OMg4 trigonal pyramids, and edges with two equivalent OMg5 trigonal bipyramids. In the seventh O2- site, O2- is bonded to four Mg2+ atoms to form OMg4 trigonal pyramids that share a cornercorner with one OMg5 trigonal bipyramid, corners with six OMg4 trigonal pyramids, and edges with three OMg5 trigonal bipyramids. In the eighth O2- site, O2- is bonded to five Mg2+ atoms to form OMg5 trigonal bipyramids that share corners with four OMg5 trigonal bipyramids, corners with four OMg4 trigonal pyramids, an edgeedge with one OMg4 tetrahedra, and edges with five OMg5 trigonal bipyramids. In the ninth O2- site, O2- is bonded to four Mg2+ atoms to form OMg4 trigonal pyramids that share a cornercorner with one OMg5 trigonal bipyramid, corners with six OMg4 trigonal pyramids, and an edgeedge with one OMg5 trigonal bipyramid. In the tenth O2- site, O2- is bonded to four Mg2+ atoms to form distorted OMg4 tetrahedra that share corners with two equivalent OMg4 tetrahedra, corners with seven OMg5 trigonal bipyramids, corners with three OMg4 trigonal pyramids, and an edgeedge with one OMg5 trigonal bipyramid. In the eleventh O2- site, O2- is bonded to five Mg2+ atoms to form OMg5 trigonal bipyramids that share a cornercorner with one OMg4 tetrahedra, corners with four OMg5 trigonal bipyramids, an edgeedge with one OMg5 trigonal bipyramid, and edges with five OMg4 trigonal pyramids. In the twelfth O2- site, O2- is bonded to five Mg2+ atoms to form OMg5 trigonal bipyramids that share corners with two equivalent OMg4 tetrahedra, corners with three OMg5 trigonal bipyramids, edges with three OMg5 trigonal bipyramids, and edges with three OMg4 trigonal pyramids.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO crystallizes in the monoclinic C2/c space group. The structure is three-dimensional. there are three inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to four O2- atoms to form a mixture of edge and corner-sharing MgO4 trigonal pyramids. There are a spread of Mg–O bond distances ranging from 1.95–2.09 Å. In the second Mg2+ site, Mg2+ is bonded in a distorted see-saw-like geometry to four O2- atoms. There are a spread of Mg–O bond distances ranging from 1.95–2.03 Å. In the third Mg2+ site, Mg2+ is bonded to four O2- atoms to form a mixture of edge and corner-sharing MgO4 trigonal pyramids. There are a spread of Mg–O bond distances ranging from 1.95–2.08 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded to four Mg2+ atoms to form a mixture of edge and corner-sharing OMg4 trigonal pyramids. In the second O2- site, O2- is bonded in a distorted see-saw-like geometry to four Mg2+ atoms. In the third O2- site, O2- is bonded to four Mg2+ atoms to form a mixture of edge and corner-sharing OMg4 trigonal pyramids.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO crystallizes in the orthorhombic Ibam space group. The structure is three-dimensional. there are two inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to five O2- atoms to form a mixture of corner and edge-sharing MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 2.03–2.11 Å. In the second Mg2+ site, Mg2+ is bonded to five O2- atoms to form a mixture of corner and edge-sharing MgO5 trigonal bipyramids. There are a spread of Mg–O bond distances ranging from 2.01–2.11 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to five Mg2+ atoms to form a mixture of corner and edge-sharing OMg5 trigonal bipyramids. In the second O2- site, O2- is bonded to five Mg2+ atoms to form a mixture of corner and edge-sharing OMg5 trigonal bipyramids.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO is Molybdenum Carbide MAX Phase-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. there are three inequivalent Mg2+ sites. In the first Mg2+ site, Mg2+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Mg–O bond lengths are 2.12 Å. In the second Mg2+ site, Mg2+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing MgO6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Mg–O bond lengths are 2.12 Å. In the third Mg2+ site, Mg2+ is bonded to six equivalent O2- atoms to form a mixture of distorted edge and corner-sharing MgO6 pentagonal pyramids. The corner-sharing octahedral tilt angles are 2°. All Mg–O bond lengths are 2.17 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded to six equivalent Mg2+ atoms to form a mixture of edge and corner-sharing OMg6 octahedra. The corner-sharing octahedral tilt angles are 0°. In the second O2- site, O2- is bonded to six Mg2+ atoms to form a mixture of face, edge, and corner-sharing OMg6 octahedra. The corner-sharing octahedra tilt angles range from 0–47°. In the third O2- site, O2- is bonded to six Mg2+ atoms to form a mixture of edge and corner-sharing OMg6 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Mg2+ is bonded to five equivalent O2- atoms to form a mixture of corner and edge-sharing MgO5 trigonal bipyramids. There are three shorter (2.03 Å) and two longer (2.12 Å) Mg–O bond lengths. O2- is bonded to five equivalent Mg2+ atoms to form a mixture of corner and edge-sharing OMg5 trigonal bipyramids.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Materials Data on MgO by Materials Project

MgO crystallizes in the cubic Pm-3n space group. The structure is three-dimensional. Mg2+ is bonded to four equivalent O2- atoms to form a mixture of distorted edge and corner-sharing MgO4 tetrahedra. All Mg–O bond lengths are 1.99 Å. O2- is bonded to four equivalent Mg2+ atoms to form a mixture of distorted edge and corner-sharing OMg4 tetrahedra.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Effect of chloride ingress on self-healing recovery of smart cementitious composite incorporating crystalline admixture and MgO expansive agent

Highlights: • Crack closure under wet/dry cycles in chloride solution is higher than the one in distilled water. • Element analysis shows that calcium is vital for crack healing either in water or chloride solution. • AFm is consumed by chloride ion to form Fs, and Fs were decomposed to AH{sub 3} by carbonation. • Soluble reactive silica reacts with calcium hydrate to form C-S-H gel to heal concrete cracks. • Expansion of magnesium hydroxide facilitates crack self-healing by interconnected network. In this study, the effect of chloride environment containing various concentrations of chloride ion (Cl{sup −}) on self-healing performance of pre-cracked cementitious composite containing crystalline admixture (CA) and MgO expansive agent (MEA) was investigated under wet-dry cycles in chloride solutions. The results revealed that the Cl{sup −} changed the mineralogy of self-healing products, and consequently, affected the crack closure ratio and mechanical strength recovery. When self-healing occurred in distilled water, a large amount of ettringite (AFt) were detected, whereas in Cl{sup −} solution, monosulfate (AFm) was consumed by Cl{sup −} to form Friedel's salt (Fs), and then the Fs was decomposed to Al(OH){sub 3}(AH{sub 3}) due to carbonation. During the multiphase conversion process, hydroxide (OH{sup −}) was released into crack solution, therefore the dissolved carbon dioxide (CO{sub 2}) concentration was increased. The carbonation of the crystals formed in cracks was accelerated with the volume expansion, which achieved rapid crack sealing but contributed little to the mechanical performance recovery.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Use of microbial carbonation process to enable self‑carbonation of reactive MgO cement mixes

The low hydration and carbonation of reactive MgO cement (RMC) under ambient conditions causes prolonged setting and low compressive strengths (~4 MPa). This study proposed a unique technique which led to the enhancement of the hydration and carbonation processes via the synergistic combination of microbial carbonation process (MCP) with a hydration agent (HA) that enabled the self‑carbonation of RMC-based mixes without using of any special curing environments. Through hydrolysing urea (CO(NH{sub 2}){sub 2}) using ureolytic bacteria, CO{sub 3}{sup 2−} ions were produced to facilitate the carbonation of dissolved Mg{sup 2+} ions to form hydrated magnesium hydroxy carbonates (HMHCs). The self‑carbonation of RMC enabled by the MCP resulted in formation of brucite with a poor crystallinity and its rapid conversion into HMHCs, which improved the setting time and compressive strength of RMC-based samples. The simultaneous use of MCP with 2 M urea and HA revealed HMHCs with improved morphologies, resulting in the highest compressive strength (~15 MPa).

36 MATERIALS SCIENCE↗

DOE OSTI2022

Atomically Dispersed Platinum in Surface and Subsurface Sites on MgO Have Contrasting Catalytic Properties for CO Oxidation

Atomically dispersed metals on metal oxide supports are a rapidly growing class of catalysts. Developing an understanding of where and how the metals are bonded to the supports is challenging because support surfaces are heterogeneous, and most reports lack a detailed consideration of these points. In this work, we report two atomically dispersed CO oxidation catalysts having markedly different metal–support interactions: platinum in the first layer of crystalline MgO powder and platinum in the second layer of this support. Structural models have been determined on the basis of data and computations, including those determined by extended X-ray absorption fine structure and X-ray absorption near edge structure spectroscopies, infrared spectroscopy of adsorbed CO, and scanning transmission electron microscopy. The data demonstrate the transformation of surface to subsurface platinum as the temperature of sample calcination increased. Catalyst performance data demonstrate the lower activity but greater stability of the subsurface platinum than of the surface platinum.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Optical Properties of Neutral F Centers in Bulk MgO with Density Matrix Embedding

The optical spectra of neutral oxygen vacancies (F 0 centers) in the bulk MgO lattice are investigated using density matrix embedding theory. The impurity Hamiltonian is solved with the complete active space self-consistent field and second-order n-electron valence state perturbation theory (NEVPT2-DMET) multireference methods. Here, to estimate defect-localized vertical excitation energies at the nonembedding and thermodynamic limits, a double extrapolation scheme is employed. The extrapolated NEVPT2-DMET vertical excitation energy value of 5.24 eV agrees well with the experimental absorption maxima at 5.03 eV, whereas the excitation energy value of 2.89 eV at the relaxed triplet defect-localized state geometry overestimates the experimental emission at 2.4 eV by only nearly 0.5 eV, indicating the involvement of the triplet–singlet decay pathway.

embedding↗

DOE OSTI2026

Enhancing Value-Added CO Production from CO 2 Hydrogenation by Tailoring the Ru-CeO 2 Interface on MgO

Catalytic CO 2 hydrogenation presents a promising route for converting CO 2 into valuable products, contributing to the mitigation of net CO 2 emissions. Supported Ru catalysts have recently gained considerable attention due to their tunability for 100% CO selectivity via the reverse water-gas shift pathway, effectively suppressing the competing methanation route. However, despite achieving full CO selectivity, the overall CO yield remains limited by low CO 2 conversion, necessitating further improvement. In this work, CeO 2 was introduced to modify a Ru/MgO single-atom catalyst for CO 2 hydrogenation. The resulting Ru-CeO 2 /MgO catalyst, featuring abundant Ru-CeO 2 interfacial sites, exhibited a favorable balance of CO 2 conversion and CO selectivity, delivering the highest CO yield (32.5% at 500 °C), which is 9.0 and 1.8 times higher than that on Ru/MgO (3.6%) and Ru/CeO 2 (18.4%), respectively. Although the CO selectivity was slightly compromised due to enhanced CO binding at Ru-CeO 2 interfacial sites, H 2 was more efficiently activated at these interfaces and readily reacted with CO 2 adsorbed on CeO 2 -MgO surfaces, thereby boosting the CO 2 hydrogenation activity and CO yield. This study underscores the critical role of Ru-metal oxide interface engineering in improving CO yield and advancing the rational design of highly efficient Ru catalysts for CO production from CO 2 hydrogenation.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Interface-driven magnetic anisotropy in relaxed La 0.7 Sr 0.3 CrO 3 /La 0.7 Sr 0.3 MnO 3 heterostructures on MgO

We investigate the structural and magnetic properties of La 0.7 Sr 0.3 CrO 3 /La 0.7 Sr 0.3 MnO 3 (LSMO) heterostructures grown on (001)-oriented MgO by molecular beam epitaxy. Due to the large film-substrate lattice mismatch, strain relaxation is found to occur within the first 2–3 unit cells (uc) of the film as evidenced by reflection high energy electron diffraction and high-resolution synchrotron x-ray reciprocal space mapping. Furthermore, we find that the presence of the LSCO spacer and capping layers leads to ferromagnetism in ultra-thin LSMO layers with thicknesses of the order of 2 uc with the magnetic easy axis oriented in the film plane. Net magnetic moments of 1.4 and 2.4 μ B /Mn are measured for (2 uc LSCO/2 uc LSMO) and (2 uc LSCO/4 uc LSMO) superlattices, respectively, by superconducting quantum interference device magnetometry. The effective magnetic anisotropy of the relaxed (2 uc LSCO/4 uc LSMO) heterostructure is found to be an order of magnitude higher than bulk LSMO highlighting the critical role of interfacial interactions in tuning magnetic anisotropy at complex oxide interfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Impact of hole polaron formation on excitonic transitions in MgO from first principles

Here, we present a first-principles investigation of the excitonic properties of magnesia (MgO), an ionic insulator known to host hole polarons. We combine a density functional theory-based approach for structural relaxation in the presence of the hole and many-body perturbation theory to describe the excitonic properties. We determine that the hole polaron introduces new in-gap occupied states 0.6–0.8 eV above the valence band maximum that lead to two low-energy peaks in the optical spectrum. The predicted redshift of the lowest-energy transition due to polaron formation of 0.8 eV agrees well with the experimental Stokes shift of 0.8–0.9 eV. Analysis of the exciton wave function indicates that the electron-hole pair consists of a localized hole and delocalized electron, but that the wave function retains its Wannier-Mott character even in the presence of the hole polaron. Our study demonstrates that combining these previously established methods allows for a relatively computationally inexpensive approach to studying the exciton polaron in materials where only one charge carrier forms a polaron.

electronic structure↗

DOE OSTI2021

Molecular beam epitaxy of PdO on MgO (001)

PdO, widely used in catalysis in powder form for decades, has been predicted recently to be a Dirac semimetal. Synthesis of high-quality single crystals of this material is thus of great interest. Here, by using ozone-assisted molecular beam epitaxy, PdO thin films were grown on a MgO (001) substrate. X-ray diffraction and transmission electron microscopy indicate the film is a / b axis oriented, with the c axis lying in plane. Fully oxygenated PdO films have a low density of holelike carriers, and are insulating at the lowest temperatures. Our density functional theory calculations using the Heyd-Scuseria-Ernzerhof exchange-correlation functional suggest a ~1.0 eV band gap at the M point, where the gap can be reduced by tensile strain along the c axis and the bands begin to invert when the tensile strain is as high as 18%. Although tensile strain of this magnitude is experimentally not viable using epitaxy, electrons can be doped by oxygen reduction. Finally, our results emphasize the need for careful consideration of electron correlation effects and stoichiometry in ab initio modeling of topological semimetals involving transition metal oxides.

36 MATERIALS SCIENCE↗

DOE OSTI2024

The structure of CaO–MgO–Al 2 O 3 –SiO 2 melts and glasses doped with FeO X –NiO

Neutron and x-ray diffraction measurements have been performed on CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) glasses doped with NiO–Fe X O at room temperature, along with x-ray measurements on aerodynamically levitated liquids at ≥2000 K. The disordered structures have been modeled using empirical potential structure refinement to investigate the relation between the aluminosilicate network and the modifying cations. The SiO 4 and AlO 4 tetrahedra are found to have wider Si–O and Al–O bond distance distributions in the glass, and the first Ca–O n coordination shell is highly distorted, redistributing different populations of long and short bonds between the liquid and the glass. The addition of Fe and Ni at low aluminosilicate content increases the number of free oxygens not bonded to AlO 4 or SiO 4 . Mg–O and Fe–O are both found to be predominantly fourfold and fivefold in the liquid and glassy states. Despite these low coordination numbers, their bond angle distributions indicate that they are predominantly in nontetrahedral-type geometries, with ferrous and ferric iron possessing similar coordination environments. The Ca–O and Mg–O average coordination numbers and enthalpies of solution are consistent with their higher reactivity within relatively acidic aluminosilicate melts.

36 MATERIALS SCIENCE↗