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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.

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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Raman and X-ray diffraction study of pressure-induced phase transition in synthetic Mg 2 TiO 4

Synthetic Mg 2 TiO 4 qandilite was investigated to 50 and 40.4 GPa at room temperature using Raman spectroscopy and X-ray diffraction, respectively. The Raman measurements showed that cubic Mg 2 TiO 4 spinel transforms to a high pressure tetragonal (I4 1 /amd, No.141) phase at 14.7 GPa. Owing to sluggish kinetics at room temperature, the spinel phase coexists with the tetragonal phase between 14.7 and 24.3 GPa. In the X-ray diffraction experiment, transformation of the cubic Mg 2 TiO 4 to the tetragonal structure was complete by 29.2 GPa, ~5 GPa higher than the transition pressure obtained by Raman measurements, owing to slow kinetics. The obtained isothermal bulk modulus of Mg 2 TiO 4 spinel is K T0 = 148(3) GPa when K T0 ’ = 6.6, or K T0 = 166(1) GPa when K T0 ’ is fixed at 4. The isothermal bulk modulus of the high-pressure tetragonal phase is calculated to be 209(2) GPa and V 0 = 270(2) Å 3 when K T0 ’ is fixed at 4, and the volume reduction on change from cubic to tetragonal phase is about 9%. The calculated thermal Grüneisen parameters (γ th ) of cubic and tetragonal Mg 2 TiO 4 phases are 1.01 and 0.63. Based on the radii ratio of spinel cations, a simple model is proposed to predict post-spinel structures.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Ba 1−x Sr x FeO 3−δ as an improved oxygen storage material for chemical looping air separation: a computational and experimental study

Chemical looping air separation (CLAS) is a promising technology to generate oxygen-rich gas streams to enable efficient carbon dioxide capture during fossil fuel combustion or gasification. CLAS relies on the capture and release of oxygen from the atmosphere using the redox properties of an oxygen-selective solid oxide carrier. This study investigates the redox characteristics of Ba 1−x Sr x FeO 3−δ (0.0 ≤ x ≤ 0.417, 0.0 ≤ δ ≤ 0.5) using a combination of density functional theory (DFT) calculations and experimental verification using X-ray diffraction, thermogravimetric analysis, and oxygen-temperature-programmed desorption. The DFT computed energies of the Ba 1−x Sr x FeO 3−δ perovskites reveal a composition-dependent transition from hexagonal to cubic phases as the Sr-concentration or oxygen vacancy concentration increases. Oxygen vacancy formation energies of the cubic perovskites are found to be lower than those of their hexagonal counterparts. A low oxygen diffusion barrier of ∼1 eV combined with the thermodynamic preference of Ba 1−x Sr x FeO 3−δ compositions that form in a cubic phase suggests them as promising candidates for oxygen storage applications. The experimental results corroborate this finding by identifying Ba 0.75 Sr 0.25 FeO 3−δ in the cubic phase as an optimal composition offering low-temperature oxygen storage capacities comparable to that of the state-of-the-art Sr 0.75 Ca 0.25 FeO 3−δ perovskite oxygen storage material at 325 °C and 350 °C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Electronic, direct optical, and phonon-assisted optical properties of 4H Si from first principles

The cubic polytype of silicon (Si) is the most commercialized semiconductor material and finds applications in numerous electronic and optoelectronic devices, such as solar cells. However, recent reports on the synthesis of the hexagonal 4H Si polytype have attracted the attention of the scientific community to understand its functional properties. Here, we report the electronic, vibrational, and optical properties of the 4H Si polytype obtained with predictive first-principles calculations, with an emphasis of phonon-assisted absorption in the indirect regime. Compared to the cubic polytype, 4H Si shows a slightly narrower indirect gap by ∼0.05 eV. The calculated phonon-assisted optical spectra show that 4H Si exhibits a stronger absorption coefficient than cubic Si across the visible and IR spectral regions. We further evaluate the short-circuit current density of textured thin-films. We demonstrate that 4H Si can achieve the same short-circuit current density for a five times thinner film compared to the cubic polytype, which mainly resulted from absorption in the indirect gap regime. Our work demonstrates the advantages of 4H Si for thin-film silicon-based solar-cell applications.

Materials Science↗
DOE OSTI2024

Optical pulse-induced ultrafast antiferrodistortive transition in SrTiO 3

The ultrafast dynamics of the antiferrodistortive phase transition in perovskite SrTiO 3 is monitored via time-domain Brillouin scattering. Using femtosecond optical pulses, we initiate a thermally driven tetragonal-to-cubic structural transformation and detect the crystal phase through changes in the frequency of Brillouin oscillations (BO) induced by propagating acoustic phonons. Coupling the measured BO frequency with a spatiotemporal heat diffusion model, we demonstrate that, for a sample kept in the tetragonal phase, deposition of sufficient thermal energy induces a rapid transformation of the heat-affected region to the cubic phase. The initial phase change is followed by a slower reverse cubic-to-tetragonal phase transformation occurring on a timescale of hundreds of picoseconds. Here, we attribute this ultrafast phase transformation in the perovskite to a structural resemblance between atomic displacements of the R-point soft optic mode of the cubic phase and the tetragonal phase, both characterized by anti-phase rotation of oxygen octahedra. The structural relaxation time exhibits a strong temperature dependence consistent with the prediction of the equation of motion describing collective oxygen octahedra rotation based on the energy landscape of the phenomenological Landau theory of phase transitions. Evidence of such a fast structural transition in perovskites can open up new avenues in information processing and energy storage sectors.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Construction of hopfion crystals

Hopfions, three-dimensional topological solitons characterized by nontrivial Hopf indices, represent a fundamental class of field configurations that emerge across diverse areas of physics. Despite extensive studies of isolated hopfions, a framework for constructing spatially ordered arrays of hopfions, i.e., hopfion crystals, has been lacking. Here, we present a systematic approach for generating hopfion crystals with cubic symmetry by combining the Hopf map with rational mapping techniques. By superposing helical waves in ℝ 4 , we construct hopfion crystals with tunable Hopf indices and controllable topology. We demonstrate simple cubic, face-centered cubic, and body-centered cubic hopfion crystals and extend our framework to create crystals of more complex topological structures, including axially symmetric tori, torus links, and torus knots with higher Hopf indices. Our results provide a foundation for searching hopfions in real materials and studying their collective phenomena.

3-dimensional systems↗
DOE OSTI2021

Probing structure–property relationship in chemical vapor deposited hybrid perovskites by pressure and temperature

Chemical vapor deposition (CVD), a low-cost and a scalable deposition technique, allows the growth of methylammonium lead iodide (MAPbI 3 ) films without the use of solvents, substantially increasing air stability while also inducing the stable cubic phase at room temperature and at pressures as low as 0.25 GPa. MAPbI 3 thin films were grown by a facile two-step low-pressure vapor deposition process in a single reactor. This method results in films, which are usually in the tetragonal phase (space group: I4/mcm) and occasionally in the cubic phase under ambient conditions. High-pressure synchrotron-based X-ray diffraction studies from CVD-grown MAPbI 3 crystallites show that the sample remains in the cubic phase (space group: Im$\bar{3}$) between 0.25 and 3.0 GPa. Temperature-dependent transport measurements show sharp anomalies, correlating with the structural changes. The transport measurements from the CVD- grown cubic MAPI 3 film is further compared with a film in the tetragonal phase.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TiCo2Sn by Materials Project

Co2TiSn is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Ti is bonded in a distorted body-centered cubic geometry to eight equivalent Co and six equivalent Sn atoms. All Ti–Co bond lengths are 2.64 Å. All Ti–Sn bond lengths are 3.04 Å. Co is bonded in a body-centered cubic geometry to four equivalent Ti and four equivalent Sn atoms. All Co–Sn bond lengths are 2.64 Å. Sn is bonded in a distorted body-centered cubic geometry to six equivalent Ti and eight equivalent Co atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on CeInAu2 by Materials Project

CeInAu2 is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Ce is bonded in a body-centered cubic geometry to eight equivalent Au atoms. All Ce–Au bond lengths are 3.09 Å. Au is bonded in a body-centered cubic geometry to four equivalent Ce and four equivalent In atoms. All Au–In bond lengths are 3.09 Å. In is bonded in a distorted body-centered cubic geometry to eight equivalent Au atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TiNi2Sb by Materials Project

TiNi2Sb is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Ti is bonded in a distorted body-centered cubic geometry to eight equivalent Ni and six equivalent Sb atoms. All Ti–Ni bond lengths are 2.65 Å. All Ti–Sb bond lengths are 3.06 Å. Ni is bonded in a body-centered cubic geometry to four equivalent Ti and four equivalent Sb atoms. All Ni–Sb bond lengths are 2.65 Å. Sb is bonded in a distorted body-centered cubic geometry to six equivalent Ti and eight equivalent Ni atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Li2CdGe by Materials Project

Li2CdGe is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Li is bonded in a body-centered cubic geometry to four equivalent Cd and four equivalent Ge atoms. All Li–Cd bond lengths are 2.79 Å. All Li–Ge bond lengths are 2.79 Å. Cd is bonded in a body-centered cubic geometry to eight equivalent Li atoms. Ge is bonded in a body-centered cubic geometry to eight equivalent Li atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on HfAlAu2 by Materials Project

HfAu2Al is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Hf is bonded in a body-centered cubic geometry to eight equivalent Au atoms. All Hf–Au bond lengths are 2.87 Å. Au is bonded in a body-centered cubic geometry to four equivalent Hf and four equivalent Al atoms. All Au–Al bond lengths are 2.87 Å. Al is bonded in a body-centered cubic geometry to eight equivalent Au atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on ScAlAu2 by Materials Project

ScAu2Al is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Sc is bonded in a body-centered cubic geometry to eight equivalent Au atoms. All Sc–Au bond lengths are 2.86 Å. Au is bonded in a body-centered cubic geometry to four equivalent Sc and four equivalent Al atoms. All Au–Al bond lengths are 2.86 Å. Al is bonded in a body-centered cubic geometry to eight equivalent Au atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on AlCrCo2 by Materials Project

Co2CrAl is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Cr is bonded in a body-centered cubic geometry to eight equivalent Co atoms. All Cr–Co bond lengths are 2.47 Å. Co is bonded in a body-centered cubic geometry to four equivalent Cr and four equivalent Al atoms. All Co–Al bond lengths are 2.47 Å. Al is bonded in a body-centered cubic geometry to eight equivalent Co atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on AlFeCo2 by Materials Project

Co2FeAl is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Fe is bonded in a distorted body-centered cubic geometry to eight equivalent Co and six equivalent Al atoms. All Fe–Co bond lengths are 2.47 Å. All Fe–Al bond lengths are 2.85 Å. Co is bonded in a body-centered cubic geometry to four equivalent Fe and four equivalent Al atoms. All Co–Al bond lengths are 2.47 Å. Al is bonded in a distorted body-centered cubic geometry to six equivalent Fe and eight equivalent Co atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on AlCrNi2 by Materials Project

CrNi2Al is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Cr is bonded in a body-centered cubic geometry to eight equivalent Ni atoms. All Cr–Ni bond lengths are 2.51 Å. Ni is bonded in a body-centered cubic geometry to four equivalent Cr and four equivalent Al atoms. All Ni–Al bond lengths are 2.51 Å. Al is bonded in a body-centered cubic geometry to eight equivalent Ni atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on HfAlCu2 by Materials Project

AlCu2Hf is Heusler structured and crystallizes in the cubic Fm-3m space group. The structure is three-dimensional. Hf is bonded in a body-centered cubic geometry to eight equivalent Cu atoms. All Hf–Cu bond lengths are 2.68 Å. Cu is bonded in a body-centered cubic geometry to four equivalent Hf and four equivalent Al atoms. All Cu–Al bond lengths are 2.68 Å. Al is bonded in a distorted body-centered cubic geometry to eight equivalent Cu atoms.

36 MATERIALS SCIENCE↗