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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Enhancing Properties with Distortion: A Comparative Study of Two Iron Phosphide Fe 2 P Polymorphs

Iron phosphide (Fe 2 P) crystallizes in its own hexagonal crystal structure type (h-Fe 2 P). As found in meteorites, orthorhombic polymorph (o-Fe 2 P) was originally reported as a high-temperature and high-pressure phase. Recently, o-Fe 2 P was described as being stable at ambient pressure, yet no synthetic methods were developed for single-crystal growth or single-phase bulk powder synthesis. Here, we report a successful method for growing o-Fe 2 P single crystals and synthesizing phase-pure polycrystalline samples using tin-flux. In situ powder X-ray diffraction studies showed that the phase transition from o-Fe 2 P to h-Fe 2 P occurs at about 873 K, and below that temperature, the formation of the o-Fe 2 P phase is favored thermodynamically rather than kinetically. Systematic comparison of transport, magnetic, and electrocatalytic properties of both h-Fe 2 P and o-Fe 2 P phases showed a substantial impact of the crystal structure on properties. The orthorhombic structural distortion resulted in considerable changes in magnetic properties, with the o-Fe 2 P phase exhibiting a 60% lower Fe magnetic moment and a substantially higher ferromagnetic Curie temperature than h-Fe 2 P. Further, electrochemical measurements toward the hydrogen evolution reaction in acidic media showed that the o-Fe 2 P phase requires an 80 mV lower overpotential than the h-Fe 2 P phase to generate a current density of -10 mA/cm 2 , and their electronic structures suggest that the higher density of states at the Fermi energy is the origin of superior catalytic activity in o-Fe 2 P.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Leveraging Polymorphism in YbCuBi to Map Transport and Elastic Properties

AMX Zintl compounds with the hexagonal ZrBeSi structure have gained significant attention for their remarkable vacancy tolerance and low thermal conductivity. Their 2D honeycomb sublattice, composed of M–X covalent bonds, is believed to contribute to high anharmonicity and unusual thermal transport properties. In this study, we explore the temperature-dependent polymorphism of YbCuBi as a model system to investigate the relationship between the structure and elastic and thermal transport properties in AMX Zintls. YbCuBi undergoes a structural transition from the “flat” Cu–Bi layers in the ZrBeSi structure to corrugated layers in the LiGaGe structure below 410 K, resulting in a distortion of its centrosymmetric structure. To probe the effects of this crystallographic transition, we employ inelastic neutron scattering and temperature-dependent resonant ultrasound spectroscopy. These experimental findings, coupled with first-principles calculations and thermal conductivity measurements, allow us to elucidate a direct relationship between corrugation of the honeycomb lattice and the observed changes in elastic and thermal transport properties. These insights can be extended to other Zintl phases with similar structure types, providing a platform for the rational design of functional materials with tailored thermal properties.

Chemical structure↗
DOE OSTI2025

Fe 0.6 Pd 0.4 Te 2 : A New Polymorph of FeTe 2 and PdTe 2 Stable at Ambient Pressure

Whereas pyrite space group is a high pressure polymorph of FeTe 2 , here we report pyrite-type single crystal Fe 0.6 Pd 0.4 Te 2 prepared using Pd substitution on Fe atomic site with ambient pressure crystal growth methods. Here, Fe 0.6(1) Pd 0.4(1) Te 2 single crystals show metal behavior above 15 K abided by Bloch-Grüneisen relation, and display resistivity upturn below 15 K due to disorder-related scattering of correlated electronic states.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

LeaPP: Learning Pathways to Polymorphs through Machine Learning Analysis of Atomic Trajectories

Understanding the mechanisms underlying crystal nucleation and growth is crucial for many technological applications. Due to the short length and time scales involved, crystal nucleation is often studied using molecular simulations. Most existing approaches to extract the nucleation mechanism from simulations focus on the analysis of static snapshots of the configurations, potentially overlooking subtle local fluctuations and the history of the particles involved in the formation of solid nuclei. Here, in this work, we propose a novel methodology called LeaPP that categorizes nucleation trajectories based on the temporal information of their constituent particles. We leverage the time evolution of the local environment of the crystallizing particles to encapsulate the relationship between the structure and dynamics and distinguish between different evolving particle paths. Identification of the distinct particle paths further enables characterizing the nucleation trajectories into different pathways. Collectively, LeaPP provides a more nuanced understanding of nucleation through an unsupervised approach with lesser dependence on traditional order parameters. Furthermore, the pathways identified by LeaPP are predictive of the resulting polymorph. We demonstrate LeaPP on three different systems─Lennard-Jones-like particles, Ni 3 Al, and water on surfaces. The general methodology underlying LeaPP─considering the time evolution of the building blocks─applies to a wide range of self-assembly problems.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Achieving Phase Control of Polymorphic Tungsten Carbide Catalysts

The polymorphism of tungsten carbide (W x C) and the challenge of selectively synthesizing pure phases have impeded a precise understanding of catalytic structure−property relationships. This study establishes a framework for phase-selective synthesis of W x C through controlling carburization kinetics. By maintaining particle sizes below 10 nm, β-W 2 C is selectively synthesized using gaseous carbon precursors (CH 4 /H 2 ) via temperature-programmed carburization (TPC). Our findings reveal that W 2 C stabilization is predominantly dictated by particle size and carburization kinetics rather than support interactions, providing a tunable approach to synthesize tungsten carbide catalysts. We elucidate the mechanistic pathway of WO x carburization, demonstrating that CH 4 activation occurs at mild temperatures via lattice oxygen. Our reactor studies establish ex situ synthesized β- W 2 C as an active and stable catalyst for the reverse water-gas shift (RWGS) reaction. However, the need for passivation and reduction pretreatment leads to a complex surface structure with diminished intrinsic activity. In contrast, our in situ synthesis protocol for β-W 2 C eliminates the need for passivation and exhibits increased CO STY during RWGS, illustrating the intrinsically higher activity compared to metallic W, WC 1−x (0.5 < x < 1), and stoichiometric WC.

CO2 conversion↗
DOE OSTI2023

Understanding the Polymorphism of Cobalt Nanoparticles Formed in Electrodeposition–An In Situ XRD Study

An advanced synchrotron-based in situ X-ray diffraction (XRD) technique is successfully developed and employed to track and monitor the formation and phase selection of cobalt (Co) in electrodeposition in real time and verify DFT computational results. The impacts of a number of controlling factors including the pH of the electrolyte and deposition overpotential are systematically studied. The results show that the yielded phase of the electrodeposited Co is controlled by both thermodynamics and kinetics. The low pH low overpotential condition favors the formation of the thermodynamically stable fcc phase. While the high pH high overpotential condition promotes the formation of the metastable hcp phase. The experimental results agree well with the nanometric phase diagram computed with DFT. Layer-by-layer alternative stacking of fcc-hcp polymorphic phases can be facilely fabricated by just varying the overpotential. This work not only offers an effective means to control the phase of electroplating of Co but also presents a new approach to reveal the fundamental insights of the formation of metals under electrochemical reduction driving force.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Unraveling two distinct polymorph transition mechanisms in one n-type single crystal for dynamic electronics

Cooperativity is used by living systems to circumvent energetic and entropic barriers to yield highly efficient molecular processes. Cooperative structural transitions involve the concerted displacement of molecules in a crystalline material, as opposed to typical molecule-by-molecule nucleation and growth mechanisms which often break single crystallinity. Cooperative transitions have acquired much attention for low transition barriers, ultrafast kinetics, and structural reversibility. However, cooperative transitions are rare in molecular crystals and their origin is poorly understood. Crystals of 2-dimensional quinoidal terthiophene (2DQTT-o-B), a high-performance n-type organic semiconductor, demonstrate two distinct thermally activated phase transitions following these mechanisms. Here we show reorientation of the alkyl side chains triggers cooperative behavior, tilting the molecules like dominos. Whereas, nucleation and growth transition is coincident with increasing alkyl chain disorder and driven by forming a biradical state. We establish alkyl chain engineering as integral to rationally controlling these polymorphic behaviors for novel electronic applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Directing polymorph specific calcium carbonate formation with de novo protein templates

Biomolecules modulate inorganic crystallization to generate hierarchically structured biominerals, but the atomic structure of the organic-inorganic interfaces that regulate mineralization remain largely unknown. We hypothesized that heterogeneous nucleation of calcium carbonate could be achieved by a structured flat molecular template that pre-organizes calcium ions on its surface. To test this hypothesis, we design helical repeat proteins (DHRs) displaying regularly spaced carboxylate arrays on their surfaces and find that both protein monomers and protein-C a2+ supramolecular assemblies directly nucleate nano-calcite with non-natural {110} or {202} faces while vaterite, which forms first in the absence of the proteins, is bypassed. These protein-stabilized nanocrystals then assemble by oriented attachment into calcite mesocrystals. We find further that nanocrystal size and polymorph can be tuned by varying the length and surface chemistry of the designed protein templates. Thus, bio-mineralization can be programmed using de novo protein design, providing a route to next-generation hybrid materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Polymorphism of garnet solid electrolytes and its implications for grain-level chemo-mechanics

Understanding and mitigating filament formation, short-circuit and solid electrolyte fracture is necessary for advanced all-solid-state batteries. Here, we employ a coupled far-field high-energy diffraction microscopy and tomography approach for assessing the chemo-mechanical behaviour for dense, polycrystalline garnet (Li 7 La 3 Zr 2 O 12 ) solid electrolytes with grain-level resolution. In situ monitoring of grain-level stress responses reveals that the failure mechanism is stochastic and affected by local microstructural heterogeneity. In this work, coupling high-energy X-ray diffraction and far-field high-energy diffraction microscopy measurements reveals the presence of phase heterogeneity that can alter local chemo-mechanics within the bulk solid electrolyte. These local regions are proposed to be regions with the presence of a cubic polymorph of LLZO, potentially arising from local dopant concentration variation. The coupled tomography and FF-HEDM experiments are combined with transport and mechanics modelling to illustrate the degradation of polycrystalline garnet solid electrolytes. The results showcase the pathways for processing high-performing solid-state batteries.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Can a shock-induced phonon up-pumping model relate to impact sensitivity of molecular crystals, polymorphs and cocrystals?

Impact sensitivity engineering of high-energy molecular crystals requires accurate predictive models. For this purpose, the promising multi-phonon based approach is selected, assessing a bit more its strengths and weaknesses. Presently used with high-quality phonon calculations of 22 molecular crystals, using a physics-based criterion to determine the phonon bath extent, the resulting intrinsic shock sensitivity index (SSI) is compared to the most common marker of impact sensitivity, h 50 , as determined from drop-weight impact tests. Selecting a data subset from experiments performed under very similar conditions (2.5 kg hammer with grit and 30–40 mg samples), the model can predict h 50 values for mono-molecular crystals with very good accuracy, including the ability to discriminate the polymorphs of HMX and CL20. This very good agreement validates an initial indirect up-pumping mechanism occurring under these conditions, where the doorway modes also interact with the phonon bath. However, the phonon bath criterion for mono-molecular crystals does not transfer well to cocrystals. Owing to the vibrational coupling of the co-molecules, it seems a broader phonon bath should be considered. Additionally recalling experimental uncertainty and various experimental factors affecting h 50 values for a given compounds, we recommend that the density of the sample, granularity and morphology be systematically considered and reported along with measurements, which will in turn allow for more systematic data and predictive capabilities for sensitivity models.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Structural stability, dihydrogen bonding, and pressure-induced polymorphic transformations in hydrazine borane

Hydrazine borane (N 2 H 4 BH 3 ) has attracted considerable interest as a promising solid-state hydrogen storage material owing to its high hydrogen content and easy preparation. In this work, pressure-induced phase transitions of N 2 H 4 BH 3 were investigated using a combination of vibrational spectroscopy, X-ray diffraction, and density functional theory (DFT) up to 30 GPa. Our results showed that N 2 H 4 BH 3 exhibits remarkable structural stability in a very broad pressure region up to 15 GPa, and then two phase transitions were identified: the first one is from the ambient-pressure Pbcn phase to a Pbca phase near 15 GPa; the second is from the Pbca phase to a Pccn phase near 25 GPa. As revealed by DFT calculations, the unusual stability of N 2 H 4 BH 3 and the late phase transformations were attributed to the pressure-mediated evolutions of dihydrogen bonding frameworks, the compressibility and the enthalpies of the high-pressure polymorphs. In conclusion, our findings provide new insight into the structures and bonding properties of N 2 H 4 BH 3 that are important for hydrogen storage applications.

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

The pressure-stabilized polymorph of indium triiodide

A layered rhombohedral polymorph of indium(iii) triiodide is synthesized at high pressure and temperature. It has an orange color, which is different from ambient pressure InI 3 , which has a monoclinic molecular structure and a light-yellow color.

Chemistry↗
DOE OSTI2024

Identification of large polarons and exciton polarons in rutile and anatase polymorphs of titanium dioxide

Titanium dioxide (TiO 2 ) is a wide-gap semiconductor with numerous applications in photocatalysis, photovoltaics, and neuromorphic computing. The unique functional properties of this material critically depend on its ability to transport charge in the form of polarons, namely narrow electron wavepackets accompanied by local distortions of the crystal lattice. It is currently well established that the most important polymorphs of TiO 2 , the rutile and anatase phases, harbor small electron polarons and small hole polarons, respectively. However, whether additional polaronic species exist in TiO 2 , and under which conditions, remain open questions. Here, we provide definitive answers to these questions by exploring the rich landscape of polaron quasiparticles in TiO 2 via recently developed ab initio techniques. In addition to the already known small polarons, we identify three species, namely a large hole polaron in rutile, a large quasi-two-dimensional electron polaron in anatase, and a large exciton polaron in anatase. These findings complete the puzzle on the polaron physics of TiO 2 and pave the way for systematically probing and manipulating polarons in a broad class of complex oxides and quantum materials.

Dai, Zhenbang↗
DOE OSTI2022

Dynamic compression of [100] MgF 2 single crystals: Shock-induced polymorphism to highly coordinated structures

AX 2 -type compounds exhibit a rich and complex series of phase transitions under compression. The high-pressure polymorphism of rutile-type AX 2 difluorides can be characterized by a typical sequence from rutile type → Ca Cl 2 type → HP – Pd F 2 type → cotunnite type . Relative to dioxides, the reduced valence and ionic radius of the F – anion in rutile-type MgF 2 (sellaite) results in lowered transition pressures, making it a useful analog for SiO 2 and other dioxides. In this work, MgF 2 single crystals were shock-compressed along the [100] direction to 24 to 120 GPa using plate impact techniques, and wave profiles were measured using laser interferometry. At low stresses (24 to 44 GPa), we observe features consistent with an elastic–inelastic response, followed by a phase transformation. Peak stress-density states in this stress range are consistent with those expected for the modified fluorite-type (HP- PdF 2 ) phase or a related structure. At higher stresses (69 to 91 GPa), we observe a two-wave structure with peak stress-density states consistent with transformation to a denser and likely more highly coordinated phase such as the cotunnite-type structure. Furthermore, at 120 GPa, only a single wave structure is observed, indicating that the wave profile features observed at lower stresses are overdriven.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗