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Results for “structural phase transitions”

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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Shape Dependence of Pressure-Induced Phase Transition in CdS Semiconductor Nanocrystals

Understanding structural stability and phase transformation of nanoparticles under high pressure is of great scientific interest, as it is one of the crucial factors for design, synthesis, and application of materials. Even though high-pressure research on nanomaterials has been widely conducted, their shape-dependent phase transition behavior still remains unclear. Examples of phase transitions of CdS nanoparticles are very limited, despite the fact that it is one of the most studied wide band gap semiconductors. Here we have employed in-situ synchrotron wide-angle X-ray scattering (WAXS) and transmission electron microscopy (TEM) to investigate the high-pressure behaviors of CdS nanoparticles as a function of particle shapes. We observed that CdS nanoparticles transform from wurtzite to rocksalt phase at elevated pressure in comparison to their bulk counterpart. Phase transitions also vary with particle shape—rod-shaped particles show a partially reversible phase transition and the onset of the structural phase transition pressure decreases with decreasing surface-to-volume ratios, while spherical particles undergo irreversible phase transition with relatively low phase transition pressure. Additionally, TEM images of spherical particles exhibited sintering-induced morphology change after high-pressure compression. Calculations of the bulk modulus reveals that spheres are more compressible than rods in the wurtzite phase. These results indicate that the shape of the particle plays an important role in determining their high-pressure properties. Our study provides important insights into understanding of the phase-structure-property relationship, guiding future design and synthesis of nanoparticles for promising applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Coexisting Multiple Martensites in Ni 57-x Mn 21+x Ga 22 Ferromagnetic Shape Memory Alloys: Crystal Structure and Phase Transition

A comprehensive study of the crystal structure and phase transition as a function of temperature and composition in Ni 57–x Mn 21+x Ga 22 (x = 0, 2, 4, 5.5, 7, 8) (at. %) magnetic shape memory alloys was performed by a temperature-dependent synchrotron X-ray diffraction technique and transmission electron microscopy. A phase diagram of this Ni 57–x Mn 21+x Ga 22 alloy system was constructed. The transition between coexisting multiple martensites with monoclinic and tetragonal structures during cooling was observed in the Ni 51.5 Mn 26.5 Ga 22 (x = 5.5) alloy, and it was found that 5M + 7M multiple martensites coexist from 300 K to 160 K and that 5M + 7M + NM multiple martensites coexist between 150 K and 100 K. The magnetic-field-induced transformation from 7M martensite to NM martensite at 140 K where 5M + 7M + NM multiple martensites coexist before applying the magnetic field was observed by in situ neutron diffraction experiments. The present study is instructive for understanding the phase transition between coexisting multiple martensites under external fields and may shed light on the design of novel functional properties based on such phase transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Quasiparticle self-consistent GW band structures and phase transitions of LiAlO 2 in tetrahedrally and octahedrally coordinated structures

In this work, a first-principles computational study is presented of various phases of LiAlO 2 . The relative total energies and equations of state of the α, β, and γ phases are determined after structural relaxation of each phase. The β and γ tetrahedral phases are found to be very close in energy and lattice volume with the γ phase having the lowest energy. The octahedral α phase is a high-pressure phase and the transition pressure from the γ and β phases to α is determined to be about 1 GPa. The electronic band structures of each phase at their own equilibrium volume are determined using the quasiparticle self-consistent (QS) GW method as well as using the 0.8 Σ approach in which the QS GW self-energy is reduced by a factor of 0.8 to correct for the underscreening of W in QS GW. The effective masses of the band edges and the nature of the band gaps are presented. The lowest energy γ phase is found to have a pseudodirect gap of 7.69 eV. The gap is direct at Γ but corresponds to a dipole forbidden transition. The imaginary part of the dielectric function and the absorption coefficient are calculated in the long-wavelength limit and the random phase approximation, without local field or electron-hole interaction effects for each phase, and their anisotropies are discussed. Si doping on the Al site is investigated as a possible n-type dopant in γ-LiAlO 2 using a 128-atom supercell corresponding to 3.125% Si on the Al sublattice in the generalized gradient approximation and a smaller 16-atom cell with 25% Si in the QS GW approximation. The Si is found to significantly perturb the conduction band and lower the gap but a clearly separated deep donor defect level is not found. However, the donor binding energy is still expected to be relatively deep, on the order of a few tenths eV in the hydrogenic effective mass approximation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Revisiting the structures and phase transitions of Ba 2 NaNb 5 O 15

The room-temperature and low-temperature structure(s) of Ba 2 NaNb 5 O 15 (BNN) have been debated since the structure was proposed in the 1960s. This work revisits the structures and phase transitions of BNN, combining high-resolution X-ray and neutron powder diffraction with density functional theory calculations. Temperature-dependent high-resolution X-ray powder diffraction patterns are collected from 4 to 918 K, and sequential batch Rietveld refinement using a symmetry mode approach to describe the structure is used to extract the main structural changes as a function of temperature. The data show that the average structure of BNN is best described by the Ama2 space group, and no other structural phase transitions were observed below the ferroelastic transition. The symmetry mode analysis, combining results from diffraction and density functional theory, shows significant octahedral tilting and corrugations of both the A1 and A2 sites along the c direction. A strong correlation between the spontaneous strain and the octahedral tilting was observed, and a potential connection with emerging microstructure at low temperatures is proposed, all enabled by the symmetry mode approach used in this work.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Evolution of the structural transition in Mo 1–x W x Te 2

The composition dependence of the structural transition between the monoclinic 1T' and orthorhombic Td phases in the Mo 1–x W x Te 2 Weyl semimetal was investigated by elastic neutron scattering on single crystals up to x ≈ 0.54. First observed in MoTe 2 , the transition from T d to 1T' is accompanied by an intermediate pseudo-orthorhombic phase, T d *. Upon doping with W, the T d * phase vanishes by x ≈ 0.34. Above this concentration, a phase coexistence behavior with both T d and 1T' is observed instead. The interlayer in-plane positioning parameter δ, which relates to the 1T' β angle, decreases with temperature as well as with W substitution, likely due to strong anharmonicity in the interlayer interactions. As a result, the temperature width of the phase coexistence remains almost constant up to x ≈ 0.54, in contrast to the broadening reported under pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Structural Manipulation of Phase Transitions by Self‐Induced Strain in Geometrically Confined Thin Films

Strain engineering is a well-known method often used to tune material properties in thin films. The most studied sources of strain are lattice mismatch and differential thermal contraction between the substrate and film. However, in materials which undergo a structural phase transition (SPT), a third and often overlooked source of strain may play a very significant role. If the substrate confines the area of the film, the SPT may induce stress which changes the evolution of the transition. This is a 2D analog of the isochoric phase transition between water and ice, where the freezing point drops below 0 °C. To illustrate this, the prototypical Mott insulator V 2 O 3 which has an SPT coupled to a metal–insulator transition is used to show how self-induced strain can drastically alter structural and electronic properties. This effect provides an elegant approach for mapping the phase diagram of the SPT and the transitions coupled to it. Moreover, the magnitude of self-straining is tunable by modifying the substrate morphology. Furthermore, this effect may be important for numerous materials which exhibit an SPT and are subjected to geometrical constraints.

Kalcheim, Yoav↗
DOE OSTI2023

Controllable superconducting to semiconducting phase transition in topological superconductor 2M-WS 2

The investigation of exotic properties in two-dimensional (2D) topological superconductors has garnered increasing attention in condensed matter physics, particularly for applications in topological qubits. Despite this interest, a reliable way of fabricating topological Josephson junctions (JJs) utilizing topological superconductors has yet to be demonstrated. Controllable structural phase transition presents a unique approach to achieving topological JJs in atomically thin 2D topological superconductors. In this work, we report the pioneering demonstration of a structural phase transition from the superconducting to the semiconducting phase in the 2D topological superconductor 2M-WS 2 . We reveal that the metastable 2M phase of WS 2 remains stable in ambient conditions but transitions to the 2H phase when subjected to temperatures above 150°C. We further locally induced the 2H phase within 2M-WS 2 nanolayers using laser irradiation. Notably, the 2H phase region exhibits a hexagonal shape, and scanning tunneling microscopy (STM) uncovers an atomically sharp crystal structural transition between the 2H and 2M phase regions. Moreover, the 2M to 2H phase transition can be induced at the nanometer scale by a 200 keV electron beam. The electrical transport measurements further confirmed the superconductivity of the pristine 2M-WS 2 and the semiconducting behavior of the laser-irradiated 2M-WS 2 . Furthermore, our results establish a novel approach for controllable topological phase change in 2D topological superconductors, significantly impacting the development of atomically scaled planar topological JJs.

2D materials↗
DOE OSTI2021

A Lattice Litany for Transition Metal Oxides

In this tribute to K Alex Müeller, I describe how his early insights have influenced future decades of research on perovskite ferroelectrics and more broadly transition metal oxides (TMOs) and related quantum materials. I use his influence on my own research journey to discuss impacts in three areas: structural phase transitions, precursor structure, and quantum paraelectricity. I emphasize materials functionality in ground, metastable, and excited states arising from competitions among lattice, charge, and spin degrees of freedom, which results in highly tunable landscapes and complex networks of multiscale configurations controlling macroscopic functions. I discuss competitions between short- and long-range forces as particularly important in TMOs (and related materials classes) because of their localized and directional metal orbitals and the polarizable oxygen ions. I emphasize crucial consequences of elasticity and metal–oxygen charge transfer

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Pressure-induced phase transition in barium hydride studied with neutron scattering

Barium hydride can undergo a structural phase transition from an orthorhombic phase to a hexagonal phase induced by high temperature or high pressure. This transition causes an immediate increase in the hydrogen diffusion rates by over an order of magnitude, and therefore, understanding the origin and details of such transition is of great interest not only for fundamental reasons but also for improving materials for future applications. In this work, the pressure evolution of the crystal structure was characterized using neutron powder diffraction up to a maximum pressure of 11.3 GPa. The pressure dependence of the unit cell volumes, lattice parameters, atomic sites, and compressibilities were determined for both phases. A structural phase transition occurred over a wide pressure range of P = 1.3 GPa–4.9 GPa. The transition to the higher density hexagonal phase reduced the volume per formula unit of BaD 2 by 13.6%, hence increasing the volumetric storage density. In addition, we investigated the hydrogen diffusion process using high pressure quasi-elastic neutron scattering up to 7.1 GPa. Our results show that the hydrogen mobility increases with pressure in the hexagonal phase. This work sheds light on the structural and dynamical aspects of barium hydride caused by the application of high pressure. Finally, the results may aid in the development of advanced metal hydride systems with increased hydrogen dynamics.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Structural transition and anisotropic magnetism in disordered Zintl phase Eu 7 Ga 6 Sb 8

Single crystals of the Zintl compound Eu 7 Ga 6 Sb 8 were synthesized using a Ga-Sb flux. We report the temperature (T) and magnetic field (H) dependence of the magnetic susceptibility (χ), magnetization (M), resistivity (ρ), specific heat (C), and thermal expansion (α). We also report high-resolution powder x-ray diffraction data that support a structural phase transition with accompanying signatures seen in C(T), ρ(T), and α(T). We find Eu 7 Ga 6 Sb 8 exhibits antiferromagnetic ordering at T N1 = 9.0 K from anomalies seen in χ(T), C(T), and α(T), as well as a potential reorientation of Eu 2+ spins at T N2 = 7.5 K and T N3 = 7.2 K. In conclusion, density functional theory calculations predict Eu 7 Ga 6 Sb 8 to be semiconducting; however, electrical resistivity measurements show bad-metal behavior that indicates the presence of disorder.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Giant linear magnetoelectric effect at the morphotropic phase boundary of epitaxial Sr 0.5 Ba 0.5 MnO 3 films

First-principles calculations are conducted to compute linear magnetoelectric coupling coefficients in epitaxial (001) Sr 0.5 Ba 0.5 MnO 3 films. A large enhancement of different linear magnetoelectric elements is found in a strained-induced morphotropic phase boundary region. Such enhancement is demonstrated to originate from the behavior of the dielectric susceptibility, thanks to a simple phenomenological model that is presently shown to be relevant and accurate. Finally, this work can thus provide a promising approach towards designing highly desired single-phase multiferroic with a colossal magnetoelectric conversion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Mott vs Kondo: Influence of various density functional based methods on the Ce isostructural phase transition mechanism

The cerium iso-structural phase transition (γ–α) is dominated by f-electron localization changes that results in a magnetic ordering change and a volume collapse. Generally, these physics are difficult to capture with ab initio and first principles methods. However, previous works have shown various methods to be successful in predicting at least some of the physics of the γ to α phase transition. Therefore, here, we perform a broad survey of density functional based methods across three levels of theory and types of functions [GGA (generalized gradient approximation), MetaGGA, and hybrid functionals] and compare the results, focusing on hydrostatic compression across the phase boundary at zero kelvin. For the methods that best reproduce experimental results, we directly probe the predicted mechanisms and frame the results in the Mott/Kondo debate, assessing how the underlying methods and assumptions of different functionals can assess the physical drivers in the phase transition, providing insight into the governing processes of this unique phase transition.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Unexpected Observation of Disorder and Multiple Phase-Transition Pathways in Shock-Compressed Zr

The response of materials under dynamic compression involves a complex interplay of various deformation mechanisms aimed at relieving shear stresses, yielding a remarkable diversity in material behavior. Here, in this Letter, we utilize femtosecond x-ray diffraction coupled with nanosecond laser compression to reveal an intricate competition between multiple shear-relieving mechanisms within an elemental metal. Our observations in shocked-compressed single-crystal Zr indicate a disorder-mediated shear relaxation at lower pressures. Above the phase-transition pressure, we observe the increasing contribution of structural phase transition in relieving shear stress. We detect not one but three concurrent pathways during the transition from the hcp to a hex-3 structure. These complex dynamics are partially corroborated through multimillion-atom molecular dynamics simulations employing a machine-learned interatomic potential. Our observation of multiple concurrent pathways and disorder during shock compression underscore the far greater intricacies in the dynamic response of metals than previously assumed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Ba₁₋ₓSrₓFeO₃₋ᵧ as an Improved Oxygen Carrier for Chemical Looping Air Separation: A Computational and Experimental Study

Chemical looping air separation (CLAS) is a promising method to generate pure carbon-dioxide from fuel combustion with a pure oxygen stream, which is produced through the capture, and targeted release, of oxygen from the atmosphere using a solid oxide carrier. The performance of this process depends on the redox characteristics of the oxide carrier. Using experimental oxygen-temperature-programmed desorption (TPD) and thermogravimetric analysis (TGA), the study shows that Ba₀․₇₅Sr₀․₂₅FeO₃ has improved oxygen storage capacity (OSC), oxidation, and reduction kinetics over pristine SrFeO₃ at temperatures ranging from 300-500 °C. The redox energetics computed by using first-principles density functional theory (DFT) calculations also depict the measured trend establishing it as an important descriptor of the measured performance. The Ba₁₋ₓSrₓFeO₃₋ᵧ depicts a Sr-substitution and oxygen stoichiometry induced structural phase transition from hexagonal at low temperatures to pseudo-cubic phase with higher OSC up to T=400 °C. Also, the mechanism leading to this structural phase transition is identified by performing electronic and vibrational structural calculations. This presentation was given on March 8, 2024 at the APS March Meeting in Minneapolis, MN.

Acharya, Shree Ram↗
DOE OSTI2020

Tuning the structural and antiferromagnetic phase transitions in UCr 2 Si 2 : Hydrostatic pressure and chemical substitution

Structural phase transitions in $\textit{f}$-electron materials have attracted sustained attention both for practical and basic science reasons, including the fact that they offer an environment to directly investigate relationships between structure and the $\textit{f}$-state. Here we present results for UCr 2 Si 2 , where structural (tetragonal → monoclinic) and antiferromagnetic phase transitions are seen at $T_S$ = 205 K and $T_N$ = 25 K, respectively. We also provide evidence for an additional second-order phase transition at $T_X$ = 280 K. We show that $T_X$, $T_S$, and $T_N$ respond in distinct ways to the application of hydrostatic pressure and Cr → Ru chemical substitution. In particular, hydrostatic compression increases the structural ordering temperature, eventually causes it to merge with $T_X$, and destroys the antiferromagnetism. In contrast, chemical substitution in the series UCr 2– x Ru x Si 2 suppresses both $T_S$ and $T_N$, causing them to approach zero temperature near $\textit{x}$ ≈ 0.16 and 0.08, respectively. The distinct $\textit{T–P}$ and ${T–x}$ phase diagrams are related to the evolution of the rigid Cr-Si and Si-Si substructures, where applied pressure semiuniformly compresses the unit cell, and Cr → Ru substitution results in uniaxial lattice compression along the tetragonal $\textit{c}$-axis and an expansion in the $\textit{ab}$-plane. These results in this work provide insights into an interesting class of strongly correlated quantum materials in which degrees of freedom associated with $\textit{f}$-electron magnetism, strong electronic correlations, and structural instabilities are readily controlled.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials preparation, single-crystal growth, and the phase diagram of the cuprate high-temperature superconductor La 1.6-x Nd 0.4 Sr x CuO 4

One branch of the La-214 family of cuprate superconductors, La 1.6-x Nd 0.4 Sr x CuO 4 (NdLSCO), has been of significant and sustained interest, in large part because it displays the full complexity of the phase diagram for canonical hole-doped, high TC superconductivity, while also displaying relatively low superconducting critical temperatures. The low superconducting TC’s imply that experimentally accessible magnetic fields can suppress the superconductivity to zero temperature. In particular, this has enabled various transport and thermodynamic studies of the T = 0 ground state in Nd-LSCO, free of superconductivity, across the critical doping p* = 0.23 where the pseudogap phase ends. The strong dependence of its superconducting properties on its crystal symmetry has itself motivated careful studies of the Nd-LSCO structural phase diagram. This paper provides a systematic study and summary of the materials preparation and characterization of both single crystal and polycrystalline samples of Nd-LSCO. Single-phase polycrystalline samples with x spanning the range from 0.01 to 0.40 have been synthesized, and large single crystals of La 1.6-x Nd 0.4 Sr x CuO 4 for select x across the region (0.07, 0.12, 0.17, 0.19, 0.225, 0.24, and 0.26) were grown by the optical floating zone method. Furthermore, systematic neutron and X-ray diffraction studies on these samples were performed at both low and room temperatures, 10 K and 300 K, respectively. These studies allowed us to follow the various structural phase transitions and propose an updated structural phase diagram for NdLSCO. In particular, we found that the low-temperature tetragonal (LTT) phase ends at a critical doping p LTT = 0.255±0.005, clearly separated from p*.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Topological phase transition to a hidden charge density wave liquid

Charge density waves (CDWs), electronic crystals that form within a host solid, have long been speculated to melt into a spatially textured electronic liquid. Though they have not been previously detected, liquid CDWs may nonetheless be fundamental to the phase diagrams of many correlated electron systems, including high temperature superconductors and quantum Hall states. In one of the most promising candidate materials capable of hosting a liquid CDW, 1T-TaS2, a structural phase transition impedes its observation. Here, by irradiating the material with a femtosecond light pulse, we circumvent the structural phase transition to reveal how topological defect dynamics govern the otherwise invisible CDW correlations. Upon photoexcitation, the CDW diffraction peaks broaden azimuthally, initially revealing a hexatic state. At higher temperatures, photoexcitation completely destroys translational and orientational order and only a ring of diffuse scattering is observed, a key signature of a liquid CDW. Our work provides compelling evidence for a defect-unbinding transition to a CDW liquid and presents a protocol for uncovering states that are hidden by other transitions in thermal equilibrium.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗