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At least 1,009 records · Page 56
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Phase transitions and octahedral rotations in epitaxial Ag(Ta x Nb 1-x )O 3 thin films under tensile strain

In this paper, epitaxial Ag(Ta 0.5 Nb 0.5 )O 3 (ATN) films under tensile strain were deposited on (Ba 0.4 Sr 0.6 )TiO 3 /LaAlO 3 (001) p and KTaO 3 (001) substrates. These films exhibited a domain structure with the c-axis aligned primarily along the in-plane direction in contrast with the poly-domain nature of bulk ATN ceramics or relaxed films. While the generic phase transition sequence of the tensile films was qualitatively similar to bulk, the tetragonal and orthorhombic phase field regions expanded by ~270 °C in ATN/(Ba 0.4 Sr 0.6 )TiO 3 /LaAlO 3 . Furthermore, the films were found to be in the M 3 (complex octahedral tilting with disordered Nb/Ta displacements) phase at room temperature with either significantly reduced tilt angles or a suppression of the long range order of the complex tilt as compared to bulk materials. It was observed that the octahedral tilt domains were oriented with the complex tilt axes lying in the plane of the film due to the tensile strain. Finally, this work demonstrates that tensile strain can be used to strain-engineer materials with complex tilt systems and thereby modify functional properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Unraveling the transformation pathway of the 𝛽 to 𝛾 phase transition in Ga 2 ⁢O 3 from atomistic simulations

Defect spinel 𝛾−Ga 2 ⁢O 3 is the least stable polymorph of Ga 2 ⁢O 3 , so its frequent appearance as a structural defect within or on the surface of monoclinic 𝛽−Ga 2 ⁢O 3 remains a mystery. Through first-principles calculations, we explore potential pathways for the phase transition from 𝛽−Ga 2⁢ O 3 to 𝛾−Ga 2 ⁢O 3 , and examine two key driving forces: tensile strain and Ga deficiency. When configurational entropy contributions to phase energies are included, the 𝛾 phase becomes energetically competitive with the 𝛽 phase, with the free energy difference between these phases diminishing even further under Ga-deficient conditions. Notably, a stability crossover occurs at room temperature at high vacancy concentrations ([V$^{3−}_{Ga}$]>3%) . A simple model 𝛽 → 𝛾 transformation pathway is identified, comprising two primary reactions, that enables the formation of the 𝛾 phase via simultaneous migration of Ga atoms from tetrahedral lattice sites to octahedral interstitial positions. The transformation barriers are prohibitively large in pristine Ga 2 ⁢O 3 , but can be substantially reduced by: (1) the presence of Ga vacancies, (2) elongational strains along the crystallographic 𝑎-axis, and (3) when volumetric relaxations are possible during transformation. These results elucidate prior experimental observations, where 𝛾−Ga 2⁢ O 3 is seen on damaged surfaces or in highly 𝑛-type 𝛽−Ga 2⁢ O 3 environments, which support Ga deficiency and mechanical strain. The insights into the driving forces and mechanisms of 𝛾−Ga 2⁢ O 3 formation enhance understanding of how localized strain and nonequilibrium defect concentrations may facilitate its formation from the 𝛽 phase.

Defects↗
DOE OSTI2018

Electron–electron interactions and the paired-to-nematic quantum phase transition in the second Landau level

In spite of its ubiquity in strongly correlated systems, the competition of paired and nematic ground states remains poorly understood. Recently such a competition was reported in the two-dimensional electron gas at filling factor ν = 5/2. At this filling factor a pressure-induced quantum phase transition was observed from the paired fractional quantum Hall state to the quantum Hall nematic. Here we show that the pressure-induced paired-to-nematic transition also develops at ν = 7/2, demonstrating therefore this transition in both spin branches of the second orbital Landau level. However, we find that pressure is not the only parameter controlling this transition. Indeed, ground states consistent with those observed under pressure also develop in a sample measured at ambient pressure, but in which the electron–electron interaction was tuned close to its value at the quantum critical point. Our experiments suggest that electron–electron interactions play a critical role in driving the paired-to-nematic transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Acoustic Detection of Phase Transitions at the Nanoscale

On page 478, N. Bassiri-Gharb and co-workers demonstrate acoustic detection in nanoscale volumes by use of an atomic force microscope tip technique. Elastic changes in volume are measured by detecting changes in resonance of the cantilever. Also, the electric field in this case causes a phase transition, which is modeled by Landau theory.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Complex Studies of the Kinetics of Induced Phase Transition in [001], [011], and [111]-Oriented Single Crystal Relaxor Solid Solutions

The kinetics of an induced phase transition in single-crystal relaxor solid solutions 33PbIn{sub 1/2}Nb{sub 1/2}O{sub 3}–35PbMg{sub 1/3}Nb{sub 2/3}O{sub 3}–32PbTiO{sub 3} and PbMg{sub 1/3}Nb{sub 2/3}O{sub 3}–29PbTiO{sub 3} under application of an electric field along three directions [001], [011], and [111] at room temperature is studied. Joint measurements of optical transmission at different wavelengths, dielectric properties, and attenuation of sound are carried out. It was found that, even in small fields (E ~ 2 kV/cm) applied in the [011] and [111] directions, the maximum changes in all three characteristics as well as the coincidence of the time dependences of the optical transmission at different wavelengths of light occur in the same time interval. It is established that, in the same interval of fields along the [001] direction, the anomalies of the optical properties do not coincide in time with the anomalies of the acoustic and dielectric properties. The results obtained are explained by the induction of single-domain and multidomain phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

A Simple Kinetic Model for the Phase Transition of the van der Waals Fluid

A simple kinetic model, which is presumably minimum, for the phase transition of the van der Waals fluid is presented. In the model, intermolecular collisions for a dense gas has not been treated faithfully. Instead, the expected interactions as the non-ideal gas effect are confined in a self-consistent force term. Collision term plays just a role of thermal bath. Accordingly, it conserves neither momentum nor energy, even globally. It is demonstrated that (i) by a natural separation of the mean-field self-consistent potential, the potential for the non-ideal gas effect is determined from the equation of state for the van der Waals fluid, with the aid of the balance equation of momentum, (ii) a functional which monotonically decreases in time is identified by the H theorem and is found to have a close relation to the Helmholtz free energy in thermodynamics, and (iii) the Cahn–Hilliard type equation is obtained in the continuum limit of the present kinetic model. Numerical simulations based on the Cahn–Hilliard type equation are also performed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Hugoniot, sound speed, and phase transitions of single-crystal sapphire for pressures 0.2–2.1 TPa

Sapphire (Al 2 O 3 ) is a major constituent of the Earth's mantle and has significant contributions to the field of high-pressure physics. Constraining its Hugoniot over a wide pressure range and identifying the location of shock-driven phase transitions allows for development of a multiphase equation of state and enables its use as an impedance-matching standard in shock physics experiments. In this paper we present measurements of the principal Hugoniot and sound velocity from direct impact experiments using magnetically launched flyers on the Z machine at Sandia National Laboratories. The Hugoniot was constrained for pressures from 0.2–2.1 TPa and a four-segment piecewise linear shock-velocity–particle-velocity fit was determined. First-principles molecular dynamics simulations were conducted and agree well with the experimental Hugoniot. Sound-speed measurements identified the onset of melt between 450 and 530 GPa, and the Hugoniot fit refined the onset to 525 ± 13 GPa. A phase diagram which incorporates literature diamond-anvil cell data and melting measurements is presented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The local phase transitions of the solvent in the neighborhood of a solvophobic polymer at high pressures

We investigate local phase transitions of the solvent in the neighborhood of a solvophobic polymer chain which is induced by a change of the polymer-solvent repulsion and the solvent pressure in the bulk solution. We describe the polymer in solution by the Edwards model, where the conditional partition function of the polymer chain at a fixed radius of gyration is described by a mean-field theory. The contributions of the polymer-solvent and the solvent-solvent interactions to the total free energy are described within the mean-field approximation. We obtain the total free energy of the solution as a function of the radius of gyration and the average solvent number density within the gyration volume. The resulting system of coupled equations is solved varying the polymer-solvent repulsion strength at high solvent pressure in the bulk. We show that the coil-globule (globule-coil) transition occurs accompanied by a local solvent evaporation (condensation) within the gyration volume.

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

Prolate-Oblate Asymmetric Shape Phase Transition in the Interacting Boson Model with SU(3) Higher-Order Interactions

Prolate-oblate shape phase transition is an interesting topic in nuclear structure, which is useful for understanding the intrinsic interactions between nucleons. Recently, the interacting boson model with SU(3) higher-order interactions was proposed, in which the prolate shape and the oblate shape are not described in a mirror symmetric way. This asymmetric description seems more realistic. The level evolutions, B(E2) values, and other important indicators showing the prolate-oblate asymmetric transitions are investigated in detail, and realistic structure evolutions from 180Hf to 200Hg are compared. A key finding is that the average deformation of the prolate shape is nearly twice the one of the oblate shape. These results, together with the successful description of the B(E2) anomaly in 168,170Os, 172Pt, the γ -soft properties of 196Pt, 82Kr, and the normal states of 110Cd, support the validity of the new model.

Wang, Tao↗
DOE OSTI2018

The Curie–Weiss model with Complex Temperature: Phase Transitions

We study the partition function of the Curie–Weiss model with complex temperature, and partially describe its phase transitions. As a consequence, we obtain information on the locations of zeros of the partition function (the Fisher zeros).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Great reduction of loss at high electric field in the polyvinylidene fluoride/aromatic polythiourea blend films along with an irreversible phase transition

In this paper, the polarization response of dielectric blend based on the polyvinylidene fluoride (PVDF) and aromatic polythiourea (ArPTU) was investigated. It is found that the conduction loss at high field has been reduced 2–4 times along with an interesting irreversible transition under high electric field. The loss at 700 MV/m can be kept below 20%, which is better than most other polar dielectric polymers. It is proved that the transition is a phase change from glass phase to polar β phase at a critical field. And the highly polar thiourea groups of ArPTU can act as traps for the electrons, reduce conduction loss greatly and prevent dielectric breakdown by stabilizing the electron energy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Phase transition kinetics and surface binding states of methylammonium lead iodide perovskite

We have presented a detailed analysis of phase transition kinetics and binding energy states of solution processed methylammonium lead iodide (MAPbI3) thin films prepared at ambient conditions and annealed at different elevated temperatures. The processing temperature and environmental conditions predominantly control the crystal structure and surface morphology of MAPbI3 thin films. The structural transformation from tetragonal to cubic occurs at 60C for 30 minutes annealing time while 10 minutes annelaed films posses tetragonal structure. The transformed phase is greatly intact even at higher annealing temperature of 150 C and time of 2 hours. Existence of charge transfer interaction between Pb 4f and I 3d oxidation states is quantified by the XPS spin orbit spilitting.

Kumar, G. R.↗
DOE OSTI2013

Diffuse phase transition in Li{sub 0.12}Na{sub 0.88}NbO{sub 3} piezoelectric ceramics

The morphotropic phase boundary composition viz. lithium sodium niobate, Li{sub 0.12}Na{sub 0.88}NbO{sub 3}, (LNN-12) was prepared by conventional solid state reaction and sintering route. The temperature dependent permittivity response near transition temperature showed a diffused phase transition (DPT). The degree of diffuseness, {gamma}, using the modified Curie-Weiss law, was found to be 1.92, indicative of almost-complete diffuse phase transition. The planar coupling constant k{sub p}, and mechanical quality factor Q{sub m}, measured by resonance-antiresonance method, were 0.17 and 413 respectively. These parameter values make LNN-12 ceramic an attractive candidate for transducers applications.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Magnetic and Structural Phase Transitions in the Spinel Compound Fe1+xCr2 xO4

Neutron diffraction, X-ray diffraction, magnetic susceptibility, and specific heat measurements have been used to investigate the magnetic and structural phase transitions of the spinel system Fe1+xCr2 xO4 (0.0 x 1.0). The temperature versus Fe concentration (x) phase diagram contains two magnetically ordered phases and four structural phases below 420 K. The complicated transitions are closely related to the change in the spin and orbital degrees of freedom induced by substitution of Fe ions for Cr ions. The systematic change in the crystal structure is explained by the combined effects of Jahn-Teller distortion, spin-lattice interaction, Fe2+-Fe3+ hopping, and disorder among Fe2+, Fe3+, and Cr3+ ions.

Ma, Jie↗
NASA NTRS1978

Solid-liquid phase transition in argon

Starting from the Lennard-Jones interatomic potential, a modified cell theory has been used to describe the solid-liquid phase transition in argon. The cell-size variations may be evaluated by a self-consistent condition. With the inclusion of cell-size variations, the transition temperature, the solid and liquid densities, and the liquid-phase radial-distribution functions have been calculated. These ab initio results are in satisfactory agreement with molecular-dynamics calculations as well as experimental data on argon.

Tsang, T.↗
DOE OSTI2015

Quantitative analysis of the local phase transitions induced by the laser heating

Functional imaging enabled by scanning probe microscopy (SPM) allows investigations of nanoscale material properties under a wide range of external conditions, including temperature. However, a number of shortcomings preclude the use of the most common material heating techniques, thereby limiting precise temperature measurements. Here we discuss an approach to local laser heating on the micron scale and its applicability for SPM. We applied local heating coupled with piezoresponse force microscopy and confocal Raman spectroscopy for nanoscale investigations of a ferroelectric-paraelectric phase transition in the copper indium thiophosphate layered ferroelectric. Bayesian linear unmixing applied to experimental results allowed extraction of the Raman spectra of different material phases and enabled temperature calibration in the heated region. Lastly, the obtained results enable a systematic approach for studying temperature-dependent material functionalities in heretofore unavailable temperature regimes.

36 MATERIALS SCIENCE↗