Crystal Transition Research Articles

Importance of the Hubbard U parameter to explore accurate electronic and optical behaviour of BiFeO3

Based upon Hubbard U corrected density functional theory (DFT), the electronic and optical responses of multiferroic Bismuth ferrite (BiFeO3; BFO) have been critically explored. Treating BFO as a strongly correlated insulating system, a Hubbard U parameter is considered for accurate description of localised Fe-3d states. However, the U parameter for the Fe-3d state is not sufficient as 2p states of O atoms are greatly influenced by Fe-3d states. Similar U parameters for Fe-3d as well as O-2p states are considered. An effective U value around 4.3 eV not only solves the band gap mismatch of BFO from both a theoretical and experimental aspect, but also predicts the exact nature of the band gap. Moreover, within this framework, the optical responses of BFO are critically investigated. Multiple transitions appearing in both theoretical and experimental absorption spectra are well explained by crystal field transitions of Fe3+ ions in a d5 high spin state, supported by the density of states curve of BFO. Therefore, based upon the DFT + U approach, accurate electronic and optical characteristics of BFO are critically explored, which is essential for its multidirectional application perspectives.

(Invited, Digital Presentation) Synthesis and Atomic Scale Characterization of 2D Layered Heterostructures Atom by Atom: An Ultra-high Resolution Aberration-corrected Electron Microscopy Study

Defects can have a profound effect on the macroscale physical, chemical, and electronic properties of nanostructures. They can lead to structural distortions, introduce extra states in the band gap and give rise to excess potential locally at buried interfaces. While defects and interfaces have been a well-studied subject for decades, little is known about their local atomic and chemical structure, sub-Angstrom structural distortions within their vicinity.Using ultra-high-resolution aberration-corrected S/TEM imaging and spectroscopy, this talk will discuss our recent efforts on the determination of the defect chemistry and sub-Angstrom relaxation effects in nanostructures around at the interfaces in the family of 2D crystal heterostructures. In the family of 2D crystal transition metal dichalcogenides (TMDs) alloys, we report how on anisotropy of the building blocks in the 2D heterostructure, such as ReS2/MoS2 and WS2/Graphene, and their epitaxy and strain at the interface can affect their nucleation and growth. This work will further uncover the structural distortions that occur in these heterostructure across various length scales and reveals the underlying physics of the formation of such heterostructures and their strain state resulting from CVD synthesis.

Suppression of phase-transition temperature in aluminium indium tungstate and aluminium indium molybdate

Aluminium indium tungstate (AlInW3O12) and aluminium indium molybdate (Al0.84In1.16Mo3O12) were synthesized by non-hydrolytic sol–gel chemistry, and their crystal structures, phase transition and thermal expansion behavior were studied using variable-temperature synchrotron powder diffraction. AlInW3O12 adopts an orthorhombic phase above 260 K and gradually transitions to a monoclinic polymorph below this temperature. Al0.84In1.16Mo3O12 also shows a gradual transition between the monoclinic and orthorhombic structures between 330 and 445 K. Both materials display much lower phase-transition temperatures than predicted on the basis of the parent compounds and Vegard's law. This suppression is attributed to the large size difference between Al3+ and In3+. Interestingly, both samples display positive thermal expansion along all unit-cell axes instead of the typically observed negative expansion of orthorhombic A 2 M 3O12 compositions.

Intensity of cooperative vibronic transitions in the Eu(III), Gd(III) and Tb(III) formates spectra.

The luminescence and absorption spectra of the lanthanide ions in solids and coordination compounds are characterized by sharp pure electronic lines, which are accompanied by much weaker lines of vibronic transitions. The vibronic spectroscopy is a good probing tool for investigations of the properties of surrounding ion ligands. The lanthanides formates are efficient luminescent crystals and can be viewed as the elementary type in the whole class of the oxygen-containing lanthanide coordination compounds. The intensity of vibronic transitions in spectra of luminescence and excitation europium (5 D0 →7 F2 , 7 F0 →5 D2 ), terbium (7 F6 →5 D4 ), gadolinium (6 P7/2 →8 S7/2 ) in anhydrous formates of the type Ln(HCOO)3 (Ln = Eu, Tb, Gd) and Y(HCOO)3 .2H2 O doped with Eu3+ and Tb3+ (C ~1 mol%) are reported. Also, the infrared and Raman spectra were obtained for the same compounds. Related integral intensity vibronic sidebands depend on the type of electronic transition of the same ion and varies for the same electronic transitions in different crystals. The obtained experimental data referring to the rate constants of vibronic transitions and intensity distribution in vibronic spectra on normal vibrations of the formate groups are in agreement with the predictions based on the Stavola-Dexter theory of cooperative vibronic transitions.

Phase transitions and physical properties by a color texture analysis: Results for liquid crystals

A new approach to investigate subtle liquid crystal transitions and some of their light wavelength dependent physical properties is developed and demonstrated. In this method color texture changes obtained from a polarizing optical microscope are processed, and their color features are exploited. The significance of color features in image processing is discussed with reference to a color extension of the GLCM method which demonstrates smooth phase transitions occurring in two types of chiral liquid crystals (where ferroelectric smectic phases are observed). In the proposed method representation of the physical light wavelength distribution is obtained from suitable image conversion from RGB color space model to the Hue-Saturation-Lightness (HSL) model. This allows the identification of local color changes caused by external factors e.g., electric field or stresses. In this way the analysis of certain physical quantities is possible. The approach was applied to liquid crystal blue phases to show changes in the light wavelength under an electric field, which are related to the electrostriction coefficients. The result from image analysis are compared to results from a conventional spectrophotometer.

Structural Phase Transitions in Anthracene Crystals.

Anthracene (C14 H10 ) and its derivatives, π-conjugated molecules in acenes, have been widely researched in terms of their reactions, physical properties, and self-assembly (or crystal engineering). These molecules can be functionalized to tune reactivities, optoelectronic properties, and self-assembling abilities. Structural changes in the molecular assemblies, solid states, and crystals have recently been discovered. Therefore, a systematic discussion of anthracene's molecular structure, packing, and optical properties based on its intermolecular structure and phase transitions is important for future chemical and structural design. In the present review, we discuss anthracene's molecular design, dimer packing, and crystal structure, focusing on the structural phase transitions of its crystals. We also provide examples of the phase transitions of anthracene crystals. Changes to edge-to-face of CH-π interaction and face-to-face packing of π-π interaction affect the thermodynamic stabilities of various crystal structures. These structures can inform the prediction of structural and physical properties.

Water-Induced Crystal Transition and Accelerated Relaxation Process of Polyamide 4 Chains in Microfibers

Microplastics have recently been identified as one of the major contributors to environmental pollution. To design and control the biodegradability of polymer materials, it is crucial to obtain a better understanding of the aggregation states and thermal molecular motion of polymer chains in aqueous environments. Here, we focus on melt-spun microfibers of a promising biodegradable plastic, polyamide 4 (PA4), with a relatively greater number density of hydrolyzable amide groups, which is regarded as an alternative to polyamide 6. Aggregation states and thermal molecular motion of PA4 microfibers without/with a post-heating drawing treatment under dry and wet conditions were examined by attenuated total reflectance-Fourier transform infrared spectroscopy and wide-angle X-ray diffraction analysis in conjunction with dynamic mechanical analysis. Sorbed water molecules in the microfibers induced the crystal transition from a meta-stable γ-form to a thermodynamically stable α-form via activation of the molecular motion of PA4 chains. Also, the post-drawing treatment caused a partial structural change of PA4 chains, from an amorphous phase to a crystalline phase. These findings should be useful for designing PA4-based structural materials applicable for use in marine environments.

Cutting characteristics of monocrystalline silicon in elliptical vibration nano-cutting using molecular dynamics method

Based on the molecular dynamics method, the model of monocrystalline silicon in elliptical vibration nano cutting (EVNC) is established. The effects of amplitude ratio (ratio of amplitude in parallel cutting direction to amplitude in vertical cutting direction) on the mechanical properties, thermal properties, crystal phase transition and morphology are systematically investigated. The results show that with the amplitude ratio increasing from 0.0 to 3.0, the hydrostatic compressive stress during cutting in the contact area decreases from 12.31 GPa to 8.31 GPa, and the tensile stress on the machined surface decreases from 5.23 GPa to 3.07 GPa. The depth of tensile stress layer after cutting is reduced from 12.02 Å to 4.8 Å. During cutting the maximum temperature of the machined surface decreases from 1419 K to 1236 K, and the chip maximum temperature increases from 1730 K to 2994 K. The number of diamond structures (bond length is 2.35 Å) is reduced during the cutting process, and the moderately increasing in the amplitude ratio can inhibit the formation of the bct-5 phase. After cutting, the number of diamond structures (bond length 2.35 Å) on the workpiece surface increases. The scratch morphology quality is effectively improved. With increasing amplitude ratio, the extrusion impact of the tool increases, resulting in the length and direction of the atomic displacement vector in the cutting area changing periodically, and the atomic kinetic energy and potential energy also change periodically, which may be the reason for EVNC to obtain better surface integrity. The research results provide theoretical reference for the application of ultrasound vibration assisted process in the field of atomic or near-atomic scale manufacturing.

Electronic structure, magnetic properties and magnetocaloric performance in rare earths (RE) based RE2BaZnO5 (RE = Gd, Dy, Ho, and Er) compounds

Nowadays, the rare earths (RE) based magnetic materials with various promising functional characterizations are playing particularly important roles in present modern society and daily life. The magnetocaloric (MC) effect which is the basis of solid state magnetic refrigeration (MR) technology has been extensively investigated in various magnetic materials. In this work, a series of high quality RE-based RE2BaZnO5 (RE = Gd, Dy, Ho, and Er) compounds have been fabricated and a combination of theoretical and experimental investigation has been performed in terms of the crystal structure, electronic structure, magnetic properties and magnetic phase transition (MPT) as well as the MC effects and MC performance. All the RE2BaZnO5 compounds are confirmed to crystallize in the orthorhombic structure belonging to Pnma space group and with a typical semi-conductor characteristic. Based on the magnetization measurements and the first principle calculations, all the RE2BaZnO5 compounds are ordering magnetically below 3 K and the ground state is of a ferromagnetic (FM) ordering for RE = Dy or Ho, whereas is of an antiferromagnetic (AFM) ordering for RE = Gd or Er, respectively. Additionally, considerable MC effects have been observed for all the RE2BaZnO5 compounds which are related to the first ordered MPT for RE = Gd or Er, whereas related to the second order MPT for RE = Dy or Ho, respectively. Moreover, considerable MC effects and promising MC performance have been realized in the present RE2BaZnO5 compounds, especially for the Dy2BaZnO5 and Ho2BaZnO5, making them considerable for cryogenic MR applications.

On the critical threshold for detonation

This paper considers the stress threshold required to shock an explosive so that it runs to full detonation. At present it is assumed that a reaction zone composes localised burning regions that increase in density as pressure is increased. However, a series of burn rate experiments and investigations into the shock response of explosives indicate a critical stress level at which response changes. The stress at which the theoretical shear strength is exceeded under shock loading is called the Weak Shock Limit. This is the gateway to the strong shock region in which a compression front travels faster than an elastic wave. In this region experiments suggest that the response becomes independent of microstructure and that there is a transition in an energetic crystal's electronic state. This results in pre-explosion conductivity and luminescence, consistent with a reduced bandgap in the homogeneous flow. The rate-limiting steps in explosive decomposition are generally accepted to follow Arrhenius kinetics, and it is hypothesised here that the activation energy then decreases above the weak shock limit. This allows reaction to move from temperature-dependant burn at mesoscale hot spots, to temperature-dependant, homogeneous reaction behind the shock wave. The shock can then quickly build to detonation even through the stress for this transition is an order of magnitude less than the Von Neumann spike pressure. This hypothesis has a range of implications that may be tested in further experiments. If explicitly proven, the weak shock limit will allow the description of the transition to, and propagation of, detonation within continuum codes. It will thus represent a vital quantity to consider for the safe handling of energetic materials.

Effect of Host Structure on Optical Freedericksz Transition in Dye-Doped Liquid Crystals.

The optical Freedericksz transition (OFT) can reversibly control the molecular orientation of liquid crystals (LCs) only by light irradiation, leading to the development of all-optical devices, such as smart windows. In particular, oligothiophene-doped LCs show the highly sensitive OFT due to the interaction between dyes and an optical-electric field. However, the sensitivity is still low for the application to optical devices. It is necessary to understand the factors in LCs affecting the OFT behavior to reduce the sensitivity. In this study, we investigated the effect of the host LC structure on the OFT in oligothiophene-doped LCs. The threshold light intensity for the OFT in trifluorinated LCs was 42% lower than that in LCs without fluorine substituents. This result contributes to the material design for the low-threshold optical devices utilizing the OFT of dye-doped LCs.

Coevolution of Superconductivity With Structure and Hall Coefficient in Pressurized NaSn2As2

A new class of van der Waals-type layered materials, ASn2Pn2 (A= Li, Na, Sr, Eu; Pn= As, P, Sb), has attracted much attention in the field of condensed matter physics because they have interesting physical properties and various ground states, as well as potential applications. Here, we are the first to report the close connection among the superconducting transition temperature Tc, crystal structure and Hall coefficient in pressurized NaSn2As2 single crystal. We found that the superconducting NaSn2As2 displays two pressure-induced crystal structure phase transitions, first from an ambient-pressure rhombohedral (R) phase to a monoclinic (M) phase starting at ∼ 12 GPa (PC1), and then to a simple cubic (C) phase starting at ∼ 33 GPa (PC2). In these phases, the Tc value and carrier concentration change correspondingly. Our results suggest that the observed three superconducting states are related to the change of structural phase and the variation of carrier concentrations.

Constitutional supercooling and corresponding microstructure transition triggered by high magnetic field gradient during directional solidification of Al-Fe eutectic alloy

Directional solidification experiments of eutectic AlFe alloys were carried out under different high magnetic field gradients. High magnetic field gradient changed the microstructure selection during solidification process, induced the solidification microstructure to undergo eutectic instability, cellular transition, single-phase instability, and dendrite transition, which is similar to the planar-cellular-dendritic crystal growth pattern transition usually induced by an increasing growth velocity without magnetic field. The single phase was proved to be formed through an independent nucleation mechanism during single-phase instability. The effect of high magnetic field gradient on solidification was analyzed. Through the coupling effect of magnetic force and Lorentz force on solute migration and diffusion during solidification, high magnetic field gradient triggered a constitutional supercooling at the front of solid-liquid interface, which led to the growth pattern transition. A solidification model of eutectic alloy under high magnetic field gradient was proposed, and the microstructure selection map of AlFe alloy under gradient magnetic field was qualitatively drawn for the first time. This work proved that an alloy material with a microstructure usually obtained at high growth velocity can be obtained at a low growth velocity by applying a high magnetic field gradient, and suggested a new potential method for the future controlling of alloy structures and properties.

Experimental determination and correlation of naproxen solubility in biodegradable low-toxic betaine-based deep eutectic solvents and water mixtures at 293.15 K to 313.15 K

For enhancement the low bioavailability of poorly water-soluble naproxen, three deep eutectic solvents (DESs) obtained from betaine as a hydrogen bond acceptor and propylene glycol, ethylene glycol or glycerol as a hydrogen bond donor (Bet/PG, Bet/EG or Bet/Gly DESs) are utilized from 0.0 to 1.0 mass fractions of DESs at T = (293.15–313.15) K under atmospheric pressure (≈ 85 kPa). The solubility values tend to enhance with enhancing mass fraction of each DES as well as increasing temperature and the maximum values are obtained in the highest mass fraction of DESs at 313.15 K. Besides, the Hansen solubility parameters are used to predict the selection of appropriate cosolvent system; the results indicated that the Bet/PG DES is a proper cosolvent for naproxen solubilization. Moreover, the equilibrated solid phase crystal of naproxen with neat solvents is characterized with X-ray powder diffraction analysis to identify no polymorphic transformation, solvate formation or crystal transition during the whole solvent crystallization process. In addition, the experimental solubilities are correlated with the temperature dependency of traditional cosolvency models including Jouyban-Acree, Jouyban-Acree-van't Hoff and modified Wilson-van't Hoff together with the activity coefficient models of NRTL, Wilson and UNIQUAC. The accuracies of the calculated solubilities are evaluated by calculation of the mean relative deviation (MRD%) and the results suggested that the Jouyban-Acree (MRD% ≤ 8.9) along with the NRTL activity coefficient model (MRD% ≤ 2.0) are more accurate than the others. Finally, the respective apparent thermodynamic quantities of the dissolution and mixing processes, namely Gibb's energy, enthalpy, and entropy are calculated based on the van't Hoff and Gibbs equations.

Effect of B2O3, K2O and Li2O as modifiers on crystal phase transition and crystallite growth of TiO2 in calcination of titanyl sulfate

Thermal phase transformation and crystal growth in calcination of titanyl sulfate with addition of B2O3, K2O and Li2O have been studied, and the structure and morphology of TiO2 product has been characterized by XRD, SEM, TEM and size analyzer. It is found that B3+ ions being doped interstitially into crystal lattice of rutile TiO2 tend to form compound of TiB0.024O2 with TiO2, which leads to increase in unit cell volume and henceforth, an increase in growth rate of the crystal plane (110) or reduction in growth rate of crystal plane (001). Change in growth rate in different crystal planes contributes to development of the plate shape and rod shape microcrystals and improvement in grain sphericity. Doping of K2O tends to inhibit the transformation of crystal form and inhibit particle sintering at high temperatures mainly by adsorption of the dopants onto crystal plane (110) and plane (100) of rutile TiO2. Li2O, which is mainly adsorbed on the surface of rutile TiO2, is found to have a weak effect on crystal phase transformation. The apparent activation energy for crystal growth of rutile TiO2 doped with B2O3, K2O and Li2O has been calculated to be 73.16 ​kJ/mol, 103.21 ​kJ/mol, and 187.24 ​kJ/mol respectively.

Crystal structure evolution in the van der Waals vanadium trihalides

Most transition-metal trihalides are dimorphic. The representative chromium-based triad, CrCl3, CrBr3, CrI3, is characterized by the low-temperature (LT) phase adopting the trigonal BiI3-type while the structure of the high-temperature (HT) phase is monoclinic of AlCl3 type (C2/m). The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis in CrCl3 and CrI3. We studied crystal structures and structural phase transitions of vanadium-based counterparts VCl3, VBr3, and VI3 by measuring specific heat, magnetization, and x-ray diffraction as functions of temperature and observed an inverse situation. In these cases, the HT phase has a higher symmetry while the LT structure reveals a lower symmetry. The structural phase transition between them shows no measurable hysteresis in contrast to CrX3. Possible relations of the evolution of the ratio c/a of the unit cell parameters, types of crystal structures, and nature of the structural transitions in V and Cr trihalides are discussed.

Interfacial structure and properties of isotactic polybutene-1/polyethylene blends

Abstract Polymer blending is one of the most economical and effective techniques for achieving products with high comprehensive performances. However, the immiscibility between polymers results in a weak interface, which is typically the position where material failure starts when an external force is applied. Therefore, understanding and controlling the interfacial structure are important for controlling the failure behavior of polymer blends and achieving advanced materials. In this study, the related work was performed on a crystal/crystal blend of isotactic polybutene-1 and polyethylene (iPB-1/PE). The results indicated that iPB-1 and PE were partially miscible in a wide temperature window (140–220°C), and the phase separation of iPB-1/PE blends was retarded at 180°C, resulting in an increase in the interfacial thickness and interfacial adhesive strength when iPB-1/PE crystallized at a low temperature. In addition, the iPB-1/high-density PE (HDPE) samples exhibited higher interfacial adhesive strength than the iPB-1/linear low-density PE, which was attributed to the relative streamline chain structure and the wide molecular weight distribution of HDPE and improved the interpenetration, crystallization, and miscibility of iPB-1 and HDPE at the interface. During storage at room temperature, the interfacial adhesive strength of iPB-1/PE decreased because of the spontaneous crystal transition of iPB-1.

Finger front propagation in smectic-A Fréedericksz transition.

Systems with multistability are characterized by exhibiting complex nonlinear waves between equilibria. Experimentally, near the smectic-A to chiral nematic transition in a liquid crystal mixture cell with planar anchoring, we observe finger fronts emerge in the smectic-A phase when applying an electric field, a reorientation transition. Finger fronts propagate in the direction orthogonal to the anchoring. Colorimetry characterization allows us to describe the molecular reorientation transition and front dynamics. We reveal that the reorientation transition is of the first-order type and determine their critical points. The front speed is determined as a function of the applied voltage. Theoretically, based on a prototype model of liquid crystal transitions, we qualitatively describe the experimental observations. We have analytically determined the bifurcation diagram and the propagation speeds of finger fronts, finding a fair agreement with the experimental observations.

Biasing crystallization in fused silica: An assessment of optimal metadynamics parameters.

Metadynamics (MetaD) is a useful technique to study rare events such as crystallization. It has been only recently applied to study nucleation and crystallization in glass-forming liquids such as silicates, but the optimal set of parameters to drive crystallization and obtain converged free energy surfaces is still unexplored. In this work, we systematically investigated the effects of the simulation conditions to efficiently study the thermodynamics and mechanism of crystallization in highly viscous systems. As a prototype system, we used fused silica, which easily crystallizes to β-cristobalite through MetaD simulations, owing to its simple microstructure. We investigated the influence of the height, width, and bias factor used to define the biasing Gaussian potential, as well as the effects of the temperature and system size on the results. Among these parameters, the bias factor and temperature seem to be most effective in sampling the free energy landscape of melt to crystal transition and reaching convergence more quickly. We also demonstrate that the temperature rescaling from T > Tm is a reliable approach to recover free energy surfaces below Tm, provided that the temperature gap is below 600K and the configurational space has been properly sampled. Finally, albeit a complete crystallization is hard to achieve with large simulation boxes, these can be reliably and effectively exploited to study the first stages of nucleation.

Constructing Anatase-Brookite TiO2 Phase Junction by Thermal Topotactic Transition to Promote Charge Separation for Superior Photocatalytic H2 Generation.

Phase junctions of photocatalysts can promote the separation of photogenerated charge carriers for efficient utilization of the carriers. Construction of phase junctions and establishing their structure-performance relationship are still required. Herein, polycrystalline TiO2 decahedral plates with different phases were synthesized by thermal treatment-induced topotactic transition of titanium oxalate crystals. The phase of TiO2 evolved from pure anatase to anatase-brookite, anatase-brookite-rutile, and then to anatase-rutile, while the morphology of the decahedral plates was well maintained. The biphase anatase-brookite was found to be most efficient in photocatalytic hydrogen generation. Specifically, the hydrogen generation rate of the biphase anatase-brookite TiO2 was nearly 2.4 times greater than that of the biphase anatase-rutile TiO2. The spatially resolved surface photovoltage measurements indicate the more efficient separation of photogenerated charge carriers and thus greater photocatalytic activity of the former. This work provides a strategy for developing efficient phase-junction photocatalysts.