Hexagonal Phase Transition Research Articles

Scalar field model applied to the lamellar to the inverse hexagonal phase transition in lipid systems

In this paper we use a phenomenological field theory model to study the first-order phase transition from the lamellar phase to the inverse hexagonal phase in specific lipid bilayers. The model is described by a real scalar field with potential that supports both symmetric and asymmetric phase conformations. We adapt the coordinate and parameters of the model to describe the phase transition, and we show that the model is capable of correctly inferring the fraction of the inverse hexagonal phase in the phase transition, suggesting an alternative way to be couple to experimental techniques generally required for HII−phase characterization.

Determination of n-alkane partitioning within phosphatidylethanolamine Lα/HII phases

Alkanes are known to promote the fluid lamellar (Lα)-to-inverted hexagonal (HII) phase transition of different phospholipids. In this work, we studied the interaction of decane and tetradecane with self-assemblies formed of 1-palmitoyl-2-oleoyl-sn-glycero-phosphoethanolamine (POPE), using sequential 2H and 31P solid-state NMR spectroscopy. This technique allowed calculating the partitioning constant of the alkanes between the Lα and HII phases of POPE. Our results show that both alkanes are preferentially distributed in the HII phase compared to the Lα phase. In the HII phase, both alkanes display a very high conformational disorder, consistent with their location in the intercylinder voids. This preferential partitioning in the HII phase is more pronounced for tetradecane than for decane. This finding is proposed to be associated with a less energetically favored insertion (limited solubility) of tetradecane in the lamellar phase. This proposition is supported by the observation that tetradecane experiences more conformational freedom than its shorter analog in POPE bilayers, suggesting that it is located, on average, closer to the middle of the bilayers. It is also established that the increase in size of the intercylinder voids, by the addition of 1-palmitoyl-2-oleoyl-sn-glycero-phosphocholine (POPC) in the HII cylinders, enhances the partitioning of decane in the HII phase compared to the Lα phase. These findings propose handles to modulate the balance of the relative Lα/HII phase stability.

Synthesis and characterization of ZnTiO3 and Ag doped ZnTiO3 perovskite nanoparticles and their enhanced photocatalytic and antibacterial activity

ZnTiO3 and ZnTiO3:Ag (x%) (x = 0.01,0.02 and 0.03) perovskite nanocrystals were synthesized by using the sol gel method. The integrated products annealed at 600 °C–800 °C yields cubic to the hexagonal phase transition of ZnTiO3, that is clearly observed by XRD patterns. The products have been described using X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM) together with Energy dispersion X-ray spectrum (EDS) analysis and UV–Vis spectroscopy analysis. The photocatalytic studies exposed that prepared samples show entire photocatalytic decolorization of methyl blue under UV-light irradiation at 30mins. The antibacterial activity of pure and Ag doped ZnTiO3 was experienced in opposition to Staphylococcus aureus and Vibrio by means of the agar well diffusion method. The report shows that the Ag doped ZnTiO3 nanoparticles play a crucial responsibility in enhancing its photo-catalytic and antibacterial properties. It acts as multifunctional product and behaves both as an antibacterial agent and a proficient material for photo-catalysis beneath UV-light.

Effect of heat treatment on structural, magnetic, elastic and optical properties of the co-precipitated Co0.4Sr0.6Fe2O4

The crystalline phase, elastic, magnetic, and optical properties of heated Co0.4Sr0.6Fe2O4 have been studied at different annealing temperatures. The samples have been scanned by (XRD), (HR-TEM), (FT-IR), (VSM), and UV–Visible spectrophotometry techniques. The cubic → hexagonal phase transition occurs at about 900 °C. During the precipitation process, the formed Sr(OH)2 exposed to atmospheric CO2 and the SrCO3 appeared as a second phase. So the precipitated powders are a mixture of cobalt ferrites and SrCO3, and the solid-state reaction between them leads to the transformation from cubic to hexagonal. Heating effects on the structural and magnetic parameters offer the possibility to modify the magnetic and electric phenomena to suit promising applications. The elastic moduli decrease by heating, whereas the average particle size increases in the range of 8–55 nm. The optical band-gap depends on the annealing temperature and was found to be ~2.78 eV, and the optical studies show a direct allowed transition in these nanostructures.

First-principles study of thermoelectric properties of Li-based Nowotony–Juza phases

We investigated the cubic–hexagonal phase transition and its effect on thermoelectric performance in Li-based Nowotny–Juza phases LiZn X (X = N, P, As, Sb, and Bi). Interestingly, other than LiZnSb, the cubic LiZnBi is found to be energetically more favorable than the hitherto reported hexagonal phase. The hexagonal phases of reported cubic LiZnP and LiZnAs are likely to be stabilized by pressure-hydrostatic pressure can be aided by internal pressure. We find that while power factor values are much improved in the proposed hexagonal phases, the values in cubic phases are also impressive. We also determine conservative estimates of the figure of merit. The ZT values of cubic and hexagonal LiZnSb at 700 K are 1.27 and 1.95, respectively. Other promising values are 1.96 and 1.49 at 700 K of hexagonal n-type LiZnP and LiZnAs, respectively. Overall, our findings suggest the good thermoelectric potential of Nowotny–Juza phases.

Phase‐Engineered Synthesis of Ultrathin Hexagonal and Monoclinic GaTe Flakes and Phase Transition Study

AbstractGaTe is an important III–VI semiconductor with direct bandgap; thus, it holds great potential in the field of optoelectronics. Although it is known that GaTe can exist both in monoclinic and hexagonal phases, current studies are still exclusively restricted to the monoclinic phase of two dimensional (2D) GaTe owing to the difficulty in the fabrication of 2D hexagonal GaTe. Both monoclinic and hexagonal GaTe are demonstrated in this work, which can be selectively synthesized via a physical vapor deposition method, under precisely controlled growth temperatures. The pristine Raman and non‐linear optical properties of hexagonal GaTe has been systematically explored for the first time; moreover, a novel selected‐area phase transition from hexagonal to monoclinic of GeTe has been achieved via fs‐laser irradiation. This work may pave the way for widely use of 2D GaTe in various fields in future.

Physiological neutral pH drives a gradual lamellar-to-reverse cubic-to-reverse hexagonal phase transition in phytantriol-based nanoparticles.

Dispersed systems of bicontinuous cubic phases, called cubosomes, show a drug release rate faster than those obtained using other liquid-crystalline phases. To minimize side effects associated with the accelerated release of incorporated drugs, compounds may be added in the dispersions to produce systems of slow initial release and then fast release only in the desired action region. This paper addresses the addition of 10.0% (w/w) of decyl betainate chloride (DBC), a cleavable surfactant, into phytantriol/Pluronic-based dispersions to generate lamellar-to-cubic-to-hexagonal phase transitions. Small-angle X-ray scattering (SAXS) was used to analyze the mesophases obtained with the addition of DBC and pH variation. Transmission electron microscopy (TEM) images confirmed the presence of niosomes after the addition of DBC. The niosomes formed in these systems are pH-responsive with lamellar-to-hexosomes transitions at pH ≥ 7.4. The system investigated herein is gastro-resistant presenting potential therapeutic role for controlled release of drugs in neutral or alkaline environments of the organism.

Mechanical property enhancement of one-dimensional nanostructures through defect-mediated strain engineering

The presence of defects is assumed to deteriorate mechanical properties, which has led to immense focus on fabricating perfect nanostructures. Herein, we prove that inherent defects can be employed to enhance the mechanical characteristics of low-dimensional materials via the molecular dynamics (MD) technique: defective nanobelts (NBs) are found to have higher critical stress and Young’s modulus compared with their perfect counterparts — up to 63% and 26%, respectively. Such an improvement has already been observed in nanowires (NWs), and can be justified by the interaction between localized Stacking fault (SF)-induced stress and surface stress. Additionally, twin boundary formation was observed to occur as an alternative stress relaxation mechanism in high densities of SFs, which delays wurtzite to hexagonal phase transition and further gives rise to structural toughness by forming a plateau in the stress–strain curve. Further, a systematic study was performed to investigate the effect of various design parameters, e.g., size, aspect ratio, and random distribution, on the mechanical response of the defective NBs.

Sugar-based bactericides targeting phosphatidylethanolamine-enriched membranes

Anthrax is an infectious disease caused by Bacillus anthracis, a bioterrorism agent that develops resistance to clinically used antibiotics. Therefore, alternative mechanisms of action remain a challenge. Herein, we disclose deoxy glycosides responsible for specific carbohydrate-phospholipid interactions, causing phosphatidylethanolamine lamellar-to-inverted hexagonal phase transition and acting over B. anthracis and Bacillus cereus as potent and selective bactericides. Biological studies of the synthesized compound series differing in the anomeric atom, glycone configuration and deoxygenation pattern show that the latter is indeed a key modulator of efficacy and selectivity. Biomolecular simulations show no tendency to pore formation, whereas differential metabolomics and genomics rule out proteins as targets. Complete bacteria cell death in 10 min and cellular envelope disruption corroborate an effect over lipid polymorphism. Biophysical approaches show monolayer and bilayer reorganization with fast and high permeabilizing activity toward phosphatidylethanolamine membranes. Absence of bacterial resistance further supports this mechanism, triggering innovation on membrane-targeting antimicrobials.

Effects of strain, defects and crystal phase transition in mechanically milled nanocrystalline In2O3 powder

In this paper, we investigated structural, optical and magnetic properties of mechanically milled In2O3 nanoparticles in the cubic bixbyite phase. It was observed that mechanical milling induces an important increase in the density of defects, strain and hexagonal phase transition for cubic In2O3 nanoparticles. Remarkably, room temperature ferromagnetism was observed after mechanical milling. The hexagonal phase of In2O3 nanoparticles (H-In2O3) which is usually obtained in a high-temperature and pressure environment was clearly observed for the sample submitted to higher milling times which considerably affects the magnetic and optical properties. The physical origin of the FM order of mechanically milled In2O3 nanoparticles has been ascribed to the increase in the density of defects and strain.

Size-controlled synthesis of nanocrystalline CdSe thin films by inert gas condensation

Size, shape and structure are considered to have significant influence on various properties of semiconducting nanomaterials. Different properties of these materials can be tailored by controlling the size. Size-controlled CdSe crystallites ranging from ∼ 04 to 95 nm were deposited by inert gas-condensation technique (IGC). In IGC method, by controlling the inert gas pressure in the condensation chamber and the substrate temperature or both, it was possible to produce nanoparticles with desired size. Structure and crystallite size of CdSe thin films were determined from Hall–Williamson method using X-ray diffraction data. The composition of CdSe samples was estimated by X-ray microanalysis. It was confirmed that CdSe thin film with different nanometer range crystallite sizes were synthesized with this technique, depending upon the synthesis conditions. The phase of deposited CdSe thin films also depend upon deposition conditions and cubic to hexagonal phase transition was observed with increase in substrate temperature. The effect of crystallite size on optical and electrical properties of these films was also studied. The crystallite size affects the optical band gap, electrical conductivity and mobility activation of nanocrystalline CdSe thin films. Mobility activation study suggested that there is a quasi-continuous linear distribution of three different trap levels below the conduction band.

Combining MD Simulations and 31P NMR Spectroscopy to Decipher Lamellar to Hexagonal Phase Transition Promoted by Diverse Lipid Types

Transition from lamellar to non-lamellar phase is an important process for biological phenomena such as vesicle fusion or lipid nano-domains organization. Finely understanding such biophysical state of lipids, especially stalk formation, will help to develop more accurate force field and better understand experimental data from NMR spectroscopy. Here, we present extensive work comparing Molecular Dynamic simulations (based on the MARTINI force field) with 31P NMR spectroscopy. This allows benchmarking the simulations on a very detail dataset of temperatures and lipid compositions (based on a variation of PE/PC lipids). We then performed calculations to emulate NMR spectra based on simulations data to directly compare with experimental results. This shed new lights on NMR results and help to explain the formation of stalks and the transition to inverse hexagonal phase. We then assessed both experimentally and computationally how insertion of diverse biologically important lipids - from cholesterol to bacterial virulence factors - may influence the phase transition. These results can then be replaced in a more biological context to understand the influence of lipids on cell membrane structuration.

One-step phase transition and thermal stability improvement of Ge2Sb2Te5 films by erbium-doping

The crystallization behavior of Er-doped Ge2Sb2Te5 phase-change materials is investigated systemically for phase change memory application. It is observed that Er dopants can serve as a center for suppression of face-centered-cubic to hexagonal phase transition of Ge2Sb2Te5 films, leading to a one-step crystallization process. The crystallization temperature, 10-year data retention ability and crystalline resistance of Ge2Sb2Te5 films can be significantly increased. Raman spectra suggest that GeTe component is mainly responsible for the phase transition in Er-doped Ge2Sb2Te5 films.

Observation of two superconducting domes under pressure in tetragonal FeS

We investigate the evolution of superconductivity and structure with pressure for the new superconductor FeS (Tc ≈ 4.5 K), a sulfide counterpart of FeSe. A rapid suppression of Tc and vanishing of superconductivity at 4.0 GPa are observed, followed by a second superconducting dome from 5.0 to 22.3 GPa with a 30% enhancement in maximum Tc. An onsite tetragonal to hexagonal phase transition occurs around 7.0 GPa, followed by a broad pressure range of phase coexistence. The residual deformed tetragonal phase is considered as the source of second superconducting dome. The observation of two superconducting domes in iron-based superconductors poses great challenges for understanding their pairing mechanism.

OH− ions-controlled synthesis and upconversion luminescence properties of NaYF4:Yb3+,Er3+ nanocrystals via oleic acid-assisted hydrothermal process

Yb3+ and Er3+ ions co-doped NaYF4 nanocrystals were synthesized with different amounts of NaOH via oleic acid (OA)-assisted hydrothermal process. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and photoluminescence spectra were used to characterize the products. The result indicated that the introduction of NaOH into the initial reaction solution effectively promoted the cubic (α-) to hexagonal (β-) phase transition of NaYF4:Yb3+,Er3+ nanocrystals, while excessive amount of NaOH favored the formation of α-NaYF4:Yb3+,Er3+ nanocrystals. Besides, with the increase of NaOH amount, the morphologies of β-NaYF4:Yb3+,Er3+ nanocrystals varied from irregular nanobranch to uniform nanorod. Further investigation revealed that the addition of NaOH could facilitate the deprotonation of OA, leading to the formation of oleate (OA−), and meanwhile increased the concentration of OH− ions, inducing consequently the phase transition and morphology evolution of NaYF4:Yb3+,Er3+ nanocrystals. Moreover, the upconversion luminescence properties of NaYF4:Yb3+,Er3+ nanocrystals were systematically investigated. It was found that the upconversion emissions not only depended on the phase and morphology but also were influenced by the surface groups.

Surface Energy Driven Cubic-to-Hexagonal Grain Growth of Ge2Sb2Te5 Thin Film

Phase change memory (PCM) is a promising nonvolatile memory to reform current commercial computing system. Inhibiting face-centered cubic (f-) to hexagonal (h-) phase transition of Ge2Sb2Te5 (GST) thin film is essential for realizing high-density, high-speed, and low-power PCM. Although the atomic configurations of f- and h-lattices of GST alloy and the transition mechanisms have been extensively studied, the real transition process should be more complex than previous explanations, e.g. vacancy-ordering model for f-to-h transition. In this study, dynamic crystallization procedure of GST thin film was directly characterized by in situ heating transmission electron microscopy. We reveal that the equilibrium to h-phase is more like an abnormal grain growth process driven by surface energy anisotropy. More specifically, [0001]-oriented h-grains with the lowest surface energy grow much faster by consuming surrounding small grains, no matter what the crystallographic reconfigurations would be on the frontier grain-growth boundaries. We argue the widely accepted vacancy-ordering mechanism may not be indispensable for the large-scale f-to-h grain growth procedure. The real-time observations in this work contribute to a more comprehensive understanding of the crystallization behavior of GST thin film and can be essential for guiding its optimization to achieve high-performance PCM applications.

Ceramide-C16 Is a Versatile Modulator of Phosphatidylethanolamine Polymorphism

Ceramide-C16 (CerC16) is a sphingolipid associated with several diseases like diabetes, obesity, Parkinson disease, and certain types of cancers. As a consequence, research efforts are devoted to identify the impact of CerC16 on the behavior of membranes, and to understand how it is involved in these diseases. In this work, we investigated the impacts of CerC16 (up to 20 mol %) on the lipid polymorphism of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), using differential scanning calorimetry, and sequential 2H and 31P solid-state nuclear magnetic resonance spectroscopy. A partial phase diagram is proposed. The results indicate that the presence of CerC16 leads to an upshift of the temperature of the gel-to-liquid crystalline (Lβ − Lα) phase transition, leading to a large Lβ/Lα phase coexistence region where gel-phase domains contain ∼35 mol % CerC16. It also leads to a downshift of the temperature of the lamellar-to-inverted hexagonal (L − HII) phase transition of POPE. The opposite influence on the two-phase transitions of POPE brings a three-phase coexistence line when the two transitions overlap. The resulting HII phase can be ceramide enriched, coexisting with a Lα phase, or ceramide depleted, coexisting with a Lβ phase, depending on the CerC16 proportions. The uncommon capability of CerC16 to modulate the membrane fluidity, its curvature propensity, and the membrane interface properties highlights its potential as a versatile messenger in cell membrane events.

Effect of high concentration of colloidal gold nanoparticles on the thermodynamic, optical, and electrical properties of 2, 3, 6, 7, 10, 11-hexabutyloxytryphenylene discotic liquid crystalline material

ABSTRACTIn this work, colloidal gold nanoparticles (GNPs) have been dispersed in a plastic columnar discotic liquid crystal (DLC), namely 2,3,6,7,10,11-hexabutyloxytryphenylene (HAT4). Thermodynamic, dielectric, and optical properties of nanocomposites verify the incorporation of the nanoparticles (NPs) into columnar matrix. Due to addition of GNPs in the triphenylene-based DLC (HAT4), columnar hexagonal to isotropic liquid phase, transition temperature appreciably decreases; however, the crystal to columnar hexagonal phase transition temperature does not change drastically. Interestingly, the ionic conductivity of nanocomposite shows an increment by two orders of magnitude due to doping of GNPs. Dielectric permittivity measured parallel to the column axis in the homeotropic aligned sample has improved. Optical study suggests that band gap has decreased due to dispersion of GNPs.

High pressure behaviour of uranium dicarbide (UC2): Ab-initio study

The structural stability of uranium dicarbide has been examined under hydrostatic compression employing evolutionary structure search algorithm implemented in the universal structure predictor: evolutionary Xtallography (USPEX) code in conjunction with ab-initio electronic band structure calculation method. The ab-initio total energy calculations involved for this purpose have been carried out within both generalized gradient approximations (GGA) and GGA + U approximations. Our calculations under GGA approximation predict the high pressure structural sequence of tetragonal → monoclinic → orthorhombic for this material with transition pressures of ∼8 GPa and 42 GPa, respectively. The same transition sequence is predicted by calculations within GGA + U also with transition pressures placed at ∼24 GPa and ∼50 GPa, respectively. Further, on the basis of comparison of zero pressure equilibrium volume and equation of state with available experimental data, we find that GGA + U approximation with U = 2.5 eV describes this material better than the simple GGA approximation. The theoretically predicted high pressure structural phase transitions are in disagreement with the only high experimental study by Dancausse et al. [J. Alloys. Compd. 191, 309 (1993)] on this compound which reports a tetragonal to hexagonal phase transition at a pressure of ∼17.6 GPa. Interestingly, during lowest enthalpy structure search using USPEX, we do not see any hexagonal phase to be closer to the predicted monoclinic phase even within 0.2 eV/f. unit. More experiments with varying carbon contents in UC2 sample are required to resolve this discrepancy. The existence of these high pressure phases predicted by static lattice calculations has been further substantiated by analyzing the elastic and lattice dynamic stability of these structures in the pressure regimes of their structural stability. Additionally, various thermo-physical quantities such as equilibrium volume, bulk modulus, Debye temperature, thermal expansion coefficient, Gruneisen parameter, and heat capacity at ambient conditions have been determined from these calculations and compared with the available experimental data.

Ionic Switch Induced by a Rectangular-Hexagonal Phase Transition in Benzenammonium Columnar Liquid Crystals.

We demonstrate switching of ionic conductivities in wedge-shaped liquid-crystalline (LC) ammonium salts. A thermoreversible phase transition between the rectangular columnar (Colr) and hexagonal columnar (Colh) phases is used for the switch. The ionic conductivities in the Colh phase are about four orders of magnitude higher than those in the Colr phase. The switching behavior of conductivity can be ascribed to the structural change of assembled ionic channels. X-ray experiments reveal a highly ordered packing of the ions in the Colr phase, which prevents the ion transport.