Hexagonal Phase Transition Research Articles

Surface engineering of TiSiO4 nano-coating for high-voltage nickel-rich ternary cathodes: An approach to improve cyclic performance

In recent years, layered ternary transition metal oxides with a high nickel content have gained substantial attention as a potential positive electrode material for lithium-ion batteries. However, its poor rate capability, cyclability, and capacity retention at high working voltages limit its use in commercial lithium-ion batteries. To overcome these challenges, we synthesize a cost-effective wet-chemical deposition of novel TiSiO4 nano-coatings over the surface of LiNi0.83Mn0.06Co0.11O2 (NMC-83). X-ray diffraction and electron microscopy confirm the presence of a coating and verify that the deposition process did not alter the crystallinity and morphology of the NMC-83 particles. In addition, during cycling, it completely protects NMC-83 with enhancements in hexagonal phase transitions between H2 and H3, resulting in excellent cycling stability at upper cut-off voltages. Likewise, when the amount of coated material is equal to 1 wt%, it provides faster ion kinetics and a stable electrode-electrolyte interface. The 1 wt% TiSiO4 coating provides 35 % retention after 250 cycles at a 0.5C rate, while the pristine NMC-83 shows only 13.7 %. From postmortem analysis, the 1 wt% TiSiO4-coated NMC-83 cathode maintains its stable polycrystalline nature compared with pristine NMC-83. Thus, this novel TiSiO4 nanocoating has significant implications for developing high-performance rechargeable batteries for energy applications.

The cytoplasmic tail of myelin protein zero induces morphological changes in lipid membranes

The major myelin protein expressed by the peripheral nervous system Schwann cells is protein zero (P0), which represents 50% of the total protein content in myelin. This 30-kDa integral membrane protein consists of an immunoglobulin (Ig)-like domain, a transmembrane helix, and a 69-residue C-terminal cytoplasmic tail (P0ct). The basic residues in P0ct contribute to the tight packing of myelin lipid bilayers, and alterations in the tail affect how P0 functions as an adhesion molecule necessary for the stability of compact myelin. Several neurodegenerative neuropathies are related to P0, including the more common Charcot-Marie-Tooth disease (CMT) and Dejerine-Sottas syndrome (DSS) as well as rare cases of motor and sensory polyneuropathy. We found that high P0ct concentrations affected the membrane properties of bicelles and induced a lamellar-to-inverted hexagonal phase transition, which caused bicelles to fuse into long, protein-containing filament-like structures. These structures likely reflect the formation of semicrystalline lipid domains with potential relevance for myelination. Not only is P0ct important for stacking lipid membranes, but time-lapse fluorescence microscopy also shows that it might affect membrane properties during myelination. We further describe recombinant production and low-resolution structural characterization of full-length human P0. Our findings shed light on P0ct effects on membrane properties, and with the successful purification of full-length P0, we have new tools to study the role of P0 in myelin formation and maintenance in vitro.

Direct observation of phase transitions in truncated tetrahedral microparticles under quasi-2D confinement

Colloidal crystals are used to understand fundamentals of atomic rearrangements in condensed matter and build complex metamaterials with unique functionalities. Simulations predict a multitude of self-assembled crystal structures from anisotropic colloids, but these shapes have been challenging to fabricate. Here, we use two-photon lithography to fabricate Archimedean truncated tetrahedrons and self-assemble them under quasi-2D confinement. These particles self-assemble into a hexagonal phase under an in-plane gravitational potential. Under additional gravitational potential, the hexagonal phase transitions into a quasi-diamond two-unit basis. In-situ imaging reveal this phase transition is initiated by an out-of-plane rotation of a particle at a crystalline defect and causes a chain reaction of neighboring particle rotations. Our results provide a framework of studying different structures from hard-particle self-assembly and demonstrates the ability to use confinement to induce unusual phases.

Transition Metal-Doped Co-Free Layered Cathode for High-Performance Li-Ion Batteries

Surging demand for high-energy-density cathodes based on Li(Ni x Co y Mn1−x−y )O2 (NCM) and Li(Ni x Co y Al1−x−y )O2 (NCA) for LIBs for EVs has tripled the price of Co in three years. As the price of Co rises rapidly, efforts to remove Co from the cathodes are increasing, but Co, even in minute amounts, plays an essential role in maintaining rate capability and structural stability while reducing cation mixing.1 So the realization of high-performance, Co-free layered cathodes poses a serious technical challenge. Recently, LiNi0.9Mn0.1O2 (NM90) was proposed as an alternative Ni-rich, Co-free cathode material.2 Although the long-term cycling stability of the NM90 is acceptable, its capacity is substantially less than that of a LiNi0.9Co0.05Mn0.05O2 (NCM90) when the NM90 is charged to a standard operating voltage (4.3 V) because the charging ends in the midst of the second hexagonal phase (H2) to the third hexagonal phase (H3) transition. In this study, we investigate to stabilizing the delithiated structure of an NM90 cathode at 4.4 V is proposed, namely, the introduction of a tertiary element into the cathode in combination with the modification of the electrolyte. The introduction of 1mol% Mo into NM90 accompanies increasing in the upper cut-off voltage to 4.4 V (versus Li+/Li) can help realize a half cell, delivering a capacity of 234 mAh g-1 that is comparable to that of NCM with similar Ni contents. The cycling stability of a full cell featuring the Li(Ni0.89Mn0.1Mo0.01)O2 (Mo–NM90) cathode is enhanced with a modified electrolyte (EF91, EMC:FEC = 9:1); retaining 86% of its initial capacity after 1,000 cycles while providing 880 Wh kgcathode −1. Mo–NM90 cathode is able to deliver a high capacity with cycling stability suitable for the long service life for electric vehicles at a reduced material cost, furthering the realization of a commercially viable Co-free cathode for LIBs. Reference s : [1] C. Delmas, I. Saadoune, A. Rougier, J. Power Sources, 44 (1993) 595-602.[2] A. Aishova, G.-T. Park, C. S. Yoon, Y.-K. Sun, Adv. Energy Mater. 10 (2019) 1903179.

Pressure-induced phase transition in cubic Yb2O3 and phase transition enthalpies

The high pressure structural evolution of cubic Yb2O3 has been studied using in situ synchrotron angle dispersive x-ray diffraction in combination with diamond anvil cell techniques up to 44.1 GPa. The XRD measurements revealed an irreversible reconstructive phase transition from cubic to monoclinic Yb2O3 at 11.2 GPa and extending up to 28.1 GPa with ∼8.1% volume collapse and a subsequent reversible displacive transition from monoclinic to hexagonal phase starting at 22.7 GPa. The monoclinic phase coexists with the hexagonal phase up to 44.1 GPa. After pressure releases, the hexagonal Yb2O3 reverts to the monoclinic structure. The second-order Birch–Murnaghan equation of state fit to the pressure–volume data yields a bulk modulus of 201 (4), 187 (6), and 200 (4) GPa for the cubic, monoclinic, and hexagonal phases, respectively. Furthermore, the effects of the hydrostatic pressure state on the diffraction patterns, bulk modulus, and onset transition pressure of Yb2O3 under high pressure have been discussed. It is concluded that the bulk modulus of the cubic Ln2O3 phase increases with decreasing cation radius due to lanthanide contraction. Another important work in this study is the determination of the enthalpies of the cubic to monoclinic and monoclinic to hexagonal phase transitions of Yb2O3 of 37.0 and 17.4 kJ/mol, respectively, based on the basic thermodynamic equations and using the onset transition pressures and corresponding volume changes obtained from high pressure XRD experiments.

A Membrane Biophysics Perspective on the Mechanism of Alcohol Toxicity.

Motivations for understanding the underlying mechanisms of alcohol toxicity range from economical to toxicological and clinical. On the one hand, acute alcohol toxicity limits biofuel yields, and on the other hand, acute alcohol toxicity provides a vital defense mechanism to prevent the spread of disease. Herein the role that stored curvature elastic energy (SCE) in biological membranes might play in alcohol toxicity is discussed, for both short and long-chain alcohols. Structure-toxicity relationships for alcohols ranging from methanol to hexadecanol are collated, and estimates of alcohol toxicity per alcohol molecule in the cell membrane are made. The latter reveal a minimum toxicity value per molecule around butanol before alcohol toxicity per molecule increases to a maximum around decanol and subsequently decreases again. The impact of alcohol molecules on the lamellar to inverse hexagonal phase transition temperature (TH) is then presented and used as a metric to assess the impact of alcohol molecules on SCE. This approach suggests the nonmonotonic relationship between alcohol toxicity and chain length is consistent with SCE being a target of alcohol toxicity. Finally, in vivo evidence for SCE-driven adaptations to alcohol toxicity in the literature are discussed.

Shear stress induced phase transitions of cubic Eu2O3 under non-hydrostatic pressures

Pressure-induced phase transitions in cubic Eu2O3 subjected to non-hydrostatic conditions have been studied by in situ high-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements up to 30.1 and 43.8 GPa, respectively. Both x-ray diffraction and Raman spectroscopy results indicate that the pressure-induced transition routines of cubic Eu2O3 depend on the nature of stress loading. In contrast to our previous high-pressure studies of cubic Eu2O3 under hydrostatic pressure, where cubic Eu2O3 transforms directly into a hexagonal structure, the x-ray diffraction data show that cubic Eu2O3 begins to transform into the monoclinic phase at a non-hydrostatic pressure of about 4.3 GPa, while the monoclinic to hexagonal phase transition is initiated at about 6.4 GPa. These phase transitions have also been confirmed by Raman spectroscopy; the hexagonal phase is stable up to at least 43.8 GPa; and the material decompressed from high pressures is composed of a monoclinic phase, showing that the cubic Eu2O3 to monoclinic phase transition is irreversible due to the constructive nature. Pressure coefficients of Raman peaks and Grüneisen mode parameters of cubic, monoclinic, and hexagonal phases followed under pressure were determined. Furthermore, this study provides evidence for the shear stress-induced cubic to monoclinic phase transition in cubic Eu2O3 and the corresponding mechanism.

Cyclic lipopeptides as membrane fusion inhibitors against SARS-CoV-2: New tricks for old dogs

With the resurgence of the coronavirus pandemic, the repositioning of FDA-approved drugs against coronovirus and finding alternative strategies for antiviral therapy are both important. We previously identified the viral lipid envelope as a potential target for the prevention and treatment of SARS-CoV-2 infection with plant alkaloids (Shekunov et al., 2021). Here, we investigated the effects of eleven cyclic lipopeptides (CLPs), including well-known antifungal and antibacterial compounds, on the liposome fusion triggered by calcium, polyethylene glycol 8000, and a fragment of SARS-CoV-2 fusion peptide (816–827) by calcein release assays. Differential scanning microcalorimetry of the gel-to-liquid-crystalline and lamellar-to-inverted hexagonal phase transitions and confocal fluorescence microscopy demonstrated the relation of the fusion inhibitory effects of CLPs to alterations in lipid packing, membrane curvature stress and domain organization. The antiviral effects of CLPs were evaluated in an in vitro Vero-based cell model, and aculeacin A, anidulafugin, iturin A, and mycosubtilin attenuated the cytopathogenicity of SARS-CoV-2 without specific toxicity.

Packing and elasticity in membrane mechanical properties.

Here we address how membrane properties involving head group size, acyl chain unsaturation, and lipid packing affect their structure and dynamics. Strong out-of-plane proteolipid couplings considered by the flexible surface model (FSM) affect protein function through elastic forces described by the spontaneous curvature and the bending rigidity. We hypothesized that the mechanical properties and the molecular packing resulting from different lipid types are captured by introducing a perdeuterated POPC-d31 probe lipid. Di-monounsaturated phosphatidylcholine and phosphatidylethanolamine (DOPC, DOPE), and different ratios of DOPC/DOPE mixtures were studied using MD simulations combined with experimental solid-state 2H NMR spectroscopy. Residual quadrupolar couplings obtained from de-Paked powder-type spectra yielded order parameter (SCD) profiles for the individual acyl segments giving insights into average membrane structural properties such as area per lipid and bilayer thickness [1]. The corresponding spin-lattice relaxation rates (R1Z) for the resolved peaks followed a theoretical square-law dependence on SCD providing the bending rigidity [2]. Furthermore, MD simulations for the lipid systems were carried out and order parameters were calculated and compared against the NMR-observed SCD values. Utilization of the probe lipid POPC-d31 (1−10 mol%)affected the lamellar to hexagonal phase transition temperature of DOPE lipid systems but did not affect the quadrupolar splittings of DOPE and DOPC systems. Higher bending rigidity was observed for DOPE membranes versus DOPC supporting the idea that the membrane bending rigidity is primarily governed by increased lipid packing and is related to the area-compressibility modulus. Use of a probe lipid to capture membrane mechanical properties of complex lipid systems allow investigation of the relationship of membrane mechanics in vital biological functions.

The Relationship between Electron Transport and Microstructure in Ge2Sb2Te5 Alloy.

Phase-change random-access memory (PCRAM) holds great promise for next-generation information storage applications. As a mature phase change material, Ge2Sb2Te5 alloy (GST) relies on the distinct electrical properties of different states to achieve information storage, but there are relatively few studies on the relationship between electron transport and microstructure. In this work, we found that the first resistance dropping in GST film is related to the increase of carrier concentration, in which the atomic bonding environment changes substantially during the crystallization process. The second resistance dropping is related to the increase of carrier mobility. Besides, during the cubic to the hexagonal phase transition, the nanograins grow significantly from ~50 nm to ~300 nm, which reduces the carrier scattering effect. Our study lays the foundation for precisely controlling the storage states of GST-based PCRAM devices.

Long chain ceramides raise the main phase transition of monounsaturated phospholipids to physiological temperature

Little is known about the molecular mechanisms of ceramide-mediated cellular signaling. We examined the effects of palmitoyl ceramide (C16-ceramide) and stearoyl ceramide (C18-ceramide) on the phase behavior of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) using differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering (SAXS, WAXS). As previously published, the presence of ceramides increased the lamellar gel-to-lamellar liquid crystalline (Lβ–Lα) phase transition temperature of POPC and POPE and decreased the Lα-to-inverted hexagonal (Lα–HII) phase transition temperature of POPE. Interestingly, despite an ~ 30° difference in the main phase transition temperatures of POPC and POPE, the Lβ–Lα phase transition temperatures were very close between POPC/C18-ceramide and POPE/C18-ceramide and were near physiological temperature. A comparison of the results of C16-ceramide in published and our own results with those of C18-ceramide indicates that increase of the carbon chain length of ceramide from 16 to 18 and/or the small difference of ceramide content in the membrane dramatically change the phase transition temperature of POPC and POPE to near physiological temperature. Our results support the idea that ceramide signaling is mediated by the alteration of lipid phase-dependent partitioning of signaling proteins.

The spontaneous polarization in CdS to enhance the piezo-PEC performance via phase transition stress engineering

The built-in electric field caused by piezoelectric polarization has emerged as one of the most effective strategies for photoelectrochemical (PEC) water splitting, but is challenged by low piezoelectric polarization efficiency. Herein, we firstly introduce residual stress into CdS through phase transition stress engineering for improving the spontaneous polarization of CdS with a view to revealing the mechanism of the piezoelectric PEC (piezo-PEC) performance of CdS is advanced by the spontaneous polarization. The spontaneous polarization of CdS caused via phase transition residual stress generated built-in electric field to advance carriers separation. Moreover, the spontaneous polarization of CdS is motived a greater potential under external force than hexagonal wurtzite CdS due to the existence of phase transition residual stress, which effectively improves the piezo-PEC performance of CdS. The photocurrent density of miscibility of cubic sphalerite and hexagonal wurtzite CdS (M−CdS) is 0.61 mA/cm2 at 1.23 V vs RHE, which is almost 1.79 times of hexagonal wurtzite (H-CdS). In addition, the value of M−CdS photocurrent density is reached 2.12 mA/cm2 at 1.23 V vs RHE after ultrasound, which is almost 6.2 times of H-CdS. Our detail work proves that this strategy not only stimulates the piezo-PEC performance of CdS, but also can be extended to other hexagonal wurtzite semiconductor phase transition piezoelectric fields.

Green Silicon Carbide (SiC) and SiC/Graphite Composite Anodes for Lithium-Ion Batteries

Arising from concerns of continuously deteriorating environmental issues, worldwide efforts are dedicated to developing more sophisticated energy storage techniques to replace traditional nonrenewable sources. Given the significant greenhouse gas emissions that occur coincident with current battery production and operation, the introduction of agricultural waste rice hull ash (RHA), a byproduct of biomass combustion, as a source for battery components could facilitate the actual realization of green and sustainable energy storage. Herein, we focus on using RHA to produce SiC with hard carbon (HC) nanocomposites and their mixtures with graphite as Li+ battery anodes.Using a simple method developed in our laboratories to control SiO2:HC ratios, the intrinsic nanoscale mixing of SiO2 and HC in RHA provides a natural starting material for carbothermal reduction producing high purity SiC without the need to add an external carbon source. The optimized SiC/HC nanocomposites contain ≈13 wt. % HC, showing BET surface areas >200 m2/g.We previously reported the SiC/HC nanocomposites derived from RHA exhibit charge/discharge capacities 3× of current graphite anodes at > 950 mAh g-1, accompanied by distinct capacity increases. In particular, SiC can be lithiated to Li1.1-1.4SiC without forming LixSi or LixC. Investigations of the lithiation/delithiation mechanisms using multiple techniques coupled with modeling suggest a cubic (3C-SiC) to hexagonal phase transition (6H-SiC) after long-term cycling, which may explain the origins of extraordinary capacity increments. Moreover, lithiation/delithiation occur without significant volume changes according to XRD and SEM, suggesting that SiC can compete with metallic Si as an anode material as free volumes equivalent to 3× the charged Si are required.Most recently, inspired by the developed concept of adding graphite to improve the performance of silicon-based anodes, we find adding graphite to SiC/HC nanocomposites results in a 3× improvement in capacity increases on cycling, reaching >950 mAh g-1 after 150 cycles. The proposed interactions within the composites will also be addressed in this presentation. It should be noted most silicon/graphite composite anodes use low Si contents of < 30 wt. % to limit the total electrode volume expansion and ensure contacts and conductivities between components, which restrains the practical energy density improvements from using Si in the anodes. In comparison, mixing only 30 wt. % graphite with SiC/HC in the current work results in specific capacities competitive with Si. Furthermore, Si metal can be synthesized via the carbothermal reduction at ≈1900 °C, while battery-grade graphite used today is produced in an environmentally harmful and expensive multistep method that involves heating to ≈3000 °C. The SiC/HC need only be heated to 1450 °C, with hard carbon forming coincidentally providing positive effects on anode performance. In this aspect, directly using SiC/HC or adding a low amount of graphite to further boosts its performance in an apparently more environmentally friendly route to alternative anodes.

Computational Insights into the Role of Cholesterol in Inverted Hexagonal Phase Stabilization and Endosomal Drug Release.

Cholesterol is a major component of many lipid-based drug delivery systems, including cationic lipid nanoparticles. Despite its critical role in the drug release stage, the underlying molecular mechanism by which cholesterol assists in endosomal escape remains unclear. An efficient drug release from the endosome requires endosomal disruption. This disruption is believed to involve a lamellar-to-inverted hexagonal (Lα-HII) phase transition upon fusion of the lipid nanoparticle with the endosomal membrane. We used molecular dynamics simulations to study the structural properties of HII systems composed of an anionic lipid distearoyl phosphatidylserine (DSPS), an ionizable cationic lipid (KC2H), and cholesterol for several hydration levels and molar ratios. This system corresponds to the lipid mixtures in the hypothesized HII structure formed upon fusion and is of interest for the rational design of ionizable cationic lipids, including KC2, for an optimal drug release. Simulations suggest a geometry- and symmetry-driven lipid sorting and cholesterol-DSPS co-location around the water cores. Cholesterol preferentially co-locates with negatively charged saturated DSPS lipids at interstitial angles. The observed cholesterol-DSPS co-location results in an overall increase in the DSPS acyl chains' order parameters, which we propose to assist in stabilizing the HII phase by stretching the DSPS acyl chains for filling the voids formed by three adjacent lipid tubules. Furthermore, a systematic increase in the cholesterol concentration increased the lattice plane spacing and the water core radius but decreased the undulations along the lipid tubule axis. We propose that cholesterol and the degree of saturation/polyunsaturation of the lipid acyl chains, and not the lipid charge, are the main contributors in facilitating the Lα-HII phase transition and stabilizing/destabilizing the formed HII phase, whereas the positive charge of the ionizable cationic lipid promotes the LNP-endosomal membrane adhesion and assists in initiating the fusion process at the local contact area. We also propose that the effect of cholesterol on the HII structure and curvature is the main underlying reason for the well-documented HII stabilization and destabilization at low and high molar concentrations of cholesterol, respectively.

Strong Moiré Excitons in High-Angle Twisted Transition Metal Dichalcogenide Homobilayers with Robust Commensuration.

The burgeoning field of twistronics, which concerns how changing the relative twist angles between two materials creates new optoelectronic properties, offers a novel platform for studying twist-angle dependent excitonic physics. Herein, by surveying a range of hexagonal phase transition metal dichalcogenides (TMD) twisted homobilayers, we find that 21.8 ± 1.0°-twisted () and 27.8 ± 1.0°-twisted () bilayers account for nearly 20% of the total population of twisted bilayers in solution-phase restacked bilayers and can be found also in chemical vapor deposition (CVD) samples. Examining the optical properties associated with these twisted angles, we found that 21.8 ± 1.0° twisted MoS2 bilayers exhibit an intense moiré exciton peak in the photoluminescence (PL) spectra, originating from the refolded Brillouin zones. Our work suggests that commensurately twisted TMD homobilayers with short commensurate wavelengths can have interesting optoelectronic properties that are different from the small twist angle counterparts.

Excess heat capacity and entropy of mixing along the hydroxyapatite-chlorapatite and hydroxyapatite-fluorapatite binaries

The heat capacity, Cp, of synthetic hydroxyapatite [Ca5(PO4)3OH–OH-Ap], as well as of ten compositions along the OH-Ap-chlorapatite (Cl-Ap) join and 12 compositions along the OH-Ap-fluorapatite (F-Ap) join have been measured using relaxation calorimetry (heat capacity option of the Physical Properties Measurement System—PPMS) and differential scanning calorimetry (DSC) in the temperature range of 5–764 K. Apatites along the Cl-OH and F-OH joins were synthesized at 1100 °C and 300 MPa in an internally heated gas pressure vessel via an exchange process between synthetic fluorapatite or chlorapatite crystals (200–500 μm size) and a series of Ca(OH)2-H2O solutions with specific compositions and amounts relative to the starting apatite. The standard third-law entropy of OH-Ap, derived from the low-temperature heat capacity measurements, is S° = 386.3 ± 2.5 J mol−1 K−1, which is ~ 1% lower than that resulting from low-temperature adiabatic calorimetry data on OH-Ap from the 1950’s. The heat capacity of OH-Ap above 298.15 K shows a hump-shaped anomaly centred around 442 K. Based on published structural and calorimetric work, this feature is interpreted to result from a monoclinic to hexagonal phase transition. Super ambient Cp up to this transition can be represented by the polynomial: C_{p}^{{text{OH - Ap}}} {}_{{298K - 442K}}left( {{text{J mol}}^{ - 1} {text{K}}^{- 1}} right) = {1013.7-13735.5T^{{ - 0.5}}} + 2.616718,10^{7} T^{{ - 2}} - 3.551381,10^{9} T^{{ - 3}} .. The DSC data above this transition were combined with heat capacities computed using density functional theory and can be given by the Cp polynomial: C_{p}^{{text{OH - Ap}}} {}_{{ >,442K}}left( {{text{J mol}}^{ - 1} {text{K}}^{- 1}} right) = {877.2-11393.7 T^{ - 0.5}} + {5.452030,10^{7}} ,T^{- {2}} - {1.394125,10^{10}} ,T^{- {3}}. Positive excess heat capacities of mixing, ∆Cpex, in the order of 1–2 J mol−1 K−1, occur in both solid solutions at around 70 K. They are significant at these conditions exceeding the 2σ-uncertainty of the data. This positive ∆Cpex is compensated by a negative ∆Cpex of the same order at around 250 K in both binaries. At higher temperatures (up to 1200 K), ∆Cpex is zero within error for all solid solution members. As a consequence, the calorimetric entropies, Scal, show no deviation from ideal mixing behaviour within a 2σ-uncertainty for both joins. Excess entropies of mixing, ∆Sex, are thus zero for the OH-Ap–F-Ap, as well as for the OH-Ap–Cl-Ap join. The Cp–T behaviour of the OH-Ap endmember is discussed in relation to that of the F- and Cl-endmembers.

Understanding defect-mediated cubic to hexagonal topotactic phase evolution in SrMnO3 thin films and associated magnetic and electronic properties

The coupling between the oxygen stoichiometry and strain-induced lattice distortions has huge bearings on the electronic and magnetic properties of $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ (SMO). In this paper we have observed oxygen stoichiometry-mediated epitaxial tetragonal to nonepitaxial-oriented hexagonal topotactic phase transition of $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ thin film on $\mathrm{SrTi}{\mathrm{O}}_{3}$ (100) substrate. The oxygen stoichiometric hexagonal SMO films are oriented in nature, while the oxygen-deficient epitaxially strained $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ films are tetragonal in nature. It is observed that oxygen vacancy is obligatory to hold on the epitaxial strain in $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin film, while oxygenated stoichiometric cubic $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films are observed to be strain free. The structural phase and stoichiometry are found to govern the nature and mechanism of magnetic interactions in these films. The emergence of unusual weak ferromagnetism in oxygen stoichiometric hexagonal SMO films established a root towards the local structural distortion as confirmed through the Raman spectroscopy and extended x-ray absorption fine structure measurements. This work proves for us the idea that the oxygen deficiency is not the necessary effect of strain; rather, it is the cause of strain for topotactic SMO film.

Structural, thermal and light emission properties of Eu, Sm, Dy, Er and Mn doped CaAl2O4 and SrAl2O4

1 mol% Re2O3/MnO – 99 mol% MAl2O4 (Re = Eu, Sm, Dy, Er and M = Ca and Sr) powder samples were prepared by solid state sintering in the temperature range of 1386–1450 °C and their structural, thermal and photoluminescence properties were compared. X-ray diffraction and Rietveld refinement studies of calcium aluminate samples confirm the formation of monoclinic CaAl2O4 along with a small concentration (1.9–4.6 wt %) of monoclinic CaAl4O7 while the strontium aluminate samples contain monoclinic SrAl2O4 and a small amount (0.7–3.5 wt %) of cubic Sr3Al2O6. The short-range structure of CaAl2O4 consists of CaO6, CaO9 and AlO4 structural units while that of SrAl2O4 contains SrO9 and AlO4 units. The Sr–O bond-lengths (2.32–3.58 Å) are longer than Ca–O bond-lengths (2.04–3.18 Å) and Al–O bond-lengths are of the range 1.35–2.12 Å. Differential scanning calorimetry studies reveal high thermal stability of calcium aluminates, while strontium aluminate samples show reversible monoclinic to - hexagonal structural phase transition at 688 °C during heating cycle. Raman spectra exhibit Al–O vibrational modes. Photoluminescence studies show reddish-orange emission in Eu, Sm and Mn-doped samples, Er doped samples show green emission and Dy-doped samples emit yellow-white light. Calcium aluminate samples show significantly higher luminescence intensity and quantum yield than strontium aluminates.

An Evidence for a Novel Antiviral Mechanism: Modulating Effects of Arg-Glc Maillard Reaction Products on the Phase Transition of Multilamellar Vesicles.

Maillard reaction products (MRPs) of protein, amino acids, and reducing sugars from many foods and aqueous extracts of herbs are found to have various bioactivities, including antiviral effects. A hypothesis was proposed that their antiviral activity is due to the interaction with the cellular membrane. Aiming to estimate the possible actions of MRPs on phospholipid bilayers, the Arg-Glc MRPs were prepared by boiling the pre-mixed solution of arginine and glucose for 60 min at 100°C and then examined at a series of concentrations for their effects on the phase transition of MeDOPE multilamellar vesicles (MLVs), for the first time, by using differential scanning calorimetry (DSC) and temperature-resolved small-angle X-ray scattering (SAXS). Arg-Glc MRPs inhibited the lamellar gel–liquid crystal (Lβ-Lα), lamellar liquid crystal–cubic (Lα-QII), and lamellar liquid crystal–inverted hexagonal (Lα-HII) phase transitions at low concentration (molar ratio of lipid vs. MRPs was 100:1 or 100:2), but promoted all three transitions at medium concentration (100:5). At high concentration (10:1), the MRPs exhibited inhibitory effect again. The fusion peptide from simian immunodeficiency virus (SIV) induces membrane fusion by promoting the formation of a non-lamellar phase, e.g., cubic (QII) phase, and inhibiting the transition to HII. Arg-Glc MRPs, at low concentration, stabilized the lamellar structure of SIV peptide containing lipid bilayers, but facilitated the formation of non-lamellar phases at medium concentration (100:5). The concentration-dependent activity of MRPs upon lipid phase transition indiciates a potential role in modulating some membrane-related biological events, e.g., viral membrane fusion.

Structural features of solid-solid phase transitions and lattice dynamics in U3O8

Triuranium octoxide $({\mathrm{U}}_{3}{\mathrm{O}}_{8})$ undergoes an orthorhombic to hexagonal structural phase transition near ${T}_{s}=305{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, and a separate nonstructural phase transition at ${T}_{c}=210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The later transition has previously been associated with temperature-induced fluctuations in the uranium oxidation state. A discontinuity in the slope of electrical conductivity versus temperature measurement at $210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ has supported this idea. The orthorhombic phase has three crystallographic sites in two distinct oxidation configurations [2 U(V) and 1 U(VI)], whereas the hexagonal phase has one distinct uranium site. High-resolution x-ray diffraction measurements eliminate the possibility of superlattice Bragg reflections to less than 0.2 ${\mathrm{e}}^{\ensuremath{-}}$ scattering power and ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ is not metallic; consequently, the presence of oxidation fluctuations is required for charge balancing. Interestingly, the order-to-disorder transition occurs at a much lower temperature than the structural transition. Using temperature-dependent x-ray diffraction and Raman spectroscopy, we show anisotropic lattice expansion in the in-plane $b$ and $c$ lattice constants. A specific discontinuity in the temperature derivatives of the $b$ and $c$ lattice constants are the first reported structural signatures of the order-to-disorder transition, suggestive of a change in local U--O coordination. Phonon frequencies of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ measured by Raman spectroscopy show significant temperature-dependent dynamics. Redshifting of several modes between 40 and $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ cannot be explained by unit cell expansion alone because the unit cell volume decreases in this region. Instead, we show that phonon frequencies are highly correlated with the anisotropic lattice expansion/contraction along specific crystallographic directions.