Structural transition and pair formation in Fe3O2BO3

We observe for the first time a structural phase transition in the oxyborate Fe3O2BO3 which occurs along three leg ladders present in this material. X-ray diffraction shows that this transition at 283 K is associated with a new phase where atomic displacements occur in alternate directions perpendicular to the axis and within the plane of the ladders. Magnetic data show that these displacements lead to the formation of singlet pairs which dissociate close to the structural transition. Anomalies in the transport properties also occur close to 283 K showing that the structural transition is related to a charge ordering phenomenon in a low dimensional structure.     

The low temperature contributions to β-uranium hydride specific heat

Recent observation of Proton Magnetic Resonance in ferromagnetic β-uranium hydride by Barash et al. led us to perform a new analysis of the specific heat data of Flotow and Obsborne for this compound. In the temperature region 1.469K?T?3.927K, the specific heat C was found to be given by the expression C=29.504T+0.099T³mJK^?1mol^?1 while, for $\text{4.332K?T?15.184K, C=28.464T+0.157T}{}^3\text{+55.906 [T}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{+}\text{4}\text{5}\text{T}{}_0\text{T}\text{1}\text{2}\text{+}\text{4}\text{15}\text{T}{}^2{}_0\text{T}{}^{\mathrm{<mtext>sol?1</mtext><mtext>2</mtext>}}$. exp (?T₀/T) in the same units, with T₀=79.3K. This result indicates that the dispersion relation for magnons in this compound has the form E=k_BT₀+Dk². The large energy gap (k_BT₀) is attributed to the high magneto-crystalline anisotropy arising from the unquenched orbital moment of the uranium ions. To our knowledge this is the first energy gap reported for magnons in an actinide compound.     

An orthogonal spline collocation alternating direction implicit method for second-order hyperbolic problems

On a rectangular region, we consider a linear second-order hyperbolicinitial-boundary value problem involving a mixed derivativeterm, continuous variable coefficients and non-homogeneous Dirichletboundary conditions. In comparison to the alternating directionimplicit Laplace-modified method of Fernandes (1997), we formulateand analyse a new parameter-free alternating direction implicitscheme in which the standard central difference formula is usedfor the time approximation and orthogonal spline collocationis used for the spatial discretization. We establish unconditionalstability of the scheme, and its optimal order in the discretemaximum norm in time and the H¹ norm in space. Numerical experimentsindicate that the new scheme, which has the same order as themethod of Fernandes (1997, Numer. Math., 77, 223–241),is more accurate. We also show that the new scheme is easilygeneralized to the second-order hyperbolic problems on rectangularpolygons. Extensions of the scheme to problems with discontinuouscoefficients, nonlinear problems, and problems with other boundaryconditions are also discussed.     

Analysis of heterocyclic aromatic amines in foods by gas chromatography-mass spectrometry as their tert.-butyldimethylsilyl derivatives

A derivatization method for the analysis of 12 heterocyclic aromatic amines (HAs) in food, by gas chromatography-electron impact mass spectrometry, was developed. The amines are derivatized in a one-step reaction with N-methyl-N-(tert.-butyldimethylsilyl)trifluoroacetamide. The derivatives are characterized by easy-to-interpret mass spectra due to the prominent ion [M-57]+ by loss of a tert.-butyldimethylsilyl group, allowing quantification in the selected-ion monitoring mode at the picogram level. The effect of temperature, time, and reagents on the formation of the derivatives was monitored in detail. Quality parameters were evaluated in the optimum working conditions. This derivatization method is not applicable to the pyridoimidazoles Glu-P-1 and Glu-P-2 and to the beta-carboline harman due to incompletely derivatization. The instability of the imidazolquinoline and imidazoquinoxaline derivatives, requiring their injection on the same working day, is a further drawback. This simple, rapid and accurate derivatization procedure is suitable for routine analysis, as illustrated by the analysis of some common foods.     

Phase transition in malonic acid: An infrared study

Malonic acid is shown to undergo an interesting phase transition at 360 K when the two non-equivalent cyclic hydrogen-bonded dimers present in the low-temperature phase become equivalent.     

Evolution of phase transformations after multiple steps of marforming in Ti-rich Ni-Ti SMA

The phase transformations associated with Shape Memory Effect (SME) can be one step, B19' (martensite) ↔ B2 (austenite), or two/multiple steps which include the intermediate R phase, depending on the thermal and thermomechanical history of the alloy. The transformation temperatures are generally observed above room temperature in Ti-rich Ni-Ti alloys, while those observed in Ni-rich alloys occur below room temperature. The goal of the present work is to investigate the phase transformations evolution in Ti-Rich Ni-Ti SMA (Ni-51 at % Ti) when subjected to two distinct thermal treatments (500°C for 30 minutes in air and 800°C for 300 minutes in vacuum) and subsequently multiple steps of marforming thermomechanical treatments intercalated with thermal treatments (500°C for 30 minutes in air) and subsequent four distinct final thermal treatments (400, 450, 500 or 600°C for 30 minutes in air). Further, the stability of phase transformations in the initial ten thermal cycles of these thermomechanically treated samples is also studied. Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) were used to identify the transformation temperatures and the phases that are present after the thermomechanical treatments.     

Cubic and hexagonal symmetries in LiCl nanoclusters

Cubic and hexagonal symmetries are observed in molecular dynamics simulations of lithium chloride unconstrained nanoclusters, using the Born-Mayer-Huggins (BMH) potential model. Phase changes between the two solid phases, and solid-liquid coexistences, are studied for LiCl clusters with a number of ions ranging from 1000 to 5292. A stability analysis of the clusters and bulk systems, at 0K, is presented, using the BMH and the Michielsen-Woerlee-Graaf (MWG) potential models. The cubic structure from the BMH model is slightly more stable than the hexagonal one for cluster sizes between 1000 and ~10 000 ions. For higher cluster sizes and bulk LiCl the opposite is true. Moreover, at 0K, the bulk cubic phase from the MWG potential is significantly more stable than the hexagonal one. Thus, the BMH potential model seems unrealistic for large clusters and the bulk as far as a comparison with experiment is concerned. Finally, a fairly good correlation of the simulation results is obtained by means of a theoretical model recently reported by us.     

Synthesis and photophysical characterization of heterocyclic dihydrotetracenes and their utility in the fluorescence imaging of HeLa cells

Aza- and oxa-dihydrotetracenes were prepared in one step from 1,4-dicyanodibenzodioxins in good yields. These molecules represent the first examples of heterocyclic dihydrotetracenes with push-pull character. Aza-dihydrotetracene 18H showed a strong red-shifted absorbance maxima at 472?nm, nearly equal in intensity to the parent band at 260?nm, in polar DMSO medium. The fluorescence emission spectrum of aza-dihydrotetracene 18H had an emission maximum at 525?nm with a broad band stretched out as far as 650?nm. The fluorescence emission quantum yield was almost as strong as the known fluorescent standard 1,3-diphenylisobenzofuran. The aza-dihydrotetracenes exhibited in vitro cytotoxicity against the HeLa cell line and were non-toxic against the normal HaCaT cell line. The fluorescence emerging from HeLa cells incubated with aza-dihydrotetracene 18H was very strong and helped detect the cells microscopically using appropriate filters.     

Chiral Stationary Phases Based on Small Molecules: An Update of the Last 17 Years

A literature survey covering the report on Pirkle-type chiral stationary phases (CSPs) from January 2000 to March 2017 is presented in this review. More than 200 CSPs comprising small molecules as chiral selectors covalently bound to the chromatographic support have been reported in this period. The chemical nature of these new chiral selectors, new insights into the development strategies and their applications in liquid chromatography were emphasized.