Identification of first- and second-order magnetic phase transitions in ferromagnetic perovskites

A criterion used for the determination of first- and second-order magnetic phase transitions from purely magnetic methods is applied to manganese perovskites of formula La_2/3(Ca_1−xSr_x)_1/3MnO₃. A crossover from first- to second-order character at a tolerance factor t=0.92 is found, which also brings about several variations in other physical properties. At t=0.92 a change from orthorhombic to rhombohedral symmetry also takes place. The impossibility of establishing static cooperative Jahn–Teller distortions in the rhombohedral symmetry is suggested as being responsible for the observed behaviour.     

Structural transformation induced by magnetic field and "colossal-like" magnetoresistance response above 313 K in MnAs

MnAs exhibits a first-order phase transition from a ferromagnetic, high-spin metal hexagonal phase to a paramagnetic, lower-spin insulator orthorhombic phase at T(C)=313 K. Here, we report the results of neutron diffraction experiments showing that an external magnetic field, B, stabilizes the hexagonal phase above T(C). The phase transformation is reversible and constitutes the first demonstration of a bond-breaking transition induced by a magnetic field. The field-induced phase transition is accompanied by an enhanced magnetoresistance of about 17% at 310 K. The phenomenon appears to be similar to that of the colossal magnetoresistance response observed in the Mn [corrected] perovskite family.     

Evaluation of a generalized regression artificial neural network for extending cadmium’s working calibration range in graphite furnace atomic absorption spectrometry

A generalized regression artificial neural network (GRANN) was developed and evaluated for modeling cadmiums nonlinear calibration curve in order to extend its upper concentration limit from 4.0 g L^–1 up to 22.0 g L^–1. This type of neural network presents important advantages over the more popular backpropagation counterpart which are worth exploiting in analytical applications, namely, (1) a smaller number of variables have to be optimized, with the subsequent reduction in development hassle; and, (2) shorter development times, thanks to the fact that the adjustment of the weights (the artificial synapses) is a non-iterative, one-pass process. A backpropagation artificial neural network (BPANN), a second-order polynomial, and some less frequently employed polynomial and exponential functions (e.g., Gaussian, Lorentzian, and Boltzmann), were also evaluated for comparison purposes. The quality of the fit of the various models, assessed by calculating the root mean square of the percentage deviations, was as follows: GRANN > Boltzmann > second-order polynomial > BPANN > Gauss > Lorentz. The accuracy and precision of the models were further estimated through the determination of cadmium in the certified reference material Trace Metals in Drinking Water (High Purity Standards, Lot No. 490915), which has a cadmium certified concentration (12.00±0.06 g L^–1) that lies in the nonlinear regime of the calibration curve. Only the models generated by the GRANN and BPANN accurately predicted the concentrations of a series of solutions, prepared by serial dilution of the CRM, with cadmium concentrations below and above the maximum linear calibration limit (4.0 g L^–1). Extension of the working range by using the proposed methodology represents an attractive alternative from the analytical point of view, since it results in less specimen manipulation and consequently reduced contamination risks without compromising either the accuracy or the precision of the analyses. The implementation of artificial neural networks also helps to reduce the trial-and-error task of looking for the right mathematical model from among the many possibilities currently available in the various scientific and statistic software packages.     

Simple variable-volume injector for flow-injection systems

A simple and versatile variable-volume injector is described for the introduction of samples in flow-injection systems. The device delivers precisely and reproducibly (usually r.s.d. < 0.9%) 4–62-μl volumes of sample and is simple to assemble. Examples of applications include the generation of a family of calibration graphs of varying sensitivity covering an extended concentration range for the analysis of real samples. The construction of a calibration graph from a single standard solution is also possible. This is useful in the routine analysis of samples with widely varying analyte concentrations.     

Ab initio analysis of some fluoride and oxide structures doped with Pr and Yb

Doping effects are analyzed by means of the ab initio perturbed ion (aiPI) method via substitution of the fluorides and oxides of zirconium, hafnium, and thorium. Lattice relaxation is simulated through the calculation of vibrational breathing modes and substitutional effects on the compound are thereby analyzed. In addition, the band gaps (in the pure species) and impurity levels (where substitutional ions Pr⁺⁴ and Yb⁺³ are considered in the doped species) of the fluorides are estimated via the transition energy calculated at the aiPI‐optimized geometry of the pure and doped crystal clusters by means of the configuration interaction with single excitations (CIS) method that accounts for electronic correlation. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Detection of several electronically metastable atomic states by gas phase EPR

Several atomic metastable species including N(²D_{$\text{5}\text{2}$}), N(²D_{$\text{3}\text{2}$}), N(²P_{$\text{3}\text{2}$}), O(¹D₂), Ne(³P₂), Ar(³P₂), Kr(³P₂ and Xe(³P₂) have been detected by gas phase electron paramagnetic resonance (EPR) in a discharge-flow system. Spectra and absolute concentration are presented.     

Evolution of carrier and optical phonon distribution in photoexcited semiconductors

A study of the time evolution of a coupled photoexcited electron-LO phonon system is presented. Zubarev's method to deal with irreversible relaxation processes is used to derive the dynamic equation for the distribution functions. We report results which allow to remove some inconsistencies and controversies concerning early models proposed for the interpretation of measurements of the ratio Stokes to anti-Stokes intensities in the LO-phonon Raman spectrum.     

Origin of the glassy magnetic behavior of the phase segregated state of the perovskites

In this Letter we demonstrate that the phase segregated state observed in many rare-earth perovskites constitutes a sort of self-generated assembly of magnetic clusters in which magnetic interaction introduces collectivity among them. We show that the observed glassy behavior (memory, aging, etc.) can be perfectly understood taking into account only the intercluster interactions. We address the fundamental question about whether this state constitutes classical spin glass or if, on the other hand, a new universality class must be defined.     

Tetra­allyl (2,2′‐bipyridyl)‐rac‐3,3,7,7‐tetra­methyl‐trans‐5‐palladatricyclo[4.1.0.02,4]­heptane‐1,2,4,6‐tetra­carboxyl­ate above and below the phase‐transition temperature of 194 K

The crystal structure of the title compound, (2,2′‐bipyridyl‐κ²N,N′)(tetra­allyl 3,3,3′,3′‐tetra­methyl‐1,1′‐bi­cyclo­propane‐1,1′,2,2′‐tetra­carboxyl­ato‐κ²C²,C^2′)­palladium(II), [Pd(C₂₆H₃₂­O₈)(C₁₀­H₈­N₂)], is disordered above 194 K. A doubling of the unit cell is observed on cooling. The structure at 143 K contains two ordered mol­ecules related by a pseudo‐translation vector of approximately (0.44,0.00,0.50) or a pseudo‐inversion center at approximately (0.22,0.00,0.25). Weak intermolecular C—H?O interactions are enhanced in the low‐temperature structure.     

Preparation and characterization of poly(2‐hydroxyethylme thacrylate)‐b‐P(N‐phenylmaleimide)‐ZnO and poly(2‐hydroxy ethylmethacrylate)‐b‐P(styrene)‐ ZnO micro/nanostructures: micro/nanocomposite particles with core‐shell morphology

The present work reports the incorporation of the ZnO doped diblock copolymer matrix and its conversion into a self‐assembled structure. The diblock P(HEMA)₈₀‐b‐P(N‐PhMI)₂₀ and P(HEMA)₉₀‐b‐P(St)₁₀ copolymers consist of a majority (HEMA) and minority (N‐PhMI or St) block. The copolymers were synthesized with a block repeat unit ratio by atom‐transfer radical polymerization (ATRP) using a poly(2‐hydroxyethylmethacrylate)‐Cl/CuBr/bipyridine initiating system. The P(HEMA)‐Cl was prepared by reverse ATRP¹. The average theoretical number molecular weight (M_n,th) was calculated from the feed capacity. The composite of the inorganic nanoparticles was achieved at room temperature in the liquid phase, using ZnCl₂ precursor dopant and wet chemical processing to convert to ZnO nanoparticle films. Thermal characterization was performed using differential scanning calorimetry (DSC) and thermogravimetry (TG). The proton/area relationship confirmed the block copolymer compositions calculated by elemental analysis, consisting of a majority and minority blocks. Morphology properties of the polymer samples were investigated by scanning electron microscopy (SEM). The microphotographs of the film's surfaces show that the film's upper surfaces were generally smooth with ordered structure morphology. FT‐IR spectroscopy confirmed the association of the ZnCl₂ precursor with the majority block and the formation of ZnO, the white SEM showed the morphology of ZnO nanoparticles' films when the surface relief changes principally due to surface loss rather than its orientation. Copyright © 2009 John Wiley & Sons, Ltd.