K Temperature Range (k + temperature_range)

Distribution by Scientific Domains


Selected Abstracts


A Crystalline Phase Transition and Optical Properties in a CoIICuII Oxamato-Bridged Ferrimagnetic Chain

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2005
Cynthia L. M. Pereira
Abstract The compound [CoCu(opba)(DMSO)3] (1) [opba = ortho -phenylenebis(oxamato)] has been synthesized and characterized. Its crystal structure has been analyzed by X-ray diffraction techniques at 100 and 298 K. A structural phase-transition has been detected at around 150 K. An orthorhombic crystalline system is found at both temperatures, with very similar unit-cell dimensions. At room temperature 1 crystallizes in the Pnam space group (, -1 phase), with a = 7.6712(2), b = 14.8003(3), c = 21.0028(5) Å, and Z = 4, whereas at low temperature it crystallizes in the Pna21 space group (, -1 phase), with a = 7.3530(2), b = 14.5928(4), c = 21.0510(7) Å, and Z = 4. Both crystalline phases consist of linearly ordered bimetallic chains with the [Cu(opba)]2, units tied by CoII ions to form a one-dimensional system. The DMSO molecules in , -1, which are coordinated to either CuII or CoII, are disordered. At low temperature, a small reorganization of the CuII and CoII environments is observed. The origin of this phase transition, which is completely reversible, is the modification of the crystalline packing with the temperature. Linear birefringence measurements were done on single crystals in the 100,300 K temperature range. Around 150 K, the linear birefringence curve shows an inflexion that is interpreted as being related to the conversion of ,-1 into , -1. Both dc and ac magnetic measurements were performed on the polycrystalline sample. The results reveal a one-dimensional ferrimagnetic behavior. Single crystal optical characterization at room temperature shows that 1 presents a very strong dichroism superposed on the linear birefringence. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]


Thermal Stability of W- xRe/TiC/SiC Systems,=,0, 5 and 25 at % Re) at High Temperature,

ADVANCED ENGINEERING MATERIALS, Issue 5 2009
Jerome Roger
To ensure the integrity and sustainability of high-temperature applications, the development of protective materials is actually one of the major challenge in materials science. In our study, we examined the effect of a TiC film interlayered between W- xRe (x,=,0, 5 or 25 at % Re) and SiC. The protection given by TiC layer was considered in the 1573,1873,K temperature range and for TiC thicknesses up to about 100,,m. [source]


Raman scattering studies of the magnetic ordering in hexagonal HoMnO3 thin films

JOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2010
Nguyen Thi Minh Hien
Abstract We present the results of the temperature dependence of the Raman spectra of hexagonal HoMnO3 thin films in the 13,300 K temperature range. The films were grown on Pt(111)//Al2O3 (0001) substrates using the laser ablation method. In the HoMnO3 thin films, we initially observedseveral broad Raman peaks at ,510, 760, 955, 1120, and 1410 cm,1. These broad Raman peaks display an anomalous behavior near the magnetic transition temperature, and the intensity difference of the Raman spectra at different temperatures shows several pairs of negative and positive peaks as the temperature is lowered below the Néel temperature. Our analyses indicate that all the broad peaks are correlated with magnetic ordering, and we have assigned the origin of all the broad peaks. Purely on the basis of the Raman analysis, we have deduced the Néel temperature and the spin exchange integrals of HoMnO3 thin films. We also investigated the effects of the growth condition on the strongest broad peak at ,760 cm,1, which is related with pure magnetic ordering. This result indicates that the oxygen defect in the HoMnO3 sample has negligible effect on magnetic ordering. Copyright © 2009 John Wiley & Sons, Ltd. [source]


Temperature-dependent Raman scattering studies of the geometrically frustrated pyrochlores Dy2Ti2O7, Gd2Ti2O7 and Er2Ti2O7

JOURNAL OF RAMAN SPECTROSCOPY, Issue 4 2008
M. Ma, czka
Abstract The temperature-dependent Raman studies of A2Ti2O7(A = Dy, Er, Gd) were performed on single crystals and polycrystalline samples in the 4.2,295 K temperature range. The Raman spectra showed softening of the majority of phonon modes upon cooling in the whole temperature range studied and large decrease of linewidths. These changes have been analyzed in terms of strong third-order phonon,phonon anharmonic interactions. Moreover, the 312 and 330 cm,1 modes of Er2Ti2O7(Gd2Ti2O7) showed hardening upon cooling down to about 130 K (100 K) and then anomalous softening below this temperature. The observed anomalous behavior of the Raman modes indicates that some important changes occur in these materials at low temperatures. However, the origin of this behavior is still not clear. Copyright © 2008 John Wiley & Sons, Ltd. [source]


Nitroxide spin probe study of probe size, hydrogen bonding and polymer matrix rigidity effects on poly(acrylic acid)/poly(ethylene oxide) complexes

MAGNETIC RESONANCE IN CHEMISTRY, Issue 7 2003
Li Tan
Abstract An electron spin resonance (ESR) spin probe study was performed on 1 : 1 by weight poly(acrylic acid) (PAA)/poly(ethylene oxide) (PEO) complex over the 100,450 K temperature range with a series of tetramethylpiperidyloxy-based spin probes. Measurements of the parameters T5mT, Ta and Td demonstrated the effects of probe size and the strength of hydrogen bonding. The probes in the series Tempone, Tempo, Tempol and Tamine (respectively 4-oxo-, unsubstituted, 4-hydroxy- and 4-amino-2,2,6,6,-tetramethylpiperidine -1-oxyl) displayed noticeable increases in the hydrogen-bonding effect, as indicated by Ta and Td. These increases correlated with increasing hydrogen bond acceptor strength. On the other hand, as the probe size became larger, T5mT gradually increased due to the free volume decrease. These effects were analyzed using the established theoretical relationship of T5mT to probe volume expressed by f. Meanwhile, in order to investigate the effect of polymer matrix rigidity, a similar study was performed with a nitroxide spin probe, 2,2,6,6-tetramethyl-1-piperidine-1-oxyl (Tempo), on PAA/PEO complexes of different weight compositions. The quantitative fast motion fraction in the composite ESR spectrum was calculated. The influence of changes in the composition of PAA on the molecular mobility was characterized by changes of the spectral parameters and ,c. The molecular mobility was shown to diminish with increasing content of PAA in PAA/PEO blends duo to the restriction of the polymer matrix rigidity increase. Copyright © 2003 John Wiley & Sons, Ltd. [source]


Metallic bundles of single-wall carbon nanotubes probed by electron spin resonance

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007
F. Simon
Abstract C59N magnetic fullerenes inside single-wall carbon nanotubes (SWCNTs) are used to probe the density of states (DOS) on the host tubes using electron spin resonance (ESR). The C59N radicals are separated by C60 fullerenes to prevent dimerization and C59N,C60 heterodimers are formed at low temperatures. The electron spin,lattice relaxation time, T1, of the heterodimers is deduced from the homogeneous ESR line-width. The analysis is supported by saturation ESR studies. The inverse of the heterodimer T1 follows a linear behavior in the 20,300 K temperature range, the so-called Korringa law, evidencing a metallic DOS on all tubes in a bundle. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


The thermostimulated exoelectron emission of NaF:U,Me compounds after electron beam irradiation

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2007
M. M. Kidibaev
Abstract The thermostimulated exoelectron emission (TSEE) of NaF:(U,Me) compounds after electron beam irradiation in the 300,800 K temperature range has been investigated. For NaF:(U,Me) bulk, fiber and nanocrystals several regular peaks of TSEE have been found. Additional effects with the sign of thermostimulated explosive electron emission have been found too. The nature of TSEE process is discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Mass spectrometric determination of the dissociation energy of Mn2F6(g)

RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 16 2002
Andrea Ciccioli
The gaseous Mn2F6 molecule has been identified for the first time in the vapor produced by MnF3 vaporization and the MnF3(g) dimerization equilibrium studied by the Knudsen cell mass spectrometry technique in the 884,1015,K temperature range. The experimental vapor pressure data were treated by the second- and third-law procedures, and the enthalpy of Mn2F6(g) dissociation has been determined as: . Thermodynamic functions of gaseous MnF3 were calculated from geometrical and vibrational parameters taken from the literature. For gaseous Mn2F6, they were evaluated by comparison with molecular parameters of Co2F6(g). Copyright © 2002 John Wiley & Sons, Ltd. [source]


Following an isosymmetric phase transition by changes in bond lengths and anisotropic displacement parameters: the case of meta -carboxyphenylammonium phosphite

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2009
El-Eulmi Bendeif
Crystal structure studies in the 100,345,K temperature range were performed to relate the molecular structure changes of meta -carboxyphenylammonium phosphite (m -CPAMP) to its first-order phase transition at Tc = 246,(2),K. Thermal displacement parameters and most bond distances show an abrupt jump at the transition. Such a structural change is related to collective effects leading to competition between intra- and intermolecular interactions. [source]


An ordered low-temperature phase of barium nitro­prusside trihydrate studied by neutron diffraction

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2003
G. Chevrier
Crystals of barium penta­cyano­nitro­syl­ferrate trihydrate (barium nitro­prusside trihydrate), Ba[Fe(CN)5(NO)]·3H2O, have been studied by neutron diffraction in order to ex­amine the structural behaviour of the compound in the 20,120,K temperature range and to determine the structure at 105,K. The results show the existence of a new crystal phase of the compound at 80,K (with a duplicated a parameter), which still exists at 20,K. The crystal structure at 105,K shows a rearrangement of the water mol­ecules, which results in an ordered structure with P1 symmetry. Two of the four independent nitro­prusside cations are rotated by 4.5° around the [100] direction. [source]


Unexpected Conformational Properties of 1-Trifluoromethyl-1-Silacyclohexane, C5H10SiHCF3: Gas Electron Diffraction, Low-Temperature NMR Spectropic Studies, and Quantum Chemical Calculations,

CHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2007
Georgiy
Abstract The molecular structure of axial and equatorial conformers of 1-trifluoromethyl-1-silacyclohexane, (C5H10SiHCF3), as well as the thermodynamic equilibrium between these species was investigated by means of gas electron diffraction (GED), dynamic nuclear magnetic resonance (DNMR) spectroscopy, and quantum chemical calculations (B3LYP, MP2, and CBS-QB3). According to GED, the compound exists as a mixture of two Cs symmetry conformers possessing the chair conformation of the six-membered ring and differing in the axial or equatorial position of the CF3 group (axial=58(12) mol,%/equatorial=42(12) mol,%) at T=293,K. This result is in a good agreement with the theoretical prediction. This is, however, in sharp contrast to the conformational properties of the cyclohexane analogue. The main structural feature for both conformers is the unusually long exocyclic bond length SiC 1.934(10),Å. A low-temperature 19F,NMR experiment results in an axial/equatorial ratio of 17(2) mol,%:83(2) mol,% at 113,K and a ,G,, of 5.5(2),kcal,mol,1. CBS-QB3 calculations in the gas-phase and solvation effect calculations using the PCM(B3LYP/6-311G*) and IPCM(B3LYP/6-311G*) models were applied to estimate the axial/equatorial ratio in the 100,300,K temperature range, which showed excellent agreement with the experimental results. The minimum energy pathways for the chair-to-chair inversion of trifluoromethylsilacyclohexane and methylsilacyclohexane were also calculated using the STQN(Path) method. [source]