Samir F. Matar

Synthese et etude des proprietes de conductivite ionique des phases appartenant aux systemes KBiF4 BiF3 et RbBiF4 BiF3

Resume Two solid solutions K1−xBixF2x+1 (0.50 ⩽ x ⩽ 0.70) and Rb1−xBixF2x+1 (0.50 ⩽ x ⩽ 0.60) and two homologous KBi3F10 and RbBi3F10 (x = 0.75) phases have been obtained. Electrical and structural data are correlated.

Influence of the substitution of manganese for iron in the Fe4N lattice on particle formation and magnetic properties

Abstract Mossbauer resonance studies on Mn-substituted Fe4N show that manganese occupies statistically both iron sites in the lattice. Replacement of Fe by Mn modifies the crystal growth process of Fe4N magnetic particles and leads to improvement of the extrinsic magnetic properties, i.e., Hc and σ r σ s , required for magnetic recording applications.

Structural, thermal, and electrical properties of CrSi2

Stoichiometric CrSi2 was prepared by arc melting and compacted by uniaxial hot pressing for property measurements. The crystal structure of CrSi2 was investigated using the powder x-ray diffraction method. From the Rietveld refinement, the lattice parameters were found to be a = 4.427 57 (7) and c = 6.368 04 (11) A, respectively. The thermal expansion measurement revealed an anisotropic expansion in the temperature range from room temperature 800 K with αa = 14.58×10−6/K, αc = 7.51×10−6/K, and αV = 12.05×10−6/K. The volumetric thermal expansion coefficient shows an anomalous decrease in the temperature range of 450–600 K. The measured electrical resistivity ρ and thermoelectric power S have similar trends with a maxima around 550 K. Thermal conductivity measurements show a monotonic decrease with increasing temperature from a room temperature value of 10 W m−1 K−1. The ZT values increase with temperature and have a maximum value of 0.18 in the temperature range studied. An analysis of the electronic band structure is provided.

Investigation of the Young's modulus of TiB needles in situ produced in titanium matrix composite

In situ Ti/TiB composites with different volume fractions of discontinuous TiB reinforcements were produced by powder metallurgy. After compacting Ti� /TiB2 powders by hot unidirectional pressure, heat treatments led to the in situ formation of distinctive needles of TiB, randomly distributed in the titanium matrix. The Young’s modulus of TiB was evaluated using the ASW computation method and experimental Vickers micro-indentation. Three point bend tests were performed on Ti/TiB composites as a function of the TiB volume fraction in order to extract the Young’s modulus of TiB from the elastic properties of the composite. The different values obtained according to these three methods were discussed and compared with the literature. # 2002 Elsevier Science B.V. All rights reserved.

Magnetic particles derived from iron nitride

Due to its high saturation magnetization ( sigma /sub s/(0K) approximately=208 emu/g) and to the ease of obtaining it as a particulate material, Fe/sub 4/N could be a suitable magnetic material for replacing highly reactive metallic iron pigments ( sigma /sub s/(0K) approximately=218 emu/g) in high-density recording. Throughout a nitriding process of properly selected precursors Fe/sub 4/N can be obtained in either of two morphologies: acicular or isotropic. Further protection of the magnetic particles can be ensured through coating of the precursor particles prior to thermal decomposition in NH/sub 3//H/sub 2/ atmosphere or through substitutions with selected elements. The obtained passivated nitrides are characterized by high magnetic properties (H/sub c/ approximately=600 Oe; sigma /sub s/ approximately=150 emu/g) and chemical stability well adapted for use as magnetic pigments for high-density recording. >

Ultrafast Light-Induced Spin-State Trapping Photophysics Investigated in Fe(phen)2(NCS)2 Spin-Crossover Crystal

Few photoactive molecules undergo a complete transformation of physical properties (magnetism, optical absorption, etc.) when irradiated with light. Such phenomena can happen on the time scale of fundamental atomic motions leading to an entirely new state within less than 1 ps following light absorption. Spin crossover (SCO) molecules are prototype systems having the ability to switch between low spin (LS) and high spin (HS) molecular states both at thermal equilibrium and after light irradiation. In the case of Fe(II) (3d(6)) complexes in a nearly octahedral ligand field, the two possible electronic distributions among the 3d split orbitals are S = 0 for the LS diamagnetic state and S = 2 for the HS paramagnetic state. In crystals, such photoexcited states can be long-lived at low temperature, as is the case for the photoinduced HS state of the [Fe(phen)2(NCS)2] SCO compound investigated here. We first show how such bistability between the diamagnetic and paramagnetic states can be characterized at thermal equilibrium or after light irradiation at low temperature. Complementary techniques provide invaluable insights into relationships between changes of electronic states and structural reorganization. But the development of such light-active materials requires the understanding of the basic mechanism following light excitation of molecules, responsible for trapping them into new electronic and structural states. We therefore discuss how we can observe a photomagnetic molecule during switching and catch on the fly electronic and structural molecular changes with ultrafast X-ray and optical absorption spectroscopies. In addition, there is a long debate regarding the mechanism behind the efficiency of such a light-induced process. Recent theoretical works suggest that such speed and efficiency are possible thanks to the instantaneous coupling with the phonons of the final state. We discuss here the first experimental proof of that statement as we observe the instantaneous activation of one key phonon mode precluding any recurrence towards the initial state. Our studies show that the structural molecular reorganization trapping the photoinduced electronic state occurs in two sequential steps: the molecule elongates first (within 170 femtosecond) and bends afterwards. This dynamics is caught via the coherent vibrational energy transfer of the two main structural modes. We discuss the transformation pathway connecting the initial photoexcited state to the final state, which involves several key reaction coordinates. These results show the need to replace the classical single coordinate picture employed so far with a more complex multidimensional energy surface.

First-principles characterisation of new ternary heterodiamond BC2N phases

Abstract In order to discover the existence of new ultrahard materials, we have performed the substitution of some carbon atoms with boron and nitrogen in two different diamond forms: cubic and hexagonal. The number of substituted carbon atoms was fixed in order to obtain BC2N heterodiamond phases isoelectronic with diamond, which is the hardest known material. After the carbon atom replacement, a full geometry relaxation was performed with a first-principle pseudopotential (PP) method to find the fundamental electronic ground state. These hypothetical ternary compounds are expected to be more thermally and chemically stable than diamond and harder than cubic boron nitride. This possibility makes them the most interesting class of compounds that can replace the expensive diamond in many mechanical applications. In the present work, we employ the PP method to predict the mechanical properties of the new BC2N phases. An estimation of the hardness is given with the calculation of the bulk and shear moduli. The relative stability between the phases has also been studied by using both the full potential linearised augmented plane waves (FP-LAPW) and the PP methods. Further, with the help of the FP-LAPW approach, the electronic properties are discussed by means of the density-of-state (DOS) analysis.

Ionic conductivity of fluorite-type fluorides

Abstract A review is given of our last results in the field of fluoride-ion conductors with fluorite-type (or related) structure. The results obtained are discussed. PbSnF 4 is the best anionic conductor so far known thanks to strong cationic polarizability. Some electrochemical applications are proposed.

New fluorine ion conductors with tysonite-type structure

New fluorine fast ion conductors with tysonite-type structure have been obtained within the BiF31bBF2 (B = Ba, Pb) systems: Bi1−yBayF3−y (0.05 ⩽ y ⩽ 0.17) and Bi1−yPbyF3−y (0.075 ⩽ y ⩽ 0.175). The good electrical performances due to high polarizability cations set them among the best so far known fast anion conductors. Correlations between transport properties and structural data have been established.

Search for ultra-hard materials: theoretical characterisation of novel orthorhombic BC2N crystals

Abstract By replacing of the carbon atoms with boron and nitrogen in the crystalline phase of fcc diamond, new candidates for ultra-hard materials have been proposed in the BC 2 N stoichiometry. Their characterisation is here performed using methods based on density functional theory in its local density approximation. Full geometry relaxation of the substituted diamond gives the possibility to discover metastable systems which are isoelectronic with diamond and cubic boron nitride (c-BN). Such a kind of alloys are expected to be thermally and chemically (i.e. versus oxidation) more stable than diamond and harder than c-BN. That is, ternary B–C–N compounds could likely supersede the expensive diamond in various applications. In the present work we also employ first-principles methods to predict the mechanical properties of the achieved three-dimensional BC 2 N phases. Calculations of the cohesive properties, bulk and elastic moduli are presented. Considering the correlation between the hardness and the shear modulus and the values of the bulk moduli, we predict that the phases I and II of BC 2 N could be harder than c-BN. Electronic properties were studied in detail by means of density of states and band structure analysis. The calculation of the C, N and B K ELNES spectra is also shown for the presented phases.