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Mesomorphic structures and order‐disorder transitions in comb copolymer-like systems obtained by hydrogen bonding of noncharged or charged homopolymers with amphiphiles are discussed. The amphiphiles consist of attractive and repulsive moieties and the emphasis will be on the situation without an additional solvent phase. Poly(4-vinyl pyridine), i.e. P4VP, hydrogen bonded to stoichiometric amounts of pentadecylphenol (PDP) is one of the characteristic examples. Small angle X-ray scattering and dynamic rheology demonstrate that this system forms a disordered melt at temperatures above the order-disorder temperature (TODT), whereas for TB TODT a lamellar microphase separated structure is observed. Further cooling renders side chain crystallization. This phase behavior can be tailored by adjusting the attraction and the repulsion between the amphiphile and the polymer. Repulsion can be increased by making the alkyl chain longer or by adding charges to the backbone. Both lead in general to a higher TODT. Order-disorder transitions can occur even in polyelectrolyte-amphiphile complexes as is demonstrated by first fully complexing P4VP with toluene sulphonic acid (TSA) to form a polymeric salt P4VP(TSA)1.0 which is further hydrogen bonded to PDP. The presented results offer guidelines to apply the concepts to other polymers as well, with an example mentioned in relation to polyamide 6. © 1999 Elsevier Science B.V. All rights reserved.

The electrical resistivity of Fe17wt%Si alloy was measured within the solid and the liquid phases up to 5 GPa in 200 ton and 1000 ton cubic anvil presses. Special attention was paid in the investigation to the challenges in resistance measurements in connection with the contaminations originating from the electrode materials and also the dominant role of the electrode resistances in the final results. The current results on Fe17Si alloys yielded insights to the manifestations of the magnetic, order-disorder and melting transitions on the electrical resistivity at high P, T. A drop in electrical resistivity in Fe17Si was observed at the melting boundary at high pressures up to 5 GPa as reported by Baum et al. (1967) at 1 atm. The liquid resistivity results from the present study provide insight on the effect of Si on the electrical resistivity of Fe-Si alloy specifically that the difference in resistivity between Fe and Fe17Si decreased with increasing pressure. The model of saturation resistivity (Mooij, 1973) describes saturation of electron-scattering where the electron mean free path approaches the interatomic distance (Ioffe-Regel criteria); the temperature coefficient of resistivity (TCR) has been shown to change sign due to compositionally-induced changes to the mean free path and interatomic distance. The results of the present study show that pressure can also provide a mechanism for resistivity saturation and change of TCR sign most likely due to reduction in interatomic distance. The present electrical resistivity results of Fe17Si were interpreted in terms of the resistivity saturation model in order to estimate the electrical resistivity of the Earth’s outer core. This yielded a range of 9.0×10-7Ωm to 9.4×10-7Ωm which is in agreement with the very recently reported studies on the electrical resistivity of the Earth’s core. Using Wiedemann-Franz law, electrical thermal conductivities were calculated to be 103 Wm−1K−1 to 109Wm−1K−1.

We have studied, in solution and in the bulk, a model system consisting of noncrystalline poly(propylene oxide) (PPO) chains end-functionalized with complementary stickers: thymine derivative (Thy) and diaminotrozaine (DAT). This system combines weak (Thy/Thy and DAT/DAT self-associations) and strong (Thy/DAT complementary association) hydrogen bonding, aromaticity of the stickers, strong repulsion between the polar stickers and the PPO spacers, and very different tendency towards crystallization for Thy and DAT. Homoditopic and heteroditopic compounds were synthesized. In solution, the solvent influences the association constants and the organizations observed. In the bulk, all materials show a relatively complex behavior, as three different phenomena concur and compete: hydrogen bonding between the stickers, phase segregation between the polymer backbone (less polar) and the stickers (more polar), and crystallization of thymines into microdomains. These phenomena impact the mesoscopic organization of the materials and thus their mechanical properties. We present the different organizations observed in solution and in the bulk and discuss the new insight our studies bring for understanding the role played respectively by hydrogen bonding, phase segregation, solvation and crystallization.; Nous avons étudié, en solution et en masse, un système modèle qui consiste en des chaînes non cristallines de poly(oxyde de propylène) (PPO) fonctionnalisées en bout de chaîne par des motifs complémentaires : un dérivé de thymine (Thy) et la diaminotriazine (DAT). Ce système combine des liaisons hydrogène faibles (auto-associations Thy-Thy et DAT-DAT) et fortes (association complémentaire Thy-DAT), l'aromaticité des motifs collants, une forte répulsion entre les motifs collants polaires et les espaceurs peu polaires PPO, et des tendances à la cristallisation très différentes pour Thy et DAT. Des composés homoditopiques et hétéroditopiques ont été synthétisés. En solution, le solvant influence la valeur des constantes d'association et les organisations observées. En masse, un comportement relativement complexe est observé, avec une compétition entre les liaisons hydrogène entre les motifs collants, la ségrégation de phase entre les chaînes espaceurs (peu polaires) et les motifs collants (assez polaires), et la cristallisation des thymines en microdomaines. Ces phénomènes influencent l'organisation mésoscopique de ces matériaux et donc leurs propriétés mécaniques. Nous avons présenté les différentes organisations observées en solution et en masse et avons discuté les idées que notre étude apporte pour comprendre les rôles joués respectivement par les liaisons hydrogènes, la ségrégation de phase, la solvatation et la cristallisation.

We have studied, in solution and in the bulk, a model system consisting of noncrystalline poly(propylene oxide) (PPO) chains end-functionalized with complementary stickers: thymine derivative (Thy) and diaminotrozaine (DAT). This system combines weak (Thy/Thy and DAT/DAT self-associations) and strong (Thy/DAT complementary association) hydrogen bonding, aromaticity of the stickers, strong repulsion between the polar stickers and the PPO spacers, and very different tendency towards crystallization for Thy and DAT. Homoditopic and heteroditopic compounds were synthesized. In solution, the solvent influences the association constants and the organizations observed. In the bulk, all materials show a relatively complex behavior, as three different phenomena concur and compete: hydrogen bonding between the stickers, phase segregation between the polymer backbone (less polar) and the stickers (more polar), and crystallization of thymines into microdomains. These phenomena impact the mesoscopic organization of the materials and thus their mechanical properties. We present the different organizations observed in solution and in the bulk and discuss the new insight our studies bring for understanding the role played respectively by hydrogen bonding, phase segregation, solvation and crystallization.; Nous avons étudié, en solution et en masse, un système modèle qui consiste en des chaînes non cristallines de poly(oxyde de propylène) (PPO) fonctionnalisées en bout de chaîne par des motifs complémentaires : un dérivé de thymine (Thy) et la diaminotriazine (DAT). Ce système combine des liaisons hydrogène faibles (auto-associations Thy-Thy et DAT-DAT) et fortes (association complémentaire Thy-DAT), l'aromaticité des motifs collants, une forte répulsion entre les motifs collants polaires et les espaceurs peu polaires PPO, et des tendances à la cristallisation très différentes pour Thy et DAT. Des composés homoditopiques et hétéroditopiques ont été synthétisés. En solution, le solvant influence la valeur des constantes d'association et les organisations observées. En masse, un comportement relativement complexe est observé, avec une compétition entre les liaisons hydrogène entre les motifs collants, la ségrégation de phase entre les chaînes espaceurs (peu polaires) et les motifs collants (assez polaires), et la cristallisation des thymines en microdomaines. Ces phénomènes influencent l'organisation mésoscopique de ces matériaux et donc leurs propriétés mécaniques. Nous avons présenté les différentes organisations observées en solution et en masse et avons discuté les idées que notre étude apporte pour comprendre les rôles joués respectivement par les liaisons hydrogènes, la ségrégation de phase, la solvatation et la cristallisation.

The evolution of the local Jahn-Teller distortion across the LaMnO3+Delta phase diagram was obtained using the perturbed angular correlation local probe technique. We found that upon doping, the local distortion decreases continuously with increasing doping and that no fully Jahn-Teller distorted Mn3+O6 octahedra are observed within the orthorhombic insulating phase. A local single-phase scenario is established for the orbital disordered orthorhombic crystallographic structure. We also show that the continuous weakening of the Jahn-Teller distortions is not limited to a single-phase environment and occurs in a similar manner within an undistorted rhombohedric matrix upon lowering the temperature.

A-b-(B-b-A)(n)-b-B multiblock copolymers composed of short middle diblock units (B-b-A)(n) and two long A-and B-outer blocks were successfully prepared. The multiblock copolymers consist of polystyrene (S) and poly(p-hydroxystyrene) (pHS) and were prepared through sequential anionic polymerization of styrene and 4-tert-butoxystyrene, followed by hydrolysis of the tert-butoxy group. A hexablock and an octablock copolymer with a low overall polydispersity were synthesized. The self-assembled structures were investigated using Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM). Due to the smaller number of inner blocks and their relatively small molar mass, a single periodic lamellar morphology was observed for the hexablock copolymer while a lamellar-in-lamellar morphology with two thin layers within successive thick layers was observed for the octablock copolymer with larger middle diblock units. These observations are in excellent agreement with existing theories.

Sintered bodies of BaM_2O_4 (M : La, Nd, Sm, Gd, Ho or Y) with CaFe_2O_4 related structure were synthesized by sintering BaCO_3 M_2O_3 mixtures. The temperature de pendence of oxygen ionic conductivity and the relation ship between the ionic radius of M^ ions in B sites and the crystal structure of BaM_2O_4 were investigated. The ionic conductivity type in BaM_2O_4 was divided into two groups; (a) BaLa_2O_4, BaNd_2O_4 and BaSm_2O_4 with ord er disorder transition and (b) BaGd_2O_4, BaHO_2O_4 and BaY_2O_4 without the transition. There were vacant spaces in the crystal structure of BaNd_2O_4 and BaSm_2O_4 with order disorder transition due to the dis tortion in the a axis direction of an oxygen octahedron. By assuming that the vacant space is an oxygen vacan cy, the ionic conductivity in BaLa_2O_4, BaNd_2O_4 and BaSln_2O_4 has been explained with a model of the order disorder transition for oxygen vacancies without sig nificant shifting of cations in the crystal lattice., rights: 社団法人日本セラミックス協会rights: 本文データは学協会の許諾に基づきCiNiiから複製したものであるrelation: IsVersionOf: http://ci.nii.ac.jp/naid/110002290617/

Conrpounds having oxygen vacancies were synthe sized, and the influence of order disorder transition of oxygen vacallcies on the reduction of NO_x to N_2 was investigated. Brownmillerite related compounds Ba_2In_2O_5, Ba_3Y_4O_9 and BaLa_2O_4 having oxygen vacan cies were synthesized. Ionic conductivity and the cata lytic activity for the reduction of NO_x to N_2 increased at the temperature of order disorder transition of the oxygen vacancies of Ba_2In_2O_5, Ba_3Y_4O_9 and BaLa_2O_4. And it was found that the catalylic activity by order disorder transition of oxygen vacancies is hardly influenced by the coexistence with oxygen or propylene (C_3H_6) . Froun these results, it was clarified that the catalytic activity for the NO_x decomposition reaction appears at a lower temperature by controning the or der disorder transition temperature of oxygen vacan cies. Moreover, it was suggested that the oxygen vacan cy is an important factor in the reduction of NO_x to N_2, and the diffusion of oxygen vacancy appeared at the or der disorder transition temperature., rights: 社団法人日本セラミックス協会rights: 本文データは学協会の許諾に基づきCiNiiから複製したものであるrelation: isVersionOf: http://ci.nii.ac.jp/naid/110002290492/

Sintered bodies of (Ba, Pb)_2In_2O_5 were prepared by substituting A site of Ba_2In_2O_5 having a brownmillerite related structure with Pb. Crystal structure and conductivity of sintered (Ba, Pb)_2In_2O_5 were investigated. The crystal structure of (Ba, Pb)_2In_2O_5 was governed by the Pb content, and changed from orthorhombic to cubic structure with an increase in Pb content. The temperature dependence of the conductivi ty of (Ba, Pb)_2In_2O_5 revealed that the order disorder transition of oxygen vacancy disappears with an increase in Pb content, and that oxygen vacancies are disordered even at room temperature to have cubic structure. (Ba_Pb_)_2In_2O_5 showed higher conductivity than Ba_2In_2O_5 m the temperature range of 673 to 1100 K. The results suggest that the changes in crystal structure and conductivity are due to the ionic polarization of Pb., rights: 社団法人日本セラミックス協会rights: 本文データは学協会の許諾に基づきCiNiiから複製したものであるrelation: isVersionOf: http://ci.nii.ac.jp/naid/110002290515/

International audience; We report the synthesis and characterization of fully saturated hydrocarbon block copolymer thermoplastic elastomers with competitive mechanical properties and attractive processing features, Block copolymers containing glassy poly(cyclohexylethylene) (C), elastomeric poly(ethylene-alt-propylene) (P), and semicrystalline poly(ethylene) (E) were produced in a CEC-P-CEC heptablock architecture, denoted XPX, by anionic polymerization and catalytic hydrogenation, The X blocks contain equal volume fractions of C and F. totaling 40%-60% of the material overall. All the XPX polymers are disordered above the melt temperature for E(T(m,E) congruent to 95 degrees C) as evidenced by SAXS and dynamic mechanical spectroscopy measurements, Cooling below results in crystallization of the E blocks, which induces microphase segregation of E, C, and P into a complex morphology with a continuous rubbery domain and randomly arranged hard domains as shown by TEM. This mechanism of segregation decouples the processing temperature from the XPX molecular weight up to a limiting value. Tensile mechanical testing (simple extension and cyclic loading) demonstrates that the tensile strength (ca. 30 MPa) and strain at break (> 500%) are comparable to the behavior of CPC triblock thermoplastic elastomers of similar molecular weight and glass content. However, in the CPC materials, processability is constrained by the order-disorder transition temperature, limiting the applications of these materials, Elastic recovery of the XPX materials following seven cycles of tensile deformation is correlated with the fraction of X in the heptablock. copolymer, and the residual strain approaches that of CPC when the fraction of hard blocks f(X)

It was used the Complex Polymerization Method to synthesize barium calcium titanate powders (BCT). Crystalline Ba 0.8 Ca 0.2 TiO 3 perovskite-type phase could be identified by X-ray diffraction and confirmed by Raman spectroscopy in the powders heat treated at 600 °C. Inherent defects, linked to structural disorder, facilitate the photoluminescence emission. The photoluminescent emission peak maximum was around of 533 nm (2.33 eV) for the Ba 0.8 Ca 0.2 TiO 3 . The photoluminescence process and the band emission energy photon showed dependence of both the structural order–disorder and the thermal treatment history. The results revealed that Ba 0.8 Ca 0.2 TiO 3 (BCT20) is a highly promising candidate material for optical applications.

International audience; We report the synthesis, phase behavior, and viscoelastic and mechanical properties of a new type of multiblock copolymer composed of glassy poly(cyclohexylethylene) (C), semicrystalline poly(ethylene) (E), and elastomeric poly(ethylene-alt-propylene) (P). Five nearly monodisperse CECEC-P hexablock terpolymers and one (CECEC)(2)-P undecablock copolymer were synthesized by sequential anionic polymerization followed by catalytic hydrogenation. These multiblock copolymers, which contain equal volume fractions of P and compositionally symmetric CECEC, microphase separate by two different processes: segregation induced by crystallization of the E blocks and through chemical incompatibility between C, E, and P. These materials contain two different complex morphologies each with two length scales determined by the local (C-E) and overall (C-E-P) order-disorder transition temperatures relative to the glass and crystallization temperatures of the C and E blocks, respectively. Structure was determined by SAXS, TEM, and mechanical spectroscopy. Tensile tests reveal that the hexablock copolymers are tough (ca. >= 750% strain at break) and exhibit high elastic recovery despite the presence of P domains comprised of loose elastomeric end blocks. The (CECEC)(2)-P undecablock, which orders from the homogeneous melt as a consequence of crystallization of the E blocks, exhibits roughly three times the stress at failure without the loss of other physical properties. These results offer new insights into the development of enhanced mechanical response based on hierarchical molecular design.

International audience; Block copolymer materials form self-assembling structures at a nanometric scale, of interest in nanotechnology. The organization process of asymmetric poly(styrene-block-methyl methacrylate) (PS-b-PMMA) copolymer thin films is studied. In a first step it is demonstrated that two consecutive mechanisms lead to the formation of a well-ordered phase. The first mechanism is the local segregation of blocks, which leads to a metastable disordered cylinder phase (C-d). The second mechanism is a transformation of the C-d phase to a vertical cylinder phase via a nucleation-growth mechanism. The influence of film thickness and surface tension on the organization is also studied. Above the natural cylinder monolayer height, h(1), the kinetics of the cylinder organization strongly depends on the initial film thickness, and below h(1) the film splits into terraces. By varying the interactions between the substrate surface and the different blocks, a disordered phase can be formed instead of terraces.

Binary blends of four different high molecular weight poly (styrene-b-isoprene) (SI) diblock copolymers with a lower molecular weight poly(styrene-b-isoprene-b-styrene) (SIS) triblock copolymer were prepared, and their morphology was characterized by transmission electron microscopy and ultra-small-angle X-ray scattering. All the neat block copolymers have nearly symmetric composition and exhibit the lamellar morphology. The SI diblock copolymers had number-average molecular weights, (M) over bar (n), in the range 4.4 x 10(5)-1.3 x 10(6) g/mol and volume fractions of poly(styrene), Phi(PS), in the range 0.43-0.49, and the SIS triblock had a molecular weight of (M) over bar (n) similar to 6.2 x 10(4) g/mol with Phi(PS) = 0.41. The high molecular weight diblock copolymers are very strongly segregating, with interaction parameter values, chi N, in the range 470-1410. A morphological phase diagram in the parameter space of molecular weight ratio (R = M(n)(diblock)/1/2M(n)(triblock)) and blend composition was constructed, with R values in the range between 14 and 43, which are higher than previously reported. The phase diagram revealed a large miscibility gap for the blends, with macrophase separation into two distinct types of microphase-separated domains for weight fractions of SI, w(SI) < 0.9, implying virtually no solubility of the much higher molecular weight diblocks in the lower molecular weight triblock. For certain blend compositions, above R similar to 30, morphological transitions from the lamellar to cylindrical and bicontinuous structures were also observed. Macromolecules

The present work attempted to uncover the structural and chemical parameters that favor superprotonic phase transitions over melting or decomposition in the MHXO4, MH2ZO4, and mixed MHXO4-MH2ZO4 classes of compounds (X=S, Se; Z=P, As; M=Li, Na, K, NH4, Rb, Cs) and to thereby gain some ability to "engineer" the properties of solid acids for applications. Three approaches are described. First, the general observation that larger cations enable superprotonic transitions was investigated in both the isostructural M2(HSO4)(H2PO4) and non-isostructural MHSO4 family of compounds. The results of these studies confirmed and explained such a cation size effect, and also supplied a crystal-chemical measure for determining the likelihood of a compound undergoing a phase transition. Second, the entropic driving force behind the transitions was explored in the mixed CsHSO4-CsH2PO4 system of compounds. From these investigations, a general set of rules for calculating the entropy change of a superprotonic transition was established and the role of entropy in the transitions illuminated. Finally, the superprotonic phase transition of CsHSO4 was simulated by molecular dynamics, with which means the transition was probed in ways not possible through experimental methods. A sufficiently general approach was utilized so as to be applicable to other (as yet un-synthesized) compounds, thereby speeding up the process of discovering novel superprotonic solid acids. All three approaches increase the fundamental understanding of which chemical/structural features facilitate superprotonic transitions and should aid attempts to create new solid acids with properties ideal for application.

This work presents the disordering process produced by mechanical milling of the Fe70Al30 alloy and the evolution of the magnetic behaviour induced by it. For that purpose X-ray diffraction (XRD) technique and magnetic measurements are used in order to describe the structure and the magnetic phenomena that take place. It can be observed that milling induces a disorder in the sample, at the same time the lattice parameter increases and the magnetic behaviour changes drastically.

We present molecular dynamics (MD) simulation results on the role of vacancy diffusion in the surface order of the Cu3Au(001) and Ni3Al(001) faces. We found that in both systems vacancy diffusion proceeds preferably by hopping along the [110] direction. In the Cu3Au(001) case, successive vacancy diffusion events induce irreversible loss of the surface order, while in the Ni3Al(001) system each vacancy hop leads to unstable atomic arrangements having a clear preference of recovering the initial surface order. These findings can be used for the understanding of the order-disorder transition occurring in the Cu3Au(001) surface and the order-order kinetics characterizing the Ni3Al(001) face. In addition, we found that in the former case the vacancy diffusion rate saturates quickly (in

We present Molecular Dynamics simulation results concerning the structural properties of Cu and Au 2D adlayers on the Cu3Au(I 10) surface. We found that at low coverage the Cu adatoms occupy mainly dumbbell positions forming "pairs" with the Cu surface atoms, giving an incommensurate I x 4 aspect in the surface layer. It appears that this "pairing" stabilizes the surface preventing the location of Cu surface atoms at anti-sites. At higher coverage the Cu atoms reside at the four-fold adatom positions forming a well-ordered new type of "contracted" 2 x I Cu adlayer. Au deposition is characterized by wetting phenomena that take place very quickly already at low coverage and temperatures. An segregation occurs via exchange diffusion with the Cu surface atoms, resulting in mixed and disordered surface and adlayer. It comes out that these phenomena persist even close to the order-disorder bulk transition temperature. (C) 2003 Elsevier Science B.V. All rights reserved. Applied Surface Science

Using an effective potential model in analogy to the tight-binding scheme in the second-moment approximation, we investigated the transport processes and the structural properties of the Cu3Au and Ni3Al(111) surfaces by molecular dynamics simulations. We found that in the case of the Cu3Au(111) surface both Cu and Au adatoms are unstable and penetrating into the bulk (already at room temperature) induce local surface disorder that is propagating into the substrate. It is interesting to note that the interlayer diffusion takes place exclusively via Cu atoms and therefore, only Au adatoms stimulate local disorder through this process. Unlike, in the Ni3Al(111) surface the adatoms are stable, they diffuse via a multitude of hopping mechanisms, the Ni being more active than Al adatom. Below T-s = 900 K exchange mechanisms are lacking for both adatoms, while above this temperature exchange diffusion of the Al adatom with Ni surface atoms is present inducing local disorder in the surface layer. Interestingly, no exchange events were detected for the Ni adatom. Consequently, below T, the presence of adatoms on this surface does not affect the surface order, in agreement with the available experimental observations referring to Ni3Al. In addition, contrary to the behaviour found in the cases of the (001) and (110) surfaces, the presence of surface vacancies has no serious effect on the surface order in either of the alloys. This is essentially attributed to the structural difference of this surface, the second layer (Au or Al) atoms appearing to be an effective restraining element of any surface vacancy hopping mechanism. (C) 2003 Elsevier B.V. All rights reserved. Surface Science

URL: http://www-spht.cea.fr/articles/S03/018 (sur invitation).; International audience; The large increase in the flux-flow voltage noise, commonly observed in the vicinity of the peak-effect in super- conductors, is ascribed to a novel noise mechanism. The mechanism consists in random injection of the strongly pinned metastable disordered vortex phase through the sample edges and its subsequent random annealing into the weakly pinned ordered phase in the bulk. This results in large critical current fluctuations causing strong vortex velocity fluctuations. The excess noise due to this dynamic admixture of two vortex phases is found to dis- play pronounced reentrant behavior. In the Corbino geometry the injection of the metastable phase is prevented and, accordingly, the excess noise disappears. Excess flux-flow noise in the peak effect regime is dominated by vortex velocity fluctuations while the density fluctuations, frequently considered in the conventional flux-flow noise models, are negligibly weak. Strong nongaussian fluctuations are associated with S-shaped current-voltage characteristics. The spectral properties of the noise reflect the first order filter-like response of the dynamically coexisting vortex phases. The cutoff frequency in the spectra corresponds to the time-of-flight of vortices through the disordered part of the sample.

We report on the phase state of two new diblock copolymer systems, poly(butadiene-b-tert-butyl methacrylate) (BBMA) and poly(ethylene-b-tert-butyl methacrylate) (EBMA). The EBMA system was synthesized from the BBMA by hydrogenation of the polybutadiene block, thus creating two systems at practically the same volume fractions and identical degrees of polymerization, but with different segment interaction parameters and conformational asymmetries. Hydrogenation of the butadiene block resulted in an increased incompatibility and to an asymmetric phase diagram. In one of the EBMA copolymers with a PE volume fraction f(PE) = 0.42, crystallization of the PE blocks was confined by the cylindrical mesophase in a fluid matrix. Upon PE crystallization, the hexagonal order was found to improve significantly following a matrix-assisted reorganization. Macromolecules

The phase diagram for the vacancy-ordered structures in the substoichiometric TiC(x) (x = 0.5-1.0) has been established from Monte Carlo simulations with the long-range pair and multisite effective interactions obtained from ab initio calculations. Three ordered superstructures of vacancies (Ti(2)C, Ti(3)C(2), and Ti(6)C(5)) are found to be ground state configurations. Their stability has been verified by full-potential total energy calculations of the fully relaxed structures.

We present molecular dynamics results on the diffusive properties of Au and Cu adatoms on the ordered Cu(3)Au(0 0 1) surface. We found that for both adatoms there are two available equilibrium positions: one over the fourfold cite and another one on top of a Cu surface atom. It turns out that although this second position is rather a stopover than a stable adatom cite, it plays an important role in adatom diffusive behavior. The Cu adatom diffuses via a variety of Arrhenius type mechanisms, among which a new type of exchange mechanism, occurring at temperatures higher than T(s) = 500 K, involving second layer Cu atoms. In addition, some interlayer Cu diffusion events were observed, induced by the Cu adatom when localized at the on top Cu position. Moreover, we found that the Cu adatom stimulates the creation of diffusive vacancies that contribute in the rearrangement of the surface atoms. Concerning the Au adatom, we found that its diffusive behavior is substantially different from the Cu adatom case. Indeed, although the Cu and Au adatoms have similar reaction paths for diffusion, the Au adatom's behavior is inscribed by the new exchange mechanism that takes place exclusively with the participation of Cu surface atoms. This mechanism, occurring more often above T(s), results in a local disorder of the surface layer, rendering the Au adatom unstable. Therefore, it seems that above T(s), the presence of Cu or Au adatoms on the Cu(3)Au(0 0 1) face stimulates segregation and diffusive phenomena that could be related with the order-disorder transition occurring at higher temperatures. (C) 2001 Elsevier Science B.V. All rights reserved. Surface Science

The evolutions of Fe-X (X=Cr,Mn,V,Mo) elemental powders mixtures during mechanical alloying have been characterized from the scale of the powder particle down to the atomic scale by laser granulometry, scanning electron microscopy, microprobe analysis, X-ray and neutron diffraction, Mossbauer spectroscopy and magnetic and susceptibility measurements. When the thermodynamic evolution of the system leads to mixing (Fe0.28Mn0.72), the combination of the elements is faster th an when it leads to unmixing (Fe0.30Cr0.70).In our milling conditions, the stationary state of Fe0.30Cr0.70 powders mixture is not a homogeneous solid solution but presents nanosized composition fluctuations. These results can only be explained in the light of a competition between a driven mixing mechanism and a thermal unmixing process. During mechanical alloying of Fe0.30Mo0.70 and Fe0.66Mo0.34 powder mixtures, an amorphous phase is formed at the interfaces between the Fe and Mo layers. This amorphisation seems to be due to the asymmetry of the diffusion coefficients of Fe and Mo added to a destabilization of the crystalline lattice linked to the driven mixing of the elements beyond a critical concentration. The ordering (A2-+B2) under annealing of Fe-V alloys synthesized by mechanical alloying has been studied by neutrons diffraction and Mossbauer spectroscopy. At 450°C, the order parameter of Fe0.53 V0.47 stabilizes at an unusual low value that can be attributed residual fragments of antiphase boundaries. Only high enough temperatures can lead to the elimination of these defects thanks to long range migration of vacancies. An A2+B2 two phase domain, predicted by the theory, has been observed in Fe0.37V0.63 alloy annealed at 450°C., Les évolutions, au cours de la mécanosynthèse, de mélanges de poudres Fe-X (X=Cr,Mn,V,Mo) ont été caractérisées de l'échelle de la particule de poudre jusqu'à l'échelle atomique par granulométrie laser, MEB, microsonde, diffraction des rayons X et des neutrons, spectrométrie Mössbauer et mesures d'aimantation et de susceptibilité. Nous avons mis en évidence une combinaison des éléments plus rapide lorsque l'évolution thermodynamique du système conduit au mélange (Fe0.28Mn0.72) que lorsqu'elle conduit à une démixtion (Fe0.30Cr0.70). Dans nos conditions de broyage, l'état stationnaire du mélange Fe0.30Cr0.70 n'est pas une solution solide homogène mais est caractérisé par des fluctuations de composition à une échelle de quelques nanomètres. Ces différents résultats ne peuvent s'expliquer que par une compétition entre un mécanisme de mélange forcé par le broyage et un processus thermique de démixtion. Une phase amorphe se forme, lors de la mécanosynthèses de mélanges de poudres Fe0.30Mo0.70 et Fe0.66Mo0.34 aux interfaces entre les lamelles de Fe et Mo. Cette amorphisation est probablement due à la fois à l'asymétrie des coefficients de diffusion des deux espèces et à une déstabilisation du réseau cristallin induite par le mélange forcé des éléments au-delà d'une certaine limite de concentration. La mise en ordre (A2-+B2), par recuit, d'alliages Fe-V élaborés par mécanosynthèse, a été étudiée par diffraction des neutrons et spectrométrie Mössbauer. À 450°C, le degré d'ordre d'un alliage Fe0.53V0.47 se stabilise à une valeur anormalement basse. Nous avons attribué ces caractéristiques à la subsistance de débris de parois d'antiphase ne pouvant s'éliminer que lorsque la température est assez élevée pour permettre une migration des lacunes à longue distance. Nous avons également confirmé l'existence d'un domaine biphasé A2+B2, prévu par la théorie, dans lm alliage Fe.037V0.63 recuit à 450°C.

We investigate the dynamics of a three-state stochastic lattice gas, consisting of holes and two oppositely "charged" species of particles, under the influence of an "electric" field, at zero total charge. Interacting only through an excluded volume constraint, particles can hop to nearest neighbor empty sites, but particle-particle exchange between oppositely charged particles is also allowed on a separate time scale. Controlled by this relative time scale, particle density and drive, the system orders into a charge-segregated state. Using a combination of Monte Carlo simulations and continuum field theory techniques, we study the order of these transitions and map out the steady state phase diagram of the system. On a single sheet of transitions, a line of multicritical points is found, separating the first order and continuous transitions. Furthermore, we study the steady-state structure factors in the disordered phase where homogeneous configurations are stable against small harmonic perturbations. The average structure factors show a discontinuity singularity at the origin which in real space predicts an intricate crossover between power laws of different kinds. We also seek for generic statistical properties of these quantities. The probability distributions of the structure factors are universal asymmetric exponential distributions. This research was supported in part by grants from the National Science Foundation through the Division of Materials Research. Ph. D.