Polyamorphisme induit par la pression dans les verres métalliques à base de cérium

Les verres métalliques suscitent un intérêt grandissant dû à leurs propriétés physico-chimiques remarquables par rapport à leurs homologues cristallins. Parmi eux, les verres métalliques à base de cérium (VM-Ce) se distinguent par l existence de polyamorphisme sous pression (transition de phase entre phases amorphes avec changement de densité et de structure locale à la transition). Cette transition de phase amorphe-amorphe, inattendue dans ces systèmes à structure compacte, engendre des changements de structure sous pression qui n ont pas été clairement identifiés. De plus, le rôle du cérium lors de cette transition semble décisif mais peu d études ont été menées afin d établir le lien entre les propriétés du cérium pur et le polyamorphisme des VM-Ce. Grâce à des mesures de diffusion élastique de rayons x à haute pression (0-15 GPa), nous avons prouvé l existence de polyamorphisme dans Ce69Al10Cu20Co1. D importantes modifications sous pression de l environnement local du cérium ont été mises en évidence lors de mesures d absorption de rayons x (EXAFS) au seuil K du cérium. D autre part, nous avons confirmé la nature électronique de cette transition (délocalisation sous pression de l électron 4f du cérium) par des mesures d absorption de rayons x (XANES) au seuil L3 du cérium. Le cérium pilotant cette transition, nous avons mené une étude complémentaire sur le cérium pur qui a montré l existence d un point critique dans le diagramme p,T du cérium cristallin lié à la délocalisation de l électron f. Cependant, aucun point critique n a été observé dans le diagramme de phase (0-15 GPa, 300-340 K) de Ce69Al10Cu20Co1 par diffusion élastique de rayons x à haute température.Metallic glasses are currently of growing interest worldwide due to their remarkable physicochemical properties with regard to their crystalline counterparts. Among them, the cerium based metallic glasses (Ce-MG) distinguish themselves by the existence of polyamorphism under pressure (phase transition between amorphous phases with change of density and local structure at transition). This transition, unexpected in these spatially compact systems, leads to structural changes under pressure which were not clearly identified. Furthermore, the role of the cerium constituent during this transition seems decisive but only few works studied the link between the properties of the pure cerium and the polyamorphism in the Ce-MG. Using x ray elastic diffusion techniques at high-pressure (0-15 GPa), we proved the existence of polyamorphism in Ce69Al10Cu20Co1 . Important modifications under pressure of the local environment of the cerium were revealing by x ray absorption measurements at Ce-K edge. On the other hand, we confirmed the electronic nature of this transition (4f electronic delocalization under pressure) by x ray absorption measurements (XANES) at Ce-L3 edge. The cerium piloting this transition, we studied the pure cerium properties along the g a transition, transition also induced by 4f electronic delocalization under pressure. This complementary work showed the existence of a critical point in the p, T diagram of crystalline cerium at the end of g a transition line. However, our x ray elastic diffusion measurement did not show the occurrence of such a critical point in VM-Ce phase diagram (0-15 GPa, 300-340 K).PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Apprentissage de la démarche scientifique et de l'esprit critique : un enseignement de Sorbonne Université pour les étudiants d'aujourd'hui, citoyens de demain

International audienceEn science, il ne suffit pas de posséder un savoir spécialisé pour donner une lecture interprétative du monde tel qu'il évolue, selon une progression technique de plus en plus rapide et invasive. L'actualité semble montrer chaque jour un peu plus que de hauts diplômes en sciences ou en ingénierie ne prémunissent pas contre une pensée magique, la théorie du complot ou même les interprétations littérales et dogmatiques. Être un chercheur reconnu par ses pairs n'immunise pas non plus contre la banalité des petits maux quotidiens, contre la simplification du discours à des fins d'autopromotion ou de course à la publication, contre les petits arrangements avec la réalité, et dans les cas extrêmes, contre la fraude scientifique. Une conséquence directe est qu'aujourd'hui, le fossé se fait grandissant entre questionnements citoyens, pressions économiques, pilotages politiques, scoops médiatiques et lenteur/rigueur de la recherche. Alors que faire de plus, si le savoir scientifique ne suffit pas ? Et comment mieux associer les citoyens aux réflexions sur la production des connaissances et leurs possibles applications ? Une des solutions que nous promouvons au sein de Sorbonne Université est d'associer notre formation scientifique à un apprentissage qui conjugue la démarche scientifique et l'esprit critique. Ce texte fait part d'un retour d'expérience de plusieurs années d'enseignement d'un cours interdisciplinaire intitulé « Démarche scientifique et esprit critique », destiné à des étudiants en Physique-Chimie-Ingénierie. Le texte discutera notamment des difficultés à élargir ce type de cours à plus d'étudiants, à d'autres filières. Plus globalement, il s'agira de questionner l'utilité d'un tel cours, sans rien omettre des difficultés rencontrées face aux forces qui semblent éloigner une partie de la société actuelle bien loin de toute pensée critique

New phase boundary and high pressure thermoelasticity of ZnO

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Propriétés élastiqueset magnétiques des alliages Invar Fe-Ni et FePt en fonction de la pression et de la température

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Pressure-induced softening of shear modes in ZnO

International audienceThe room temperature elastic moduli C IJ corresponding to the pure longitudinal and transverse modes in single crystals of the wurtzite phase of ZnO have been determined from ultrasonic wave velocities measurements as a function of pressure up to 10 GPa. All the moduli exhibit a linear dependence on pressure, with positive values for the longitudinal moduli (dC 11 /dP = 5.32 and dC 33 /dP = 3.78) but negative values for the shear moduli (dC 44 /dP = −0.35 and dC 66 /dP = −0.30). All modes exhibit anomalous travel time above P T r = 7.5 GPa, indicating the onset of a transition to the rocksalt (B1) phase. Using recent improvements for ultra-sonic measurements in multi-anvil apparatus, this experimental study on the phase transformation mechanism is extended to simultaneous high pressures and high tem-

High-pressure Raman spectroscopy study of wurtzite ZnO

International audienceThe high pressure behavior of optical phonons in wurtzite zinc oxide (w-ZnO) has been studied using room temperature Raman spectroscopy and ab-initio calculations based on a plane wave pseudopotential method within the density functional theory. The pressure dependence of the zone-center phonons (E2, A1 and E1) was measured for the wurtzite structure up to the hexagonal→cubic transition near 9 GPa. Above this pressure no active mode was observed. The only negative Grüneisen parameter is that of the E low 2 mode. E1(LO) and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born's transverse dynamic charge e * T is experimentally found to increase under compression like e * T (P) = 2.02 + 6.4 · 10 −3 .P whereas calculations give e * T (P) = 2.09−2.5·10 −3 .P (in units of the elementary charge e, P in GPa). In both cases, the pressure variation is small, indicating a weak dependence of the bond ionicity with pressure. The pressure dependence of the optical mode energies is also compared with the prediction of a model that treats the wurtzite-to-rocksalt transition as an homogeneous shear strain. There is no evidence of anomaly in the E2 and A1 modes behavior before the phase transition. PACS numbers: PACS: 78.30.-j, 64.70.Dv, 91.60.GF Zinc oxide belongs to the wide band-gap semiconductor family with large ionic characters of chemical bonds 2. The ionic size or relative electronegativity has been used to explain high pressure structure changes in A N B 8−N semiconductors. First principles calculations have shown that the zinc-blende (or wurtzite)→rocksalt→ β-Sn transition sequence is a common feature for most semiconductors. However, recent experimental identification of new intermediate phases (like cinnabar in CdTe for example) and the systematic absence of some phases (rocksalt (rs) for covalent compounds and β-Sn for all except the most covalent) invalidate the traditional transition sequence and, consequently, question the standard theoretical approach. In a recent Letter, Ozoli¸nšOzoli¸nš and Zunger 3 suggested that phase transitions are accompanied by phonon softening, whose instability has to be taken into account in the calculation to correctly predict the phase diagram. The latter outcome can also be discussed in terms of the transition mechanism, where the presence of negative Grüneisen parameters of phonon modes may play an important role. Recently, a new intermediate structure has been proposed in the wurtzite to rocksalt transformation path of GaN, along which the optical A 1 and E high 2 modes are expected to be affected by the new bond formations 4. Wurtzite ZnO, which transforms under pressure into the rs phase at 9 GPa, may provide a good example of this trend. Wurtzite ZnO belongs to the C 4 6v (P6 3 mc) space group. The primitive cell includes two formula units, with all atoms occupying 2b sites of symmetry C 3v. At the Γ point of the Brillouin zone, group theory predicts the existence of the following optics modes: Γ opt =A 1 +2B 1 +E 1 +2E 2. Frequency from the B low 1 and B high 1 silent modes has been calculated at 260 and 540 cm −1 respectively. A 1 , E 1 and E 2 modes are Raman active. In addition A 1 and E 1 are infrared active, and therefore split into longitudinal and transverse components (LO and TO). The mode assignment at ambient conditions is well established in the literature 5. To the best of our knowledge, no calculation and only two experimental attempts have been made to study the w-ZnO phonon frequency shift under pressure 6,7. In the first reference , the authors report the evolution of two phonon frequencies, E high 2 and E low 2 , over a relative small pressure range (0-1 GPa). In the second one, the pressure dependence of four Raman modes (E high

High pressure transformations in liquid rubidium

International audienceAn electronic-driven liquid-liquid phase transition in rubidium is revealed by picosecond acoustic measurements combined with ab initio calculations. Picosecond acoustics were used to measure the melting line up to 10 GPa, finding the maximum in the melting curve at 7 GPa and 555 K. We observe the onset of a continuous liquid-liquid phase transition beginning around the melting maxima through until 16 GPa. Sound velocity shows a softening similar to that reported for liquid caesium, caused by a change in the bulk modulus during a crossover from the low-density to the high-density liquid. Guided by the ab initio calculations, we relate the changes in the thermo-elastic properties to the progressive localization of the valence electrons in the pressure range of 6-16 GPa. At high pressure rubidium forms an electride liquid quantified by the appearance of interstitial quasi atoms (ISQs) localised in the valence electron density

High-pressure transformations in liquid rubidium

International audienceAn electronic-driven liquid-liquid phase transition in rubidium is revealed by picosecond acoustic measurements combined with ab initio calculations. Picosecond acoustics were used to measure the melting line up to 10 GPa, finding the maximum in the melting curve at 7 GPa and 555 K. We observe the onset of a continuous liquid-liquid phase transition beginning around the melting maxima through until 16 GPa. Sound velocity shows a softening similar to that reported for liquid caesium, caused by a change in the bulk modulus during a crossover from the low-density to the high-density liquid. Guided by the ab initio calculations, we relate the changes in the thermoelastic properties to the progressive localization of the valence electrons in the pressure range of 6-16 GPa. At high pressure rubidium forms an electride liquid quantified by the appearance of interstitial quasiatoms localized in the valence electron density

Polyamorphic transition of germanium under pressure

International audiencePressure-induced transformations in the atomic and electronic structure of amorphous germa-nium (a-Ge) have been investigated by using x-ray absorption spectroscopy (XAS) combined with energy-scanning x-ray diffraction. Our data show that an abrupt change in the local structure and in the electron states near the Fermi level occurs in evaporated aGe at a pressure of about 8 GPa. The transformation is clearly detectable by a change in the shape and energy shift in the near-edge structures and by an increase of the average first-neighbor distance measured by XAS. The occurrence of this polyamorphic transition is discussed in light of the recent advances in the study of multiple dense fluid phases