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    Denis Machon, Patrice Mélinon
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    ABSTRACT: Below a critical particle size, some pressurized compounds (e.g. TiO2, Y2O3, PbTe) undergo a crystal-to-amorphous transformation instead of a polymorphic transition. This effect reflects the greater propensity of nanomaterials for amorphization. In this work, a panorama of thermodynamic interpretations is given: first, a descriptive analysis based on the energy landscapes concept gives a general comprehension of the balance between thermodynamics and kinetics to obtain an amorphous state. Then, a formal approach based on Gibbs energy to describe the thermodynamics and phase transitions in nanoparticles gives a basic explanation of size-dependent pressure-induced amorphization. The features of this transformation (amorphization occurs at pressures lower than the polymorphic transition pressure!) and the nanostructuration can be explained in an elaborated model based on the Ginzburg-Landau theory of phase transition and on percolation theory. It is shown that the crossover between polymorphic transition and amorphization is highly dependent on the defect density and interfacial energy, i.e., on the synthesis process. Their behavior at high pressure is a quality control test for the nanoparticles.
    Physical Chemistry Chemical Physics 11/2014; · 4.20 Impact Factor
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    ABSTRACT: Perfectly crystalline solids are excellent heat conductors. Prominent counterexamples are intermetallic clathrates, guest-host systems with a high potential for thermoelectric applications due to their ultralow thermal conductivities. Our combined experimental and theoretical investigation of the lattice dynamics of a particularly simple binary representative, Ba_{8}Si_{46}, identifies the mechanism responsible for the reduction of lattice thermal conductivity intrinsic to the perfect crystal structure. Above a critical wave vector, the purely harmonic guest-host interaction leads to a drastic transfer of spectral weight to the guest atoms, corresponding to a localization of the propagative phonons.
    Physical Review Letters 07/2014; 113(02):5506. · 7.73 Impact Factor
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    ABSTRACT: The effect of an applied high pressure on the optical and acoustic vibrations of small anatase TiO2 nanoparticles is studied using Raman scattering. All the Raman peaks show a significant variation of their frequency with pressure, except for the low-frequency peak which is due to acoustic vibrations confined in the nanoparticles. These variations (or lack thereof) are compared to first-principles calculations of the stiffness tensor and phonons of bulk anatase TiO2 as a function of pressure. In particular, the variation of the shape of the low-frequency peak is explained by the increase of the elastic anisotropy of anatase TiO2 as pressure is increased.
    The Journal of Physical Chemistry C 04/2014; · 4.84 Impact Factor
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    ABSTRACT: The effects of surface and interface on the thermodynamics of small particles require a deeper understanding. This step is crucial for the development of models that can be used for decision-making support to design nanomaterials with original properties. Based on experimental results for phase transitions in compressed ZnO nanoparticles, we show the limitations of classical thermodynamics approaches (Gibbs and Landau). We develop a new model based on the Landau-Ginzburg theory that requires the consideration of several terms, such as the interaction between nanoparticles, pressure gradients, defect density, and so on. This phenomenological approach sheds light on the discrepancies in the literature as it identifies several possible parameters that should be taken into account to properly describe the transformations. For the sake of clarity and standardization, we propose an experimental protocol that must be followed during high-pressure investigations of nanoparticles in order to obtain coherent, reliable data that can be used by the scientific community.
    Nano Letters 12/2013; · 13.03 Impact Factor
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    ABSTRACT: Nanoparticles have more propensities for amorphization than their bulk counterparts, opening the opportunity to achieve the amorphous states in well-known poor glass-former compounds. Classical size effects are often invoked to explain such phenomenon. However, this argument is not sufficient to cover all experimental results such as functionalization effect. In this work, Y2O3 nanoparticles of 7 nm diameter are investigated under pressure. Special care is taken on the surface state of the particles by comparing the pressure-induced transformation of nanoparticles stored under different conditions (atmospheric vs argon environment). A clear difference is reported as one batch shows a transformation to an amorphous state, whereas the second undergoes a crystal-to-crystal transition. These results are discussed in terms of interface energy taking into account not only the usual surface energy but also the surface state contribution. The link between different types of amorphization (pressure-, mechanical-, and radiation-induced amorphization) in nanoparticles is underlined as a critical defect density is required to achieve the crystal-to-amorphous transformation. Defect creation may arise from multiple sources: irradiation, functionalization, and reconstructive transition.
    The Journal of Physical Chemistry C 05/2013; 117(21):11133-11140. · 4.84 Impact Factor
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    Denis Machon
    The Journal of Physical Chemistry C 04/2013; 117:11133. · 4.84 Impact Factor
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    ABSTRACT: Pressure-induced amorphization (PIA) is a phenomenon that involves an abrupt transition between a crystalline material and an amorphous solid through application of pressure at temperatures well below the melting point or glass transition range. Amorphous states can be produced by PIA for substances that do not normally form glasses by thermal quenching. It was first reported for ice Ih in 1984 following prediction of a metastable melting event associated with the negative initial melting slope observed for that material. The unusual phenomenon attracted intense interest and by the early 1990’s PIA had been reported to occur among a wide range of elements and compounds. However, with the advent of powerful experimental techniques including high resolution synchrotron X-ray and neutron scattering combined with more precise control over the pressurization environment, closer examination showed that some of the effects previously reported as PIA were likely due to formation of nanocrystals, or even that PIA was completely bypassed during examination of single crystals or materials treated under more hydrostatic compression conditions. Now it is important to understand these results together with related discussions of polyamorphic behavior to gain better understanding and control over these metastable transformations occurring in the low temperature range where structural relaxation and equilibration processes are severely constrained. The results will lead to useful new high-density amorphous materials or nanocrystalline composites containing metastable crystalline varieties and the experiments have driven new theoretical approaches to modeling the phenomena. Here we review the incidence and current understanding of PIA along with related phenomena of density- and entropy-driven liquid-liquid phase transitions (LLPT) and polyamorphism. We extend the discussion to include polymeric macromolecules and biologically-related materials, where the phenomena can be correlated with reversible vs irreversible unfolding and other metastable structural transformations.
    Progress in Materials Science 01/2013; · 23.19 Impact Factor
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    ABSTRACT: The vibrational and structural properties of modified double-wall carbon nanotubes (DWNTs) were investigated by high-pressure resonance Raman scattering. We studied bromine-intercalated DWNTs grown by chemical vapor deposition (CVD) and 13 C 60 peapod-derived DWNTs in comparison with pristine CVD-grown DWNTs. The effects of chemical modification, carbon interwall geometry, and inhomogeneous filling on the high-pressure evolution of the DWNTs have been investigated. We find that the mechanical resistance of the DWNT system is affected both in the case of bromine-intercalated CVD-DWNTs and also for the 13 C 60 peapod-derived DWNTs, thus lowering the onset of collapse pressure P (onset) c compared with pristine CVD-DWNTs. For bromine CVD-DWNTs, P (onset) c was observed to be 13 GPa, well below the 21 GPa found for pristine CVD-DWNTs. Uniaxial constrains in the interstitial regions of the DWNT bundle due to the presence of bromine arrangements explains this mechanical instability rather than a charge transfer process. Isotopic 13 C enrichment of the inner tube reduces the frequency of its tangential contribution to the G-band Raman spectrum, which appears to be an effective method to separate the contribution of inner-and outer-tube G + components during pressure evolution. P (onset) c was found to be 12 GPa for the 13 C 60 -derived DWNT system. In this case, the instability of the DWNT is mainly due to the high inhomogeneous filling of the outer tube, as a consequence of the conversion method used to produce the inner-wall nanotube from the peapods, which produces inner tubes which are usually shorter than the outer tubes, leading to the outer tube not being completely filled.
    Physical review. B, Condensed matter 11/2012; 86(19):195410. · 3.66 Impact Factor
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    The Journal of Physical Chemistry C 09/2012; 116:22043-22050. · 4.84 Impact Factor
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    ABSTRACT: In this work, we study the adhesion forces between atomic force microscopy (AFM) tips and superficial dentin etched with phosphoric acid. Initially, we quantitatively analyze the effect of acid etching on the surface heterogeneity and the surface roughness, two parameters that play a key role in the adhesion phenomenon. From a statistical study of the force-distance curves, we determine the average adhesion forces on the processed substrates. Our results show that the average adhesion forces, measured in water, increase linearly with the acid exposure time. The highest values of such forces are ascribed to the high density of collagen fibers on the etched surfaces. The individual contribution of exposed collagen fibrils to the adhesion force is highlighted. We also discuss in this paper the influence of the environmental medium (water/air) in the adhesion measurements. We show that the weak forces involved require working in the aqueous medium.
    Journal of Colloid and Interface Science 03/2012; 376(1):262-8. · 3.55 Impact Factor
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    ABSTRACT: Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp(3) bonds and it provides an explanation to several features in experimental x-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that Z-carbon is a transparent wide band-gap semiconductor with a hardness comparable to diamond.
    Physical Review Letters 02/2012; 108(6):065501. · 7.73 Impact Factor
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    ABSTRACT: We investigate the interface energy impact on phase stability using the shining example of TiO2 nanoparticles under pressure. We revisit the previously reported phase diagram of this system and propose a new mechanism allowing the control of pressure-induced amorphization of TiO2 ultrafine particles. We demonstrate that the size effect is necessary for stabilizing the amorphous state but is not sufficient in the sense that surface chemical functionalization of nanoparticles is determinant. This discovery opens the possibility to select the high-pressure phase in nanomaterials and, consequently, the recovered structure under ambient conditions.
    The Journal of Physical Chemistry C 11/2011; 115(45):22286-22291. · 4.84 Impact Factor
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    ABSTRACT: Exfoliated graphene and few layer graphene samples supported on SiO(2) have been studied by Raman spectroscopy at high pressure. For samples immersed on a alcohol mixture, an electron transfer of ∂n/∂P ∼ 8 × 10(12) cm(-2) GPa(-1) is observed for monolayer and bilayer graphene, leading to giant doping values of n ∼ 6 × 10(13) cm(-2) at the maximum pressure of 7 GPa. Three independent and consistent proofs of the doping process are obtained from (i) the evolution of the Raman G-band to 2D-band intensity ratio, (ii) the pressure coefficient of the G-band frequency, and (iii) the 2D band components splitting in the case of the bilayer sample. The charge transfer phenomena is absent for trilayer samples and for samples immersed in argon or nitrogen. We also show that a phase transition from a 2D biaxial strain response, resulting from the substrate drag upon volume reduction, to a 3D hydrostatic compression takes place when going from the bilayer to the trilayer sample. By model calculations we relate this transition to the unbinding of the graphene-SiO(2) system when increasing the number of graphene layers and as function of the surface roughness parameters. We propose that the formation of silanol groups on the SiO(2) substrate allows for a capacitance-induced substrate-mediated charge transfer.
    Nano Letters 08/2011; 11(9):3564-8. · 13.03 Impact Factor
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    ABSTRACT: The vibrational properties of double-walled carbon nanotubes (DWNTs) is investigated by high-pressure resonance Raman scattering up to 30 GPa in two different pressure-transmitting media (PTM): paraffin oil and NaCl. The protection effect on the outer tube during compression is verified .The collapse of DWNTs is experimentally observed for the first time, showing to be two-step: the onset of the outer 1.56 nm diameter tube collapse at 21 GPa is followed by the collapse of the inner 0.86 nm diameter tube at a higher pressure of 25 GPa. This observation is supported by calculations. We show that filling a tube with another tube leads to a pressure stabilization against collapse, in strong opposition to what is observed when filling a tube with fullerenes or iodine. The collapse pressure in DWNTs appears to follow a 1/dtav3 law, where dtav is the average diameter from the inner and outer tubes, in agreement with predictions [Yang, X.; Appl. Phys. Lett. 2006, 89, 113101]. Contrary to SWNTs and peapods, for DWNTs, the observed collapse pressure is independent of the PTM nature. Those differences are discussed in terms of tube filling homogeneity and of the separate roles of inner and outer tubes: the outer tube offers chemical screening to the inner tube, whereas the inner tube guarantees mechanical support to the outer one. This leads to high collapse pressure independent of the DWNT environnment: a characteristic that makes DWNTs ideal fillers for composite nanomaterials for high load mechanical support.
    The Journal of Physical Chemistry C. 03/2011; 115(13).
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    ABSTRACT: The high pressure stability of the silicon type-III clathrate Ba24Si100 has been studied by x-ray diffraction (XRD) up to a maximum pressure of 37.4 GPa. The high pressure behavior of this Si type-III clathrate appears to be analogous to the structural type-I parent Ba8Si46. An isostructural volume collapse is observed at ~23 GPa, a value higher than for Ba8Si46 (13-15 GPa). The crystallinity of the structure is preserved up to the maximum attained pressure without amorphization, which appears to be in contradiction with the interpretation given in a Raman spectroscopy study [Shimizu , Phys. Rev. B 71, 094108 (2005)]. Nevertheless, the XRD analysis shows the appearance of a type-III disordered nanocaged-based crystalline structure after the volume collapse. Moreover, we find that the volume collapse transformation is (quasi)reversible after pressure release. Additionally, a low pressure transition first evidenced by Raman spectroscopy is also observed in our XRD study at 5 GPa: The variation of the isotropic thermal factors of Ba atoms shows a clear discontinuity at this pressure while the average positions of Ba atoms remain identical.
    Physical review. B, Condensed matter 01/2011; 83. · 3.66 Impact Factor
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    ABSTRACT: We identify a mechanism responsible for amorphization of rare-earth molybdate isostructural compounds by means of x-ray absorption spectroscopy (XAS) and Raman spectroscopy as well as first-principles calculations. Our present Raman spectra show that both β′ and α-Eu2(MoO4)3 undergo a pressure-induced amorphization. Both first-principles calculations and XAS measurements explain the process of amorphization by a spatial self-reorganization of the oxygen clouds around the Mo and Eu subnetworks as the pressure is increased inducing a change in the coordination number of the Mo atoms. This latter transforms the eg crystal field of the tetrahedra into a series of stabilization-degenerated peaks, which energetically favors a disordered overlapping of the former MoO4 tetrahedra instead of a mere ordered compression and rotation of these fragments.
    Physical review. B, Condensed matter 01/2011; 83(21). · 3.66 Impact Factor
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    ABSTRACT: The magnetostructural bcc-to-hcp phase transition in iron is analysed theoretically in the framework of the Landau theory of phase transitions. In contrast to recent interpretations which emphasize the driving role of magnetism at the transition, the collapse of the ferromagnetic order in ε-Fe is interpreted as resulting from the large spontaneous strains and the magnitude of the displacive order-parameter involved in the Burgers reconstructive transition mechanism. It yields a direct first-order transition from the ferromagnetic α-phase to the non-magnetic ε-phase, without going across an intermediate magnetic structure.
    Journal of Physics Condensed Matter 11/2010; 22(46):466002. · 2.22 Impact Factor
  • Denis Machon, Friese, Breczewski, Grzechnik
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    ABSTRACT: Tl2MoO4 has been studied under high-pressure by X-ray diffraction, Raman spectroscopy, and optical absorption measurements. A first-order phase transition is observed at 3.5±0.5 GPa. The nature (ordered vs. disordered) of the high-pressure phase strongly depends on the local hydrostatic conditions. Optical absorption measurements tend to show that this transition is concomitant with an electronic structure transformation. Prior to the transition, single crystal X-ray diffraction shows that pressure induces interactions between MoO4 fragments and the Mo coordination number tends to increase. In addition, the stereoactivity of the lone-pair electrons on the three symmetrically independent Tl-sites is not uniform; while for two sites the stereoactivity decreases with increasing pressures for the third site the stereoactivity increases.
    Journal of Solid State Chemistry 11/2010; 183:2558. · 2.04 Impact Factor

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