RESUMEN
Nanophononic materials are characterized by a periodic nanostructuration, which may lead to coherent scattering of phonons, enabling interference and resulting in modified phonon dispersions. We have used the extreme ultraviolet transient grating technique to measure phonon frequencies and lifetimes in a low-roughness nanoporous phononic membrane of SiN at wavelengths between 50 and 100 nm, comparable to the nanostructure lengthscale. Surprisingly, phonon frequencies are only slightly modified upon nanostructuration, while phonon lifetime is strongly reduced. Finite element calculations indicate that this is due to coherent phonon interference, which becomes dominant for wavelengths between ~ half and twice the inter-pores distance. Despite this, vibrational energy transport is ensured through an energy flow among the coherent modes created by reflections. This interference of phonon echos from periodic interfaces is likely another aspect of the mutual coherence effects recently highlighted in amorphous and complex crystalline materials and, in this context, could be used to tailor transport properties of nanostructured materials.
RESUMEN
High-Entropy Alloys (HEAs) are a new family of crystalline random alloys with four or more elements in a simple unit cell, at the forefront of materials research for their exceptional mechanical properties. Their strong chemical disorder leads to mass and force-constant fluctuations which are expected to strongly reduce phonon lifetime, responsible for thermal transport, similarly to glasses. Still, the long range order would associate HEAs to crystals with a complex disordered unit cell. These two families of materials, however, exhibit very different phonon dynamics, still leading to similar thermal properties. The question arises on the positioning of HEAs in this context. Here we present an exhaustive experimental investigation of the lattice dynamics in a HEA, Fe20Co20Cr20Mn20Ni20, using inelastic neutron and X-ray scattering. We demonstrate that HEAs present unique phonon dynamics at the frontier between fully disordered and ordered materials, characterized by long-propagating acoustic phonons in the whole Brillouin zone.
RESUMEN
In this work we present a molecular dynamics investigation of thermal transport in a silica-gallium nitride nanocomposite. A surprising enhancement of the thermal conductivity for crystalline volume fractions larger than 5% is found, which cannot be predicted by an effective medium approach, not even including percolation effects, the model systematically leading to an underestimation of the effective thermal conductivity. The behavior can instead be reproduced if an effective volume fraction twice larger than the real one is assumed, which translates into a percolation effect surprisingly stronger than the usual one. Such a scenario can be understood in terms of a phonon tunneling between inclusions, enhanced by the iso-orientation of all particles. Indeed, if a misorientation is introduced, the thermal conductivity strongly decreases. We also show that a percolating nanocomposite clearly stands in a different position than other nanocomposites, where thermal transport is dominated by the interface scattering and where parameters such as the interface density play a major role, differently from our case.
RESUMEN
As ultrastable metallic glasses (UMGs) are promising candidates to solve the stability issues of conventional metallic glasses, their study is of exceptional interest. By means of x-ray photon correlation spectroscopy, we have investigated the stability of UMGs at the atomic level. We find a clear signature of ultrastability at the atomic level that results in slower relaxation dynamics of UMGs with respect to conventional (rapidly quenched) metallic glasses, and in a peculiar acceleration of the dynamics by near T_{g} annealing. This surprising phenomenon, called here anti-aging, can be understood in the framework of the potential energy landscape. For all samples, the structural relaxation process can be described with a highly compressed shape of the density fluctuations, unaffected by thermal treatments and regardless of the ultrastability of the glass.
RESUMEN
Engineering lattice thermal conductivity requires to control the heat carried by atomic vibration waves, the phonons. The key parameter for quantifying it is the phonon lifetime, limiting the travelling distance, whose determination is however at the limits of instrumental capabilities. Here, we show the achievement of a direct quantitative measurement of phonon lifetimes in a single crystal of the clathrate Ba7.81Ge40.67Au5.33, renowned for its puzzling 'glass-like' thermal conductivity. Surprisingly, thermal transport is dominated by acoustic phonons with long lifetimes, travelling over distances of 10 to 100 nm as their wave-vector goes from 0.3 to 0.1 Å-1. Considering only low-energy acoustic phonons, and their observed lifetime, leads to a calculated thermal conductivity very close to the experimental one. Our results challenge the current picture of thermal transport in clathrates, underlining the inability of state-of-the-art simulations to reproduce the experimental data, thus representing a crucial experimental input for theoretical developments.Phonon lifetime is a fundamental parameter of thermal transport however its determination is challenging. Using inelastic neutron scattering and the neutron resonant spin-echo technique, Lory et al. determine the acoustic phonon lifetime in a single crystal of clathrate Ba7.81Ge40.67Au5.33.
RESUMEN
We report a study on charge-neutral crystal-field (dd) excitations in NiO as a function of applied pressure up to 55 GPa, using resonant inelastic x-ray scattering spectroscopy at the Ni K edge. We find distinct signatures of the pressure-induced modifications to the 3d orbital energies as a function of pressure. These modifications are experimentally evidenced by a subtle splitting of the dd-excitation resonance energies. We compare the experimental results to a charge-transfer cluster-model calculation, and a LSDA + U calculation of the ground state as a function of lattice constant. We thus show how resonant inelastic x-ray scattering spectroscopy is able to give insights into the manifold of excited states even in conditions that are difficult to access with many traditional experimental techniques.
RESUMEN
We present an extended investigation of phase I of carbon dioxide by x-ray diffraction and spectroscopic techniques at simultaneous high pressure and high temperature, up to 12 GPa and 800 K. Based on the present and literature data, we show that a Mie-Grüneisen-Debye model reproduces within experimental uncertainties the equation of state of CO(2) over the entire range of stability of phase I. Using infrared and Raman spectroscopy, we have determined the frequencies of the zone-center lattice modes as a function of pressure and temperature. We have then extracted the volume and temperature dependencies of the optical lattice mode frequencies and their respective Grüneisen parameters. We find a large difference between the thermodynamic Grüneisen parameter obtained from the P-V-T data and those associated with the optical lattice modes. This suggests, within the quasiharmonic approximation, that acoustic modes have a dominant contribution to the anharmonicity of the system.
RESUMEN
We report the oxygen K-edge spectra of ices Ih, VI, VII, and VIII measured with X-ray Raman scattering. The pre-edge and main-edge contributions increase strongly with density, even though the hydrogen bond arrangements are very similar in these phases. While the near-edge spectral features in water and ice have often been linked to hydrogen bonding, we show that the spectral changes in the phases studied here can be quantitatively related to structural changes in the second coordination shell. Density-functional theory calculations reproduce the experimental results and support the conclusion. Our results suggest that non-hydrogen-bonded neighbors can have a significant effect also in the liquid water spectrum. We discuss the implications of the results for the actively debated interpretation of the liquid water spectrum in terms of local structure.
RESUMEN
The existence of "intermediate bonding states" in solid CO2, separating the low-pressure molecular phases from the high-pressure polymeric forms, has been the matter of a long-standing debate. Here we determine the structure of CO2-IV using x-ray diffraction of single crystals grown inside a diamond anvil cell at 11.7 GPa and 830 K. It is rhombohedral, space group R3[over ]c, and is composed of individual, linear CO2 molecules with bond lengths of 1.155(2) A at 15 GPa. This shows that CO2 remains a purely molecular solid in this P-T range, and thus invalidates the intermediate bonding state scenario. First-principles calculations confirm the stability of the proposed structure and match very well observations, including the Raman and IR spectra. Furthermore, these results evidence a striking similarity between the high-pressure polymorphs of solid CO2 and N2.
RESUMEN
The current status of phonon-dispersion studies at high pressure using very high energy resolution inelastic X-ray scattering is discussed. A brief description of the instrumental apparatus is given, together with an illustration of the high-pressure facilities available at the IXS beamlines ID16 and ID28 of the ESRF. Some selected examples of recent studies on crystalline and liquid samples in a diamond anvil cell are then presented.
RESUMEN
We report on an inelastic x-ray scattering investigation of the high frequency dynamics of liquid cesium at 493 K and 1 GPa, which corresponds to a density 23% higher than that at the room pressure melting point. The analysis of the spectra, performed within the framework of the memory function approach suggests the existence of two different relaxation processes, i.e., the structural relaxation and a second faster one. The dynamical properties thus obtained are compared to those reported in the literature for room pressure conditions in order to investigate their dependence on pressure. The increase in density is found to largely affect the structural relaxation process, though leaving the acoustic damping almost unperturbed.
RESUMEN
On the macroscopic scale, the wavelengths of sound waves in glasses are large enough that the details of the disordered microscopic structure are usually irrelevant, and the medium can be considered as a continuum. On decreasing the wavelength this approximation must of course fail at one point. We show here that this takes place unexpectedly on the mesoscopic scale characteristic of the medium range order of glasses, where it still works well for the corresponding crystalline phases. Specifically, we find that the acoustic excitations with nanometric wavelengths show the clear signature of being strongly scattered, indicating the existence of a cross-over between well-defined acoustic modes for larger wavelengths and ill-defined ones for smaller wavelengths. This cross-over region is accompanied by a softening of the sound velocity that quantitatively accounts for the excess observed in the vibrational density of states of glasses over the Debye level at energies of a few milli-electronvolts. These findings thus highlight the acoustic contribution to the well-known universal low-temperature anomalies found in the specific heat of glasses.
RESUMEN
We present an extensive analysis of the proposed relationship [T. Scopigno et al., Science 302, 849 (2003)] between the fragility of glass-forming liquids and the nonergodicity factor as measured by inelastic x-ray scattering. We test the robustness of the correlation through the investigation of the relative change under pressure of the speed of sound, nonergodicity factor, and broadening of the acoustic exitations of a molecular glass former, cumene, and of a polymer, polyisobutylene. For polyisobutylene, we also perform a similar study by varying its molecular weight. Moreover, we have included new results on liquids presenting an exceptionally high fragility index m under ambient conditions. We show that the linear relation, proposed by Scopigno et al. [Science 302, 849 (2003)] between fragility, measured in the liquid state, and the slope alpha of the inverse nonergodicity factor as a function of T/T(g), measured in the glassy state, is not verified when increasing the data base. In particular, while there is still a trend in the suggested direction at atmospheric pressure, its consistency is not maintained by introducing pressure as an extra control parameter modifying the fragility: whatever is the variation in the isobaric fragility, the inverse nonergodicity factor increases or remains constant within the error bars, and one observes a systematic increase in the slope alpha when the temperature is scaled by T(g)(P). To avoid any particular aspects that might cause the relation to fail, we have replaced the fragility by other related properties often evoked, e.g., thermodynamic fragility, for the understanding of its concept. Moreover, we find, as previously proposed by two of us [K. Niss and C. Alba-Simionesco, Phys. Rev. B 74, 024205 (2006)], that the nonergodicity factor evaluated at the glass transition qualitatively reflects the effect of density on the relaxation time even though in this case no clear quantitative correlations appear.
RESUMEN
We report the observation of a roughening transition in carbon dioxide along the melting line of phase I, which we call reverse as faceting appears with increasing temperature. The characteristics of the transition are discussed in light of modern theories of roughening and the causes of its reverse behavior investigated. We propose that high temperature faceting is related to a pressure-induced increase of the surface stiffness.
RESUMEN
The melting curve and fluid equation of state of carbon dioxide have been determined under high pressure in a resistively heated diamond anvil cell. The melting line was determined from room temperature up to 11.1+/-0.1 GPa and 800+/-5 K by visual observation of the solid-fluid equilibrium and in situ measurements of pressure and temperature. Raman spectroscopy was used to identify the solid phase in equilibrium with the melt, showing that solid I is the stable phase along the melting curve in the probed range. Interferometric and Brillouin scattering experiments were conducted to determine the refractive index and sound velocity of the fluid phase. A dispersion of the sound velocity between ultrasonic and Brillouin frequencies is evidenced and could be reproduced by postulating the presence of a thermal relaxation process. The Brillouin sound velocities were then transformed to thermodynamic values in order to calculate the equation of state of fluid CO2. An analytic formulation of the density with respect to pressure and temperature is proposed, suitable in the P-T range of 0.1-8 GPa and 300-700 K and accurate within 2%. Our results show that the fluid above 500 K is less compressible than predicted from various phenomenological models.
RESUMEN
High-temperature IR absorption spectra of solid CO2 in phases II and IV were measured in a resistive heated diamond anvil cell up to 30 GPa. The spectral structures of the bending mode, observed in high quality thin crystalline samples, and of the IR lattice phonons, measured for the first time between 80 and 640 K, are discussed using group theory arguments. According to this analysis the claimed bent molecular geometry of CO2 in phase IV can be unambiguously ruled out. Furthermore, the structures of both phases II and IV have been identified, among those so far proposed, as orthorhombic.
