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International audience; The negative thermal expansion material potassium cadmium dicyanoargentate, KCd[Ag(CN) 2 ] 3 , is studied at high pressure using a combination of X-ray single-crystal diffraction, X-ray powder diffraction, infrared and Raman spectroscopy, and density functional theory calculations. In common with the isostructural manganese analogue, KMn[Ag(CN) 2 ] 3 , this material is shown to exhibit very strong negative linear compressibility (NLC) in the crystallographic c direction due to structure hinging. We find increased structural flexibility results in enhanced NLC and NTE properties, but this also leads to two pressure-induced phase transitionsto very large unit cells involving octahedral tilting and shearing of the structurebelow 2 GPa. The presence of potassium cations has an important effect on the mechanical and thermodynamic properties of this family, while the chemical versatility demonstrated here is of considerable interest to tune unusual mechanical properties for application. ■ INTRODUCTION Materials where external stimuli induces an anomalous response, such as expansion upon cooling (negative thermal expansion, NTE 1−3) or on application of hydrostatic pressure (negative linear compressibility, NLC 4,5), have recently received considerable attention. These materials that "break the rules"as a result of specific, and therefore rare, elastic anomalies 6−8 are curious from a fundamental point of view but might also be revolutionary for a range of technologies including interferometric pressure sensors, pressure-controlled or pressure-sensitive electronic devices, and smart actua-tion. 2,4,9−11 The ability to design materials with unusual properties is therefore a key focus of the field: can we understand these elastic anomalies so that we can tune the magnitude and/or range of negative responses? Negative thermal expansion may occur in one, two, or all principal directions without violating thermodynamics. 12,13 Thus, for example, the volume coefficient of thermal expansion (CTE), () V V V T p 1 α = ∂ ∂ , takes large negative values in zirconium tungstate ZrW 2 O 8 and zinc(II) cyanide Zn(CN) 2 over large temperature ranges. 14 Under hydrostatic pressure, however, volume must always reduce and so the volume compressibility (() K V V V p T 1 = − ∂ ∂ , conventionally formulated as the bulk modulus, B = (K V) −1), must be definite and positive. 15 Expansion under pressure can occur in one or two directions, as long as this is coupled to volume reduction. 5,12,16 Therefore, negative linear compressibility (NLC)the expansion in one principal direction of a material under hydrostatic pressureis conceptually and empirically related to linear-NTE. In both cases, anomalous response relies on mechanical anisotropy. The so-called "wine-rack" model is often invoked where mechanical hinging allows linear expansion coupled to volume reduction on increasing pressure or decreasing temperature. 5,8 Attention in this field has focused on a class of materials known as coordination polymers (CPs), in particular due to the very large magnitude of responses found in comparison to conventional oxide-based materials. 7 Examples that show large NTE and NLC include cyanide frameworks such as Ag 3 [Co-(CN) 6 ] (refs 17 and 18), KMn[Ag(CN) 2 ] 3 (refs 19 and 20), Zn[Au(CN) 2 ] 2 (refs 21 and 22), and metal−organic frameworks 23 (MOFs) such as MIL-53 (ref 24), UTSA-16 (ref 25), ZAG-4 (ref 26), and [NH 4 ][Zn(HCOO) 3 ] (ref 27). In many cases, mechanical response can be predicted in these systems by considering the framework as a set of rigid connectors and hinges, sodepending on geometry and topologyNTE or

In this paper, we present the different characterization techniques used to measure the mechanical properties of silica aerogels. The mechanical behaviour of aerogels is generally described in terms of elastic and fragile materials (such as glasses or ceramics) but also in terms of plastic media in compression testing. Because of these very different mechanical behaviors, several types of characterization techniques are proposed in the literature. We first describe the dynamic characterization techniques such as ultrasounds, Brillouin scattering, dynamic mechanical analysis (DMA) to measure the elastic properties: Young's modulus (E) , shear modulus (G), poisson ratio (υ) but also attenuation and internal friction. Thanks to "static" techniques such as three-point bending, uniaxial compression, compression we also access to the elastic modulus (E) and to the rupture strength (σ). The experimental results show that the value of the elastic and fracture moduli measured is several orders of magnitude lower than that of a material without porosity. With regard to the brittleness characteristics, Weibull's analysis is used to show the statistical nature of the fracture resistance. We also present the SENB (single edge notched beam technique) technique to characterize toughness (K1C) and the stress corrosion mechanisms, which are studied in ambient conditions and temperature by the double-cleavage drilled compression experiment (DCDC). In the last part of the paper, we show how, during the isostatic compression test, aerogels behave like plastic materials.The data allow calculating the bulk modulus (K), the amplitude of the plastic deformation and the yield strength (σel), which is the boundary between the elastic and plastic domains. These different techniques allow understanding which parameters influence the overall mechanical behavior of aerogels, such as pore volume, but also pore size, internal connectivity and silanol bounds content. It is shown that pore size plays a very important role; pores can be considered as flaws in the terms of fracture mechanics.

The negative thermal expansion material potassium cadmium dicyanoargentate, KCd[Ag(CN)2]3, is studied at high pressure using a combination of X-ray single-crystal diffraction, X-ray powder diffraction, infrared and Raman spectroscopy, and density functional theory calculations. In common with the isostructural manganese analogue, KMn[Ag(CN)2]3, this material is shown to exhibit very strong negative linear compressibility (NLC) in the crystallographic c direction due to structure hinging. We find increased structural flexibility results in enhanced NLC and NTE properties, but this also leads to two pressure-induced phase transitions—to very large unit cells involving octahedral tilting and shearing of the structure—below 2 GPa. The presence of potassium cations has an important effect on the mechanical and thermodynamic properties of this family, while the chemical versatility demonstrated here is of considerable interest to tune unusual mechanical properties for application.

International audience; The Raman modes of α-quartz-type boron phosphate were found to be extremely stable in frequency over a large temperature range from 300–1000 K. In order to determine the origin of this behavior, the material was also studied at high pressure up to 6 GPa. Upon compression, a classical behavior was observed with mode Grüneisen parameters ranging from −0.08(2) to 3.0(1). The present results indicate that the high temperature behavior in this material is an example of an unusual compensation effect between phonon–phonon interactions and implicit contributions due to thermal expansion. Phonon stability is of utmost importance for materials properties that are dependent on it such as dielectric and piezoelectric properties, for example. Boron phosphate belongs to the important class of α-quartz-type piezoelectric materials for which temperature stability is of primordial importance for resonator and sensor applications.

$\ensuremath{\alpha}$-Quartz-type gallium phosphate and representative compositions in the $\mathrm{AlP}{\mathrm{O}}_{4}\text{\ensuremath{-}}\mathrm{GaP}{\mathrm{O}}_{4}$ solid solution were studied by x-ray powder diffraction and absorption spectroscopy, Raman scattering, and by first-principles calculations up to pressures of close to 30 GPa. A phase transition to a metastable orthorhombic high-pressure phase along with some of the stable orthorhombic $Cmcm\mathrm{CrV}{\mathrm{O}}_{4}$-type material is found to occur beginning at 9 GPa at $320{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ in $\mathrm{GaP}{\mathrm{O}}_{4}$. In the case of the $\mathrm{AlP}{\mathrm{O}}_{4}\text{\ensuremath{-}}\mathrm{GaP}{\mathrm{O}}_{4}$ solid solution at room temperature, only the metastable orthorhombic phase was obtained above 10 GPa. The possible crystal structures of the high-pressure forms of $\mathrm{GaP}{\mathrm{O}}_{4}$ were predicted from first-principles calculations and the evolutionary algorithm USPEX. A predicted orthorhombic structure with a $Pmn{2}_{1}$ space group with the gallium in sixfold and phosphorus in fourfold coordination was found to be in the best agreement with the combined experimental data from x-ray diffraction and absorption and Raman spectroscopy. This method is found to very powerful to better understand competition between different phase transition pathways at high pressure.

AlPO4-17, known as the oxide with the highest negative thermal expansion (NTE), was studied under high pressure by angle-dispersive X-ray diffraction (XRD), mid- and far-infrared (IR) spectroscopy. Upon increasing pressure, the closure of the (P–O–Al) angle destabilizes the porous AlPO4-17 structure, which drives the amorphization process. On the basis of the decrease in intensity of the XRD lines and broadening of the IR modes, the material was found to begin to amorphize near 1 GPa. XRD, mid- and far-IR analysis evidenced pressure-induced framework softening and complete irreversible amorphization near 2.5 GPa corresponding to the collapse of the pores. The bulk modulus and its first pressure derivative (B0 = 31.2(5) GPa and B′0 = −10.1(3)) at ambient temperature were determined by fitting a third order Birch–Murnaghan equation of state (EOS) to the pressure–volume data. The material is extremely compressible and exhibits an elastic instability. Anomalous (negative) values of B′0 are very rare and have been observed previously for cyanides and metal–organic frameworks. Such an instability appears to be characteristic of materials, which exhibit strong NTE behavior and indicates a link between NTE and anomalous compressibility behavior. Mid-IR, far-IR, nuclear magnetic resonance, and pair distribution function analysis of the new amorphous form allow an amorphization mechanism to be proposed corresponding to a collapse of the structure around its pores retaining the columns built up of cancrinite cages and hexagonal prisms, based on alternating AlO4 and PO4 tetrahedra. An increase in coordination number of 10% of the Al atoms was observed. The pressure-induced amorphization in the strong NTE material AlPO4-17 opens the door to the development of new technological applications as crystal–amorphous nanocomposites with zero or specifically selected thermal expansion coefficients.

Brillouin scattering experiments have been performed from 5 to 1600 K in vycor, a porous silica glass. The acoustic velocity and attenuation at hypersonic frequencies are compared to those of bulk silica and others porous silica samples. The experimental evidence for the influence of porosity on the scattering by acoustic waves is compared to calculations. The correlation between internal friction and thermal conductivity at low temperature is discussed.

Microporous AlPO4-54·xH2O, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 A, was investigated at high pressure by X-ray powder diffraction and Raman spectroscopy in diamond anvil cells. The material was found to begin to amorphize near 2 GPa using either a nonpenetrating pressure transmitting medium (PTM) silicone oil or no PTM. When H2O is used as a PTM, amorphization begins at a lower pressure of 0.9 GPa. In this case, superhydration effects are observed and higher relative unit cell volumes are observed prior to the beginning of pressure-induced amorphization (PIA) as compared to the experiment in silicone oil due to insertion of the H2O molecules in the pores. In all cases, in these experiments at room temperature, amorphization was irreversible. Ex situ experiments were used to investigate the local structure of pressure-amorphized AlPO4-54·xH2O by nuclear magnetic resonance and by X-ray absorption spectroscopy, which show that, upon increasing pressure,...

International audience; The counterintuitive phenomenon of negative linear compressibility (NLC) is a highly desirable but rare property exploitable in the development of artificial muscles1, actuators2 and next-generation pressure sensors3. In all cases, material performance is directly related to the magnitude of intrinsic NLC response. Here we show the molecular framework material zinc(II) dicyanoaurate(I), Zn[Au(CN)2]2, exhibits the most extreme and persistent NLC behaviour yet reported: under increasing hydrostatic pressure its crystal structure expands in one direction at a rate that is an order of magnitude greater than both the typical contraction observed for common engineering materials4 and also the anomalous expansion in established NLC candidates3. This extreme behaviour arises from the honeycomb-like structure of Zn[Au(CN)2]2 coupling volume reduction to uniaxial expansion5, and helical Au...Au 'aurophilic' interactions6 accommodating abnormally large linear strains by functioning as supramolecular springs.

International audience; The results obtained by high pressure neutron powder diffraction and single-crystal x-ray diffraction for the P4mm–Pm $\bar{3}$ m phase transition in the prototype ferroelectric perovskite lead titanate are shown. Neutron diffraction is found to be strongly sensitive to the dipolar moment in the PbTiO3 unit cell due to the gradual reduction of the displacement of the Ti and O atoms from centrosymmetric positions in the cubic perovskite structure which exhibits anti-phase scattering of Pb, Ti and O atoms. From applying both techniques, the anomalously high Debye–Waller factor for the lead atoms confirms the disordered character of the cubic phase. High pressure single crystal x-ray diffraction also perfectly describes the ferroelectric–paraelectric transition and will be the technique of choice to solve higher pressure structures for PbTiO3.

Microporous AlPO4-54, which exhibits the largest pores among zeolites and aluminophosphates with a diameter of 12.7 Å, was investigated at high pressure by X-ray powder diffraction (XRD), mid- and far-infrared (IR) spectroscopy in diamond anvil cells. The material undergoes a phase transition beginning around 0.8 GPa. The amount of AlPO4-8 gradually increases with pressure and the phase transition is complete between 2 and 3 GPa. The closure of the (P—O—Al) angle destabilizes the structure of AlPO4-54, which drives the transition to AlPO4-8. The pressure-induced phase transformation of AlPO4-54 to AlPO4-8 is associated with a symmetry reduction from hexagonal to orthorhombic and with a change in the unidirectional ring channel parallel to the c-axes from 18 to 14 AlO4 and PO4 tetrahedra. An abrupt decrease along the b direction is linked to the formation of 4 new rings of 6 tetrahedra with significant structural reorganization. The transition is followed by irreversible amorphization beginning around 3.5 GPa due to the collapse of the pores. The amorphization of AlPO4-8 was detected from the disappearance of the XRD lines, abrupt shifts, and strong broadening of the mid-IR modes, and by changes in the pressure dependence of the mid- and far-IR modes, indicating a lower compressibility for the more dense amorphous form. Far-IR spectra indicate that the new amorphous form retains the local structure of AlPO4-8.

International audience; Gallium arsenate (GaAsO4) has the highest piezoelectric properties among α-quartz-type materials. We describe first-principles calculations of the linear and second-order nonlinear optical properties of GaAsO4 combined with measurements of its second harmonic generation (SHG). Our calculations show that GaAsO4 has a SHG efficiency between 7.1 (LDA) and 12.3 pm/V (GGA), which is in agreement with our experiments (7.5 ± 1.5 pm/V) using the technique of Maker fringes. This SHG efficiency, attributed to an electronic transition from O 2p to As 4 s states, is higher than that observed in GaPO4 due to a better polarizability of As atoms. Thus, the thermal stability of GaAsO4 (no phase transition up to its thermal decomposition at 1303 K) associated with its high laser damage threshold (0.93 GW/cm2) make this compound a promising bifunctional material for piezoelectric and nonlinear optical applications at high temperatures.

In order to point out the influence of thermal history (fictive temperature) and OH content on the elastic properties of silica glass, we have performed high resolution in situ Brillouin spectra of SiO2 glass from room temperature to the supercooled liquid at 1773K across the glass transition. The well known anomalous increase of elastic modulus in the glassy state and in the supercooled liquid regime is observed. No change of slope in the elastic moduli of silica appears as a characteristic of glass transition, on the contrary to what happens in various other glasses. We show that thermal history has a weak effect on elastic moduli in the glass transition regime for silica glass. The effect of water content in silica glass is more efficient than the fictive temperature effect and gives larger changes in the amplitude of elastic modulus for the same thermal dependence. A singular decrease above 1223K is also observed in the shear moduli for hydrated samples. Different models explaining elastic properties temperature dependence in relationship with frozen-in density fluctuations or with the structure are discussed., Comment: 22pages, 11 figures

Small-angle X-ray measurements of the density fluctuations as a function of temperature and thermal history are performed in silica and float glass. Structural relaxation in the glass transition range is observed for the samples stabilized at high temperature. The amplitude of the density fluctuations in the glassy state depends on the thermal history for samples of same composition. It increases with fictive temperature for silica as well as for float glass. Our results are compared to previous results in oxide glasses or polymers and discussed in relation with the concept of fragility i.e. the temperature sensitivity of viscosity in the liquid state.

We present a neutron spin echo investigation of the high temperature dynamics of two related glass-forming systems based on poly(oxypropylene): namely the molecular liquid triol and the polyurethane gel. Measurements performed at the first maximum of the static structure factor, on a rather large time window, allow one to analyse precisely the relaxation characteristics of these systems and to compare them with previous neutron time of flight and dielectric results. We discuss the differences in relaxation times and stretching exponents obtained for both samples as a function of their microstructure: molecular nature and architecture (effects of branching or/and three-dimensional network).

International audience; Confined H$_2$O molecules act as local probes for depressurization phenomena during the pressure induced amorphisation of faujasite NaX at which the OH stretching frequency first decreases and then increases almost to its room pressure value upon further compression. Pair distribution function (PDF) analysis provides evidence that amorphisation corresponds to a collapse of the structure around hydrated sodium cations with strong distortion of the secondary building units (double six-membered rings, sodalite cages). Both the use of guest molecules as local probes in far- and mid-infrared spectroscopy, where we correlate intermolecular water H bonding vibrations and internal mode behaviour under confinement, and PDF analysis could be of great use to study the mechanical behaviour of other hydrated materials.

International audience; Relaxor ferroelectric perovskites are highly polarizable and can exhibit giant coupling between elastic strain and an applied electric field. Here, we report an in situ extended X-ray absorption fine structure (EXAFS) study of a PbZn1/3Nb2/3O3 (PZN) single crystal as a function of the electric field. We show that the strong dipoles in the NbO6 octahedra bonds are aligned along the four < 011 > directions close to the orientation of the electric field, while a small reversible polar shift occurs for Zn in the direction of the electric field, i.e., positive or negative. This reversible Zn-O polar shift is proposed to play an important role in both the "easy" switching of the ferroelectric polarization and the giant piezoelectric effect in PZN.

A1. Solubility A1. Hydrothermal crystal growth A1. Single crystal growth A2. Growth from solutions B1. Arsenates B2. Piezoelectric materials abstract GaAsO4 crystals were grown by a hydrothermal epitaxial process. Microcrystalline GaAsO4 powder was first synthesized by hydrothermal methods by using GaAs as a starting material in a sulfuric acid solvent under oxidizing conditions. Experimental conditions were optimized and the yield of the nutrient synthesis reached 88%. GaAsO4 powder was then used as the nutrient for growing GaAsO4 single crystals by epitaxy on (21 oriented GaPO4, AlPO4 and GaAsO4 plates. Crystal growth was performed in a horizontal PTFE-lined autoclave divided into two parts using sulfuric acid as the solvent under a slow heating gradient. Hydrothermal conditions were optimized in order to improve the crystal quality. The influence of the nature and orientation of the seeds on the growth rate was studied. From the solubility curve, the solute supply ΔS as a function of temperature during the growth process was calculated. Transparent crystals were obtained with ΔS between 0.002 and 0.009 mol/L. Large crystals (3.9 � 1.8 � 1.4 cm 3 ) of gallium arsenate were obtained. These crystals were cut into (210), (010) and (001) plates. Mapping using Raman spectroscopy was performed in order to study the growth recovery on the seed. GaAsO4 crystals were characterized by infrared spectroscopy in order to quantify the amount of OH groups in the crystal structure. A low value of the absorption coefficient α at 3300 cm � 1 was measured 0.067 cm � 1 indicating a good crystal quality. Optical measurements were performed on GaAsO4 crystals by UV-vis-NIR spectrophotometry. Gallium arsenate exhibits the highest value of birefringence in the α-quartz group with Δn¼0.033. The transmission in the spectral region above 250 nm yields a bandgap of 5.85 eV. Piezoelectric properties were measured on resonators. An electromechanical coupling coefficient of 20% was measured which is the highest in the α-quartz group. A quality factor QF of 3.2 � 10 10 was obtained on a Y-cut plate. The C66 elastic constant was found

Extreme conditions permit unique materials to be synthesized and can significantly update our view of the periodic table. In the case of group IV elements, carbon was always considered to be distinct with respect to its heavier homologues in forming oxides. Here we report the synthesis of a crystalline CO2–SiO2 solid solution by reacting carbon dioxide and silica in a laser-heated diamond anvil cell (P=16–22 GPa, T>4,000 K), showing that carbon enters silica. Remarkably, this material is recovered to ambient conditions. X-ray diffraction shows that the crystal adopts a densely packed α-cristobalite structure (P41212) with carbon and silicon in fourfold coordination to oxygen at pressures where silica normally adopts a sixfold coordinated rutile-type stishovite structure. An average formula of C0.6(1)Si0.4(1)O2 is consistent with X-ray diffraction and Raman spectroscopy results. These findings may modify our view on oxide chemistry, which is of great interest for materials science, as well as Earth and planetary sciences., Novel materials synthesized under extreme conditions can challenge long-held views of fundamental chemistry. Santoro et al. combine fluid CO2 and solid SiO2 to create a new crystalline compound, via experimentation at ultra-high pressures and temperatures, which is stable at ambient conditions.

X-ray absorption spectra in the near-edge region (XANES) contain information about the structural organization of materials over distances up to about 10 A. In glasses, the structural information over such a large distance is rather complex and difficult to obtain. One possible method for extracting this information is to use multiple scattering (MS) calculations in real space to mimic X-ray absorption spectra in the XANES region. Such calculations for disordered systems require that a good structural model be assumed. We present full MS calculations at the silicon K-edge of amorphous silica, performed using atomic co-ordinates taken from molecular dynamics (MD) calculations. The spectra corresponding to several clusters are averaged in order to account for the disorder. The experimental spectrum is well reproduced by the calculations, showing the validity of the combination of MD and MS calculations. The results show an influence of medium-range order up to more than 5 A. Two different structural models taken from ab initio and classical MD are compared for this study.

The authors therefore wish to retract this Article due to content related concerns.

Brillouin light scattering allows the measurement of sound velocity and elastic moduli in transparent materials. The ability to select a small scattering volume and to use specific scattering configurations gives important information about the gradient and anisotropy of mechanical properties. Brillouin experiments are often used to measure the second-order elastic constants. When a high accuracy in frequency measurements is achieved, Brillouin scattering may allow the determination of third-order elastic constants in pre-stressed media. Hence, Brillouin scattering provides in principle, a method for the analysis of stress fields in tempered glasses. In order to validate the technique, samples of float soda-lime–silica glass submitted to controlled stresses by four-point flexion were investigated. The results show the expected profile for the velocity of sound waves propagating in directions parallel and perpendicular to the surface of the samples, respectively. They allow the determination of several third-order elastic constants of the investigated glass. This technique was applied to several samples of tempered glass corresponding to different values of the surface stress. The main result is the observation of the expected general trend, namely, through the thickness of the sample, a parabolic variation of the sound velocity whose amplitude increases with the magnitude of the surface stress.

We present an experimental investigation of the relaxational processes related to the glass transition in a family of polymers where the molecular architecture can be varied. The samples are crosslinked polyurethanes where the crosslink density, as well as the length between crosslinks, can be modified. Inelastic neutron scattering was used to determine the relaxation times in the 10−8-10−12s range. The times measured by this technique split from the structural relaxation determined at lower frequencies. Inelastic neutron scattering is sensitive to a secondary relaxation process, which is in the continuity of the β process probed by dielectric spectroscopy. Moreover, the dependence on the microscopic parameters of this process suggests that the corresponding relaxing entity can be identified as a smali portion of the arms of the triol, in agreement with the explanation suggested independently for the relaxational process investigated by dielectric measurements.

Inelastic neutron scattering in a polymeric glass-forming gel probes a relaxation related to the βslow process observed by dielectric measurements. As observed previously for several liquids, this process splits from the structural relaxation around the critical temperature Tc, corresponding to a square-root singularity in the Debye–Waller factor.

An experimental study of the relaxational processes related to the glass transition in polyurethanes is presented. In these polymers, the crosslink density can be modified by changing the ratio of the two constituents (a three arm triol and a di-isocyanate) and a viscoelastic crossover is observed from a sol to a gel. The length between cross-links can be varied by changing the length of the arms of the triol. We used inelastic light and neutron scattering to determine the relaxation times, τ , in the 10 −8 –10 −12 s range. The high frequency relaxation splits from the structural relaxation determined from the lower frequency measurements. For both techniques we found that τ decreases when the length of the arms increases and τ depends on the crosslink density when the arms of the triol are short; the variations of τ with the crosslink density are correlated to the one of the glass transition temperature.

A Brillouin scattering investigation was conducted on aerogels densified under uniaxial pressure. Sound velocity and elastic modulus as a function of the applied pressure and resulting density were examined. No evidence of elastic anisotropy was observed. The frequency of the phonon–fracton crossover and fractal persistence length were determined as a function of density and a comparison was made with series of mutually self-similar samples.

Elastic properties of densified aerogels were investigated by Brillouin light scattering in silica aerogels densified by viscous sintering and isostatic compression. Sound velocity and acoustic attenuation characterize the evolution of the elastic properties vs. the bulk density for the two sets of samples. Elastic modulus increases strongly during sintering while the attenuation α decreases, which is coherent with a larger connectivity in the solid network. Viscous flow sintering creates new siloxane bonds, eliminates pores and as expected, the aerogel stiffens. Compressed aerogel has a completely different behavior. In the low pressure range, elastic modulus decreases and α rises. This is attributed to breakage of links between clusters during compression. Weakening of the aerogels is the consequence of large strain of the solid network. For higher pressure, the density increase is accompanied by stiffening, suggesting that condensation occurs more than link breakage.

We report on a low-frequency Raman scattering study of base-catalyzed silica aerogels (BCSA) and partially densified BCSA prepared by heat treatment (HT), hydrostatic pressure (HP) and uniaxial pressure (UP). Low-frequency Raman susceptibility scales with frequency, χ ( ω )∼ ω − φ , within a large range of frequencies whose extent decreases with densification. Changes in the spectra are correlated with densification-induced changes in structure and connectivity of BCSA. φ is a constant independent of density for all as-prepared samples, in agreement with their mutual self-similarity. φ also remains essentially constant for HT samples while it decreases for HP and UP samples. φ values are discussed in terms of fractal and spectral dimensions of the samples in the light of the theoretical predictions of Alexander, Courtens and Vacher (ACV) and recent Brillouin scattering results.

We present the results of inelastic light scattering measurements (Brillouin and low-frequency Raman scattering) and neutron scattering in two oxide glasses (one phosphosilicate and one heavy-metal oxide) and in polyurethane samples in which the number of cross-links has been varied. These measurements have been performed across the glass transition in solid and undercooled liquid phases. The results showed vibrational and relaxational contributions arising in the so-called boson peak and in the quasielastic contributions respectively. The softening of the vibrational contribution with increasing temperature must be taken into account in order to extract the relaxational contribution. The correlations between the position of the boson peak measured by Raman or neutron scattering and the hypersound velocity measured by Brillouin scattering on the one hand and between the quasielastic intensity and the hypersonic attenuation on the other hand are seen in the oxide glasses. In polyurethane the relax...

A detailed Raman and Brillouin scattering investigation on a strong glass over a wide range of temperature across the glass transition temperature is presented. The aim of the study is to determine whether the low-frequency vibrational behaviour is correlated with the elastic properties. A careful comparison of several models is carried out in order to determine the reliability of theoretical predictions devoted to the understanding of the dynamics of glass-forming systems. A clear correlation between the position of the boson peak and the hypersonic velocity in both polarizations is demonstrated; in particular, the connection is in better accordance if one considers the transverse velocity. On the other hand, the quasielastic Raman scattering has been analysed in terms of excitations directly coupled to the acoustic phonons. The correlation between the quasielastic Raman intensity and the hypersonic attenuation predicted for the low-temperature regime has been found to hold over a broad range below and above . We found a strong correlation between the Raman features (at ) and the Brillouin data (at ), suggesting that the same mechanism is responsible for the vibrational dynamics within the range. Moreover, we emphasize that the softening of the vibrational modes responsible for the boson peak occurs significantly below , while an increase of the quasielastic intensity sets in, indicating some similarity between the origin of the boson peak and the origin of the growth of the quasielastic line.

International audience; Zeolites exhibit an immense range of applications, such as those in the chemical industry, electronics and photonics among others. We used non-catalytic zeolites in an entirely new fashion. In fact, high pressure (0.5-26 GPa) chemical reactions of simple molecules in the pores of a pure SiO2 zeolite, silicalite were performed in the diamond anvil cell to obtain unique nano-composites with drastically modified properties. These materials were investigated using a combination of X-ray diffraction and optical spectroscopy. We will first show how silicalite can be easily filled by simple molecules at high pressures and how this filling deactivates pressure induced amorphization of the silica framework. We will then present a silicon carbonate phase synthesized by reacting silicalite and molecular CO2 that fills the nano-pores, at 18-26 GPa and 600-980 K; the resulting compound is slightly metastable at room pressure. On the other hand, a nano-composite, which is stable at room temperature and pressure, is obtained by photo-polymerizing ethylene at 0.5-1.5 GPa under UV (351-364 nm) irradiation in the channels of silicalite. The structure of this material is characterized by single polyethylene chains adapting very well to the confining channels, which significantly modifies the physical properties of the silicalite framework. These findings may pave the way to the high pressure synthesis of a unique generation of technological materials

We have investigated the phonon modes of Ag3[Co(CN)6] using first-principles based methods and high-pressure infrared spectroscopy to understand the origin of its colossal thermal expansion. Grüneisen parameters of its zone-center vibrational modes were obtained from the calculations and compared to the experimental ones. We found that optical phonon modes below 100 cm-1 mainly contribute to its negative thermal expansion, and they also have a significant mixing with acoustic branches. These modes have been assigned to translational motion of the Co-CN-Ag-NC-Co linkages. We also identify a pressure-induced softening of zone-center phonon modes that could account for the experimentally observed phase transition in terms of a first-order displacive instability from the trigonal P3̄1m space group to the acentric Cm space group. © 2013 American Chemical Society.

α-Quartz-type Ga1−xFexPO4 solid solutions (0.04 ≤ x ≤ 0.28) and pure GaPO4 and FePO4 end members were studied by X-ray diffraction (XRD) and Raman spectroscopy to investigate the influence of iron content on the GaPO4 structure, and to have a better understanding of the lattice dynamics of the Ga1−xFexPO4 solid solutions. The unit cell parameters and structural distortion (i.e. inter-tetrahedral bridging θ, and tetrahedral tilt δ angles) of Ga1−xFexPO4 solid solutions were found to vary linearly with the iron content. Coupled and decoupled vibrational modes were identified using Raman spectroscopy and the calculation of the localization entropy using density functional theory. The GaPO4 decoupled modes are centered at 98, 237, 365 and 183 cm−1 and respectively assigned to E(TO1), E(TO5), E(TO7) and A1, while the vibrational modes located between 400 and 1200 cm−1 could be coupled. The coupled A1 bending mode centered at 457 cm−1 in pure α-GaPO4 was found to vary linearly as a function of iron content at room temperature. This mode could be therefore used to determine the iron content in Ga1−xFexPO4 solid solutions.

A Brillouin scattering study of the relaxational processes above the calorimetric glass transition temperature (in the 1.2 - 1.6 temperature range) on polyurethane gels is presented. The specificity of this work is to investigate several samples corresponding to various crosslink densities. The relaxational process observed by Brillouin scattering is compared to low-frequency measurements of the relaxational processes by other techniques: in contrast to low-frequency determinations, the Brillouin scattering relaxation time exhibits an Arrhenius behaviour for all the samples; however, the characteristic parameter of the time distribution width is the same for the two processes. The low-frequency relaxation and the high-frequency Brillouin scattering relaxation seem to merge for temperatures higher than 1.6 and these data are discussed on the basis of different models for the glass transition. The influence of the crosslink density yields an increase of the mean relaxation time with increasing connectivity and small variations of the apparent activation energy. In contrast, the distribution time width is not composition dependent. Nevertheless, the most striking results are that a rescaling to fully accounts for the variations of the mean relaxation time with the crosslink density and a correspondence between the crosslink density and the inverse of the temperature is suggested.

Brillouin and Raman scattering experiments have been performed on vitreous silica samples in order to check the validity of several models proposed for the interpretation of the low-frequency part of the Raman spectrum of glasses including the so-called ``boson peak.'' Samples of various origins, various fictive temperatures, and impurities contents as well as irradiated silica have been considered. It is demonstrated that the softening of acoustic modes responsible for the variation of the location of the boson peak maximum ${\mathrm{\ensuremath{\omega}}}_{\mathrm{max}}$ is not fully accounted by the variation of the sound velocity V. This indicates that the characteristic length l (related to V and ${\mathrm{\ensuremath{\omega}}}_{\mathrm{max}}$ in several models) is not a constant. Moreover, a clear quantitative correlation between the hypersound attenuation of the phonons and the low-frequency Raman intensity is evidenced. \textcopyright{} 1996 The American Physical Society.

The dissolution of α-FePO(4) and the α-Ga(0.75)Fe(0.25)PO(4) solid solution with α-quartz-type structures under hydrothermal conditions in 1 M HNO(3) aqueous solution was investigated by in situ X-ray absorption spectroscopy (XAS) at the Fe K-edge. The solubility of α-FePO(4) increases with temperature and is higher at 25 MPa than at 50 MPa. The Fe(3+) cation in solution is 6-fold coordinated with an average Fe-O distance close to 2.0 Å. A similar experiment was performed with a solid solution of α-quartz-type Ga(0.75)Fe(0.25)PO(4) as the starting phase under a pressure of 25 MPa. By varying the temperature from 303 K up to 573 K a single crystal was grown with 23% Fe(3+) with the α-quartz-type structure. These results show that the crystallization of pure α-quartz-type FePO(4) by the hydrothermal method is not possible due to the formation of very stable Fe(3+) hexa-aquo complexes [Fe(H(2)O)(6)](3+) and to the absence of FeO(4) tetrahedra in solution. Ga(3+) cations in solution induce the formation of gallophosphate complexes at the solid-liquid interface, which are at the origin of the nuclei for crystallization. We propose a crystallization mechanism in which the Fe(3+) substitutes Ga(3+) with a 4-fold coordination in mixed (iron/gallo)-phosphate complexes that leads to the growth of an α-quartz-type Ga(0.77)Fe(0.23)PO(4) single crystal.

In Cd(S,Se)‐doped glasses the optical properties are strongly dependent on the size of the nanocrystals, but can be also largely modified by changes in the crystal stoichiometry; however, the information on both stoichiometry and size is difficult to obtain in crystals smaller than 10 nm. The intensity scattered at small angles is classically used to get information about nanoparticles sizes. Moreover the variation of amplitude of this intensity with the energy of the x ray—‘‘the anomalous effect’’—near the selenium edge is related to stoichiometry. Anomalous small‐angle x‐ray scattering has been used as a tentative method to get information about stoichiometry in nanocrystals with size lower than 10 nm. Experiments have been performed on samples treated for 2 days at temperatures in the range 540–650 °C. The samples treated at temperatures above 580 °C contain crystals with size larger than 4 nm. For all these samples the anomalous effect has nearly the same amplitude, and we found the stoichiometry x=0....

A new effect on the X-ray absorption line shape is described which is proportional to the nearest-neighbor spin-spin correlation function. We present theory and demonstrate the use of linear polarized x-ray absorption spectroscopy to study the temperature dependence of the long-range order parameter and the nearest-neighbor spin-spin correlation function in antiferro-magnetic NiO.

In the low-frequency regime, the Raman spectrum of most glasses is dominated by a strong broad line which is usually called the Boson peak. The physical origin of this peak is still a matter for discussion. Several attempts have been made in order to connect the Boson peak to the vibrational density of states. Various interpretations have been proposed in recent years, in which the Boson peak is related to structural characteristics. In order to investigate the relationship between the vibrational dynamics and the medium-range order in glasses, we have studied the Brillouin and Raman scattering of several samples of oxide glasses prepared and stabilized under very different conditions. A study of the first sharp diffraction peak (FSDP) on the same samples shows that the two features, FSDP and Boson peak, are not clearly correlated. The results are discussed on the basis of the relationship between the mean free path of phonons and the low-frequency Raman intensity.

A comparison is made between X-ray absorption spectra at the sulphur K edge for CdS in wurtzite and zincblende structures. Differences between the polarization dependences of the X-ray spectra are expected due to the differences in symmetry for the two modifications. Multiple-scattering calculations in both structures are compared to published spectra and allow us to interpret a strong dichroic effect in the wurtzite structure. The intensity of a specific resonance is related to the middle-range order around sulphur atoms and potential applications of this fingerprint analysis are proposed.

International audience

Non molecular CO 2 has been an important subject of study in high pressure physics and chemistry for the past decade opening up a unique area of carbon chemistry. The phase diagram of CO 2 includes several non molecular phases above 30 GPa. Among these, the first discovered was CO 2 -V which appeared silica-like. Theoretical studies suggested that the structure of CO 2 -V is related to that of β-cristobalite with tetrahedral carbon coordination similar to silicon in SiO 2 , but reported experimental structural studies have been controversial. We have investigated CO 2 -V obtained from molecular CO 2 at 40–50 GPa and T > 1500 K using synchrotron X-ray diffraction, optical spectroscopy, and computer simulations. The structure refined by the Rietveld method is a partially collapsed variant of SiO 2 β-cristobalite, space group , in which the CO 4 tetrahedra are tilted by 38.4° about the c -axis. The existence of CO 4 tetrahedra (average O-C-O angle of 109.5°) is thus confirmed. The results add to the knowledge of carbon chemistry with mineral phases similar to SiO 2 and potential implications for Earth and planetary interiors.

Silicalite, a high-silica zeolite with MFI topology, undergoes pressure-induced amorphisation (PIA) when compressed with nonpenetrating pressure media. PIA is prevented by penetrating pressure media. In several zeolites, links have recently been found between pressure-induced phase transitions and the flexibility window, a geometric property of the framework identified by geometric simulation. We have analyzed structural data from compression experiments on silicalite, and we find that PIA occurs while the structure lies within its flexibility window. Penetrating media, which prevent PIA, push the structure outside the flexibility window. Thus framework flexibility is required for PIA to occur in silicalite. This link between amorphisation and flexibility is further evidence of the deep connections between framework geometry, flexibility, and the physical properties of zeolites.

poster

Freezing of water in hydrophilic nanopores ($D=1.2\text{ }\text{ }\mathrm{nm}$) is probed at the microscopic scale using x-ray diffraction, Raman spectroscopy, and molecular simulation. A freezing scenario, which has not been observed previously, is reported; while the pore surface induces orientational order of water in contact with it, water does not crystallize at temperatures as low as 173 K. Crystallization at the surface is suppressed as the number of hydrogen bonds formed is insufficient (even when including hydrogen bonds with the surface), while crystallization in the pore center is hindered as the curvature prevents the formation of a network of tetrahedrally coordinated molecules. This sheds light on the concept of an ubiquitous unfreezable water layer by showing that the latter has a rigid (i.e., glassy) liquidlike structure, but can exhibit orientational order.