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International audience; In this study, lead metasilicate glasses (PbSiO3) were densified at different maximum pressures in a diamond anvil cell (DAC) at room temperature. The densified glass samples were investigated ex-situ by Raman spectroscopy to probe their pressure-induced plastic deformation limit and permanent structural modifications on the Q(n) distribution. With a high Pb content, this glass exhibits a low elastic limit at similar to 4 GPa, which is linked to an initial compact structure. Spectral curve fitting of the high-frequency region of the Raman spectra, consisting of symmetric Si-O stretching modes, exhibit subtle Q(n) population modifications with maximum pressure. This reveals silica network depolymerization where the proportion of non-bridging oxygens (NBO) increases at the expense of bridging oxygens (BO). Possible densification mechanisms are discussed in contrast to those known in other silicate glasses.

International audience; In situ X-ray scattering monitoring is carried out during temperature annealing on different densified SiO2 glasses. Density fluctuations and intermediate range coherence from X-ray scattering (small-angle X-ray scattering) and diffraction (wide angle X-ray scattering) evidence a maximum in their evolution at the same relaxation time. These extrema confirm the existence of an intermediate transitory disordered state between the two more ordered high and low density amorphous states. We propose that the existence of this transitory state confirms the existence of two mega basins in the energy landscape and therefore an amorphous-amorphous transition. Including older Raman results, we show that this intermediate disorder state implies similar mechanisms at all length scales from a few angstroms to 5 nm.

International audience; Silicate glasses show widely varying changes in their longitudinal sound velocities below 10 GPa. These changes are often attributed to structural changes in the glass (or liquid) network. This study reports both sound velocities and structural analysis of CaAl2Si2O8 (anorthite) glass in situ up to 20 GPa, based on Brillouin and Raman spectroscopy results. In situ high-pressure Brillouin spectra of CaAl2Si2O8 glass were taken during two compression-decompression cycles. The second compression-decompression cycle up to 12 GPa displayed a perfectly elastic behavior indicating that 8% faster sound velocity arose from permanent densification during the first cycle. The longitudinal sound velocity was calculated from previously reported refractive index data and displayed distinct changes in behavior at 2 and 5 GPa. Anorthite (CaAl2Si2O8) glass displays an anomalous decrease in the longitudinal sound velocity up to ∼2 GPa. Above this pressure its longitudinal sound velocity is insensitive to pressure until 5 GPa and thereafter it displays a positive pressure dependence. The longitudinal sound velocity of CaAl2Si2O8 glass is quite distinct from both polymerized (e.g. silica, albite) and depolymerized (e.g. diopside) silicate glasses.Raman spectroscopy reveals that below 2 GPa there is a rapid decrease in the inter-tetrahedral angle within the aluminosilicate network. In fact, the sigma parameter, indicative of the overall intertetrahedral angle, displays three distinct pressure regimes comparable to the longitudinal sound velocity. The lowest pressure regime, In contrast to known polymerized (e.g. silica, albite) and depolymerized (e.g. diopside, enstatite) silicate glasses, CaAl2Si2O8 glass displays a weak negative pressure dependence, as found in polymerized compositions, but a high overall longitudinal sound velocity, as found in depolymerized systems. These structure-property relationships suggest that fragility is a better measure of the high-pressure behavior of silicate glasses.

Energy dissipation in amorphous coatings severely affects high-precision optical and quantum transducers. In order to isolate the source of coating loss, we performed an extensive study of Raman scattering and mechanical loss of a thermally-treated sputtered silica coating. Our results show that loss is correlated with the population of three-membered rings of Si-O$_4$ tetrahedral units, and support the evidence that thermal treatment reduces the density of metastable states separated by a characteristic energy of about 0.5 eV, in favour of an increase of the states separated by smaller activation energies. Finally, we conclude that three-fold rings are involved in the relaxation mechanisms only if they belong to more complex chain-like structures of 10 to 100 tetrahedra., Comment: 5 pages, 3 figures

From initial calcium aluminosilicate glass, transparent glass-ceramics have been successfully synthesized under simultaneous high pressure and temperature (SHPT). Possible homogeneous volumetric crystallization of this glassy system, which was not achieved previously by means of conventional heat treatment, has been put in evidence with a SHPT procedure. Structural, mechanical, and optical properties of glass and glass-ceramic obtained were investigated. Raman spectroscopy and X-ray diffraction allowed to identify two main crystalline phases: merwinite [Ca3Mg(SiO4)2] and diopside [CaMgSi2O6]. A Raman scanning profile showed that the formation of merwinite is quite homogeneous over the bulk sample. However, the sample surface also contains significant diopside crystals. Instrumented Berkovich nanoindentation was applied to determine the effect of SHPT on hardness from glass to glass-ceramic. For Eu-doped samples, the broadband emission due to 4f65d1 → 4f7 transition of Eu2+ was studied in both host systems. Additionally, the 5D0 → 7FJ transition of Eu3+ was used as an environment probe in the pristine glass and the glass-ceramic.

article i nfo Article history: Received 13 July 2012 Available online xxxx Low frequency vibrations, a universal feature of amorphous solids which is responsible for thermodynamical anomalies at low temperature, are complicated to record in high pressure device, notably because of the close- ness of the elastic line. We first present an experimental protocol allowing to record the in situ low-frequency Raman scattering of samples under high pressure in the diamond anvil cell apparatus with a high quality. This protocol is particularly adapted to study the evolution of the boson peak of glassy materials. The second part is dedicated to the study of the boson peak of a-GeO2, a typical strong glass. The results, which clearly show the non-Debye behaviour of this material at the beginning of the compression where an anomalous compression takes place, are compared to previous measurements on a-SiO2. Interpretation in terms of local structural transitions and discussions about the elastic nanoheterogeneities of the amorphous state are addressed.

We study the changes in the Raman optical vibrations of pure silica glass under high-pressure up to 4.3 GPa and room temperature, namely in the elastic domain. Several mechanical anomalies, as the decrease of bulk modulus between 0 and 2 GPa, have been revealed many years ago (P.W. Bridgmann, Am. J. Sci 10 (1925) 359), but no physical experiments have explained what happens at the atomic scale. Our experiments show that gradual structural reversible rearrangement from 0 to 2 GPa leads to a more flexible material, in good agreement with molecular dynamics (MD) simulations (L. Huang, J. Kieffer, Phys. Rev. B 69 (2004) 224203). Above 2 GPa, a fast homogenization occurs.

Normal silica glass is usually referred to as low density amorphous silica as it can be converted to high density amorphous silica by a hydrostatic pressure (polyamorphic transition). In this work in situ Raman experiments are performed in a diamond anvil cell up to 18 GPa. The pressure effects on the structure of silica after successive compression decompression experiments are analyzed. The mode Gruneisen parameters corresponding to the elastic compression of high density amorphous silica are obtained and compared with those of normal silica. A reorganization of the high density amorphous silica below 3 GPa is evidenced.

Polyamorphism is defined as the possibility for an amorphous material to exist with different structures. This has been demonstrated in solids for ice [O. Mishima, Y. Suzuki, Nature 419 (2002) 599], amorphous silicon [P.F. McMillan, M. Wilson, D. Daisenberger, D. Machon, Nature 4 (2005) 680], amorphous germanium and several oxide glasses [M. Grimsditch, Phys. Rev. 52 (1984) 2379, J.P. Itie, A. Polian, G. Calas, J. Petiau, A. Fontaine, H. Tolentino, Phys. Rev. 63 (1989) 398, C.H. Polsky, K.H. Smith, G.H. Wolf, J. Non-Cryst. Solids 248 (1999) 159, J. Nicholas, S Sinogeikin, J. Kieffer, J. Bass, Phys. Rev. Lett. 92 (2004) 215701, O. Majerus, L. Cormier, J.P. Itie, L Galoisy, D.R. Neuville, G. Calas, J. Non-Cryst. Solids 345&346 (2004) 34]: SiO2, GeO2, B2O3, SiO2–GeO2. Here we discuss the hysteresis curves corresponding to the evolution of the Raman bands as function of pressure for oxide glasses and compare it with ab-initio or molecular dynamics simulations [L. Huang, J. Kieffer, Phys. Rev. B 69 (2004) 224203, L. Huang, J. Kieffer, Phys. Rev. B 69 (2004) 244204, M. Durandurdu, Phys. Rev. B 73 (2006) 035209]. We show that these curves correspond for most of the glass studied to an irreversible change in the medium range order at ambient pressure. For a particular three component glass La2O3–B2O3–GeO2 (LBG) we demonstrate that this irreversibility occurs if the pressure is much higher than the in situ phase transformation pressure. It is predicted that this behaviour is general for glasses, leading to the possibility to recover new high density glass polymorphs in ambient conditions of temperature and pressure.

Glass isotropy could be broken by using the electric field of the 355 nm wavelength delivered by a nanosecond YAG: Nd 3+ laser. In this way, pulsed laser-induced birefringence was obtained in the La 2 O 3 –B 2 O 3 –GeO 2 (LBG) glassy system. Buried lines behaving like waveguides were written in glasses using this laser irradiation process. Micro Raman spectra performed in irradiated points of glasses revealed a rotation of the light polarization in agreement with previous results obtained on thermally-poled LBG glasses. However, no orientation effects of irradiated points related to the direction of the laser wave polarization (UV poling) could be clearly evidenced as suggested in our last paper. The interaction of the glass with the electromagnetic field of the UV laser beam can be more probably described by mechanisms based on a densification process.

Densified silica can be obtained by different pressure and temperature paths and for different stress conditions, hydrostatic or including shear. The density is usually the macroscopic parameter used to characterize the different compressed silica samples. The aim of our present study is to compare structural modifications for silica glass, densified from several routes. For this, densified silica glasses are prepared from cold and high temperature (up to 1020 °C) compressions. The different densified glasses obtained in our study are characterized by micro-Raman spectroscopy. Intertetrahedral angles from the main band relative to the bending mode decrease and their values are larger for densified samples from high temperature compression than those samples from cold compression. The relative amount of 3-membered rings deduced from the D2 line area increases as a function of density for cold compression. The temperature increase during the compression process induces a decrease of the 3 fold ring population. Moreover, 3 fold rings are more deformed and stressed for densified samples at room temperature at the expense of those densified at high temperature. Temperature plays a main role in the reorganization structure during the densification and leads to obtaining a more relaxed structure with lower stresses than glasses densified from cold compression. The role of hydrostatic or non-hydrostatic applied stresses on the glass structure is discussed. From the Sen and Thorpe central force model, intertetrahedral angle average value and their distribution are estimated.

Light scattering in glasses is due to inhomogeneities described as density fluctuations and concentration fluctuations. Both fluctuations are characterized by fictive and chemical fictive temperatures, Tf and T f ′ respectively. A SiO2–Al2O3–P2O5 fiber amplifier glass doped with Er3+ is studied. From Raman scattering the fictive temperature of a fiber is determined to be 130 °C higher than in a preform. In situ small angle X-rays scattering (SAXS) measurements on a sample with an initial fictive temperature of 1100 °C show a structural relaxation below Tg and no evidence of evolution of concentration fluctuations up to 1400 °C. These measurements demonstrate that in a strong glass such as that considered in this work, Tf and T f ′ can be monitored separately allowing a high T f ′ favouring the dispersion of the doping ions and a low Tf decreasing the Rayleigh scattering and increasing the transmission of the signal.

Elastic (Rayleigh) and inelastic (Brillouin and Raman) scatterings are altogether measured in the same experimental conditions for germanium doped silica glass (5wt% GeO2). Taking into account that density and concentration fluctuations are frozen-in respectively at the fictive temperature Tf and at the chemical fictive temperature T f ′ , a quantitative comparison of the different scattering processes demonstrates that the concentration fluctuations are the main reasons for the increase of the scattering. However the depolarized Rayleigh scattering is much stronger than in pure silica and also contributes to the increase of the total scattering whereas the increase of the Raman cross section due to the GeO2 introduction is much smaller.

We studied the influence of germanium atom introduction into the silica network and the influence of heat treatements on the glass structure by doing Raman and infra-red spectra of vitreous germanium doped silica (5% wt GeO 2 , 95% wt SiO 2 ). Germanium incorporates into the network by substituting Si atoms and consequently, we observed band shifts and broadening of defect peaks D 1 and D 2 . The peak positions are directly correlated with the structural state characterized by the glass fictive temperature T F of the different samples.

The homogeneous/inhomogeneous structure of glasses is still a debated question. Hydrostatic high pressure experiments allow us to determine if a glass behaves as an elastic continuous random network or if a nanometer scale heterogeneity has to be taken into account. In order to get information on the homogeneous/inhomogeneous structure of glasses, in situ high pressure Raman experiments are performed on silica in the elastic domain up to 4.7 GPa. A strong decrease of the Boson peak intensity is observed between 1 bar and 3 GPa. We show that this decrease does not correspond quantitatively to the effect of pressure on a homogeneous elastic medium. From the interpretation of the narrowing of the main Raman band width under pressure as a narrowing of the θ inter-tetrahedral Si–O–Si angle distribution it is shown that the decrease of the Boson peak intensity is correlated to the decrease of the intrinsic inhomogeneity of the silica glass. These results confirm the occurrence of an intrinsic inhomogeneity at a nanometer scale even in a single component glass like SiO2 which is very important for the interpretation of the optical or mechanical properties of the glasses.

Polyamorphic transitions in silica are studied under pressure up to 26.2 GPa by Raman and Brillouin spectrocopies. The first reversible polyamorphic transition is characterized by an abrupt decrease of the Si–O–Si inter-tetrahedral angle θ at 2.5 GPa corresponding to the elastic anomaly of silica. This result is interpreted as a reversible transition between two Low Density Amorphous phases LDA β ↔ LDA α similar to the β–α transition of cristobalite crystal under pressure. The second polyamorphic transition is observed for silica glass samples compressed up at different maximum pressures P max between 12 GPa and 26.2 GPa and recovered at ambient pressure. It corresponds to an irreversible and progressive transformation from the LDA α phase to a High Density Amorphous (HDA) phase. This HDA phase is characterized by an increase of the statistic distribution of 4- and 3-membered rings of SiO 4 tetrahedra with a narrowing of inter-tetrahedral angle distribution. A vanishing of the elastic anomaly in the HDA phase is observed.

Eu3+-doped yttrium oxide (Eu:Y2O3) films were grown on fused-silica substrates by laser ablation. Depending on oxygen pressure, substrate temperature and laser energy density, the crystalline structure of the films, determined by X-ray diffraction and Raman spectroscopy, changes from monoclinic to cubic. The crystalline structure of the films is confirmed by Eu3+ fluorescence spectra and refractive indices, measured by m-line spectroscopy. The cubic crystalline films show low-loss waveguiding properties.

Vitreous GeO2 has been compressed at high temperature to investigate the effect of thermal activation on the structural reorganization during compression. The measurements were performed in-situ using micro Raman spectroscopy under pressure up to 6 GPa and temperature up to 400 °C. The evolution of the Raman shift of the main band (400–500 cm−1) with temperature during compression evidences a pressure window around 3 GPa within which temperature has a remarkable influence on the structure, in particular, the intermediate range order. We find that this change is well correlated with previous ex-situ density measurements from high pressure-high temperature densifications. Moreover, coordination changes from tetrahedrally (GeO4) to octahedrally (GeO6) coordinated GeO2 are accelerated with the heating during the compression.

Pressure induced densification in a molecular arsenic sulfide glass is studied at ambient temperature using x-ray scattering, absorption and Raman spectroscopic techniques in situ in a diamond anvil cell. The relatively abrupt changes in the position of the first sharp diffraction peak, FSDP, and the pressure-volume equation of state near ∼2 GPa suggest a phase transition between low- and high-density amorphous phases characterized by different densification mechanisms and rates. Raman spectroscopic results provide clear evidence that the phase transition corresponds to a topological transformation between a low-density molecular structure and a high-density network structure via opening of the constituent As(4)S(3) cage molecules and bond switching. Pressure induced mode softening of the high density phase suggests a low dimensional nature of the network. The phase transformation is hysteretically reversible, and therefore, reminiscent of a first-order phase transition.

International audience; The elastic and plastic behaviors of silica glasses densified at various maximum pressure reached (12 GPa, 15 GPa, 19 GPa, and 22 GPa), were analyzed using in situ Raman and Brillouin spectroscopies. The elastic anomaly was observed to progressively vanish up to a maximum pressure reached of 12 GPa, beyond which it is completely suppressed. Above the elastic anomaly the mechanical behavior of silica glass, as derived from Brillouin measurements, is interpreted in terms of pressure induced transformation of low density amorphous silica into high density amorphous silica.

International audience; Micro-Raman spectroscopy was used to map the Vickers indentation-induced densification on a window glass (soda-lime silicate glass), from the correlation between Raman spectrum and density. Both top surface and cross-section of the residual imprints were investigated. It is shown that the sample preparation needed for the investigation of the in-depth densification does not alter the results. The maximum densification ratio measured is about 3.8%, which is 60% of the maximum densification measured after a high-pressure hydrostatic loading. Within the experimental errors, densification maps are self-similar for the two indentation loads used 10 N and 20 N. A comparison with finite element results based on a constitutive model developed for silica is discussed.

International audience; In addition of a flow, plastic deformation of structural glasses (in particular amorphous silica) is characterized by a permanent densification. Raman spectroscopic estimators are shown to give a full account of the plastic behavior of silica under pressure. While the permanent densification of silica has been widely discussed in terms of amorphous-amorphous transition, from a plasticity point of view, the evolution of the residual densification with the maximum pressure of a pressure cycle can be discussed as a density hardening phenomenon. In the framework of such a mechanical aging effect, we propose that the glass structure could be labelled by the maximum pressure experienced by the glass and that the saturation of densification could be associated with the densest packing of tetrahedra only linked by their vertices.

International audience; In this paper, we present results on UV pulsed laser-induced birefringence in La2O3- B2O3-GeO2 (LBG) glasses. Samples are irradiated at a wavelength of 355 nm delivered by an Nd-YAG laser operating in the nanosecond regime. After irradiation, glasses are analyzed by Micro Raman spectroscopy and Small Angle X-ray Scattering (SAXS). Raman spectra show figures characteristic of a light polarization rotation effect in agreement with an anisotropic distribution of the scattering intensity observed by SAXS measurements. These results are interpreted as the interaction of the glass with the electromagnetic field of the UV beam (UV poling).

We present low-frequency Raman scattering of pure GeO(2) glass under pressure up to 4 GPa, corresponding to an elastic transformation. Intensity variation and frequency shift of the boson peak are analysed and compared to the Debye model. The decrease of the boson peak intensity scaled by the Debye energy is correlated to the elastic anomalous properties under pressure up to 1.5 GPa, and interpreted as an elastic homogenisation process at the local scale. We emphasize similarities between a-GeO(2) and a-SiO(2) behavior under pressure, and compare our results to other experiments, numerical studies, and predictions of several models concerning amorphous systems.

Raman micro-spectroscopy is used to analyse the plastic behaviour of window glass (a soda-lime silicate glass) under high hydrostatic pressure and Vickers indentation. We show pressure-induced irreversible structural changes, notably an increase of Q(2) species at the expense of Q(3). For the first time, a very accurate [Formula: see text] calibration curve has been established. Local density variations of a Vickers indented window glass have been characterized by micro-Raman mapping using a high spatial resolution device. The effects of glass depolymerization on indentation and hydrostatic compression are discussed. Differences between window glass and pure SiO(2) glass behaviour under high stresses are also highlighted and analysed at a local scale.

Micro-Raman spectra are performed at different radial positions in order to determine the fictive temperature distribution in pure silica optical fibers. As-received and annealed optical fibers are compared and Raman results reveal a large decrease in the fictive temperature after annealing. The fictive temperature gradient between the core and the edge of the optical fiber can be eliminated after annealing and the spectroscopic decoupling between stress and the fictive temperature variation is discussed. The micro-Raman is a good probe in determining the spatial distribution of the fictive temperature in the micrometer length scale.

In situ micro-Raman spectroscopy was performed on lanthanum borogermanate (LBG) glasses, compressed in a diamond anvil cell at ambient temperature. Up to 5.6 GPa the structural changes are reversible, whereas experiments performed at 10 GPa and higher are characterized by hysteresis loops. A noticeable change of evolution of the main Raman band at 800 cm(-1) has been evidenced around 8 GPa. Indeed, at such a pressure, this Raman band is shifted in the opposite direction while the pressure is still increasing. This change of slopes may be the sign of a pressure-induced coordination number change. Upon decompression the Raman shift of this band follows a different path from the one during compression. When the sample is returned to ambient pressure, it shows a shifted and lightly modified Raman spectrum, suggesting that a new amorphous phase for LBG glass is reached under high pressure and still exists at atmospheric pressure. However, a comparison with LaBGeO(5) crystals with the same composition shows that this material has a full elastic behaviour in the same pressure range.