Diffusion hyper-Raman et relaxeurs ferroélectriques

National audienceLa diffusion hyper-Raman (DHR) est une spectroscopie optique des vibrations qui présente des règles de sélection différentes du celles du Raman et de l'infrarouge. Après avoir décrit les principes de la DHR, nous ferons un état de l'art de la dynamique vibrationnelle (IR, Raman, neutrons) du système relaxeur modèle Pb1/3Mg2/3NbO3 (PMN). Nous insisterons sur le comportement des modes mous polaires dont l'observation est uniquement possible par diffusion hyper-Raman. Les résultats ont été capturés dans un modèle qui offre une vision cohérente des propriétés dynamiques de toute une famille de matériaux. Ils relient par exemples les vibrations à la structure polaire locale, et donnent une origine microscopique aux propriétés diélectriques géantes de ces systèmes

Soft mode dynamics of ferroelectric relaxors

International audienceHyper-Raman scattering is a non-linear inelastic spectroscopy sensitive to polar excitations of solids whatever the crystalline symmetry. This selection rule is very attractive in particular for centrosymmetric structures for which polar modes are inactive in Raman. HRS has been performed in the cubic relaxors PbMg1/3Nb2/3O3 (PMN) and PbMg1/3Ta2/3O3 (PMT), with particular attention to the low frequency region, down to about 2 cm-1. Spectra have been recorded over a wide temperature range (900K-30K in PMN) and the results compared to existing infrared and neutron scattering data. This complete set of vibrational information provides the framework for a detailed analysis of the polarization dynamics in these systems. In particular, in both materials, the soft mode response exhibits a doublet structure up to the highest temperature investigated [1,2] emphasizing therefore a common property of cubic relaxors. The lowest frequency component is assigned to the primary soft mode of symmetry F1u, while the second likely originates from a local disorder persisting until very high temperatures, and which lifts the cubic selection rules. On cooling, the soft F1u-mode of PMN becomes overdamped at the onset of the Burns temperature but very interestingly, it splits between 600K and 400K into two components which harden on decreasing further the temperature. This behavior likely highlights the onset of a local anisotropy of the polarization and provides therefore new insight about the relaxor nature

Lowering of ground state induced by core-shell structure in strontium titanate

International audienc

Vibrational properties of sodosilicate glasses from first-principles calculations

The vibrational properties of three sodosilicate glasses have been investigated in the framework of density functional theory. The pure vibrational density of states has been calculated for all systems and the different vibrational modes have been assigned to specific atoms or structural units. It is shown that the Na content affects several vibrational features as the position and intensity of the R band or the mixing of the rocking and bending atomic motions of the Si-O-Si bridges. The calculated Raman spectra have been found to agree with experimental observations and their decomposition indicated the dominant character of the nonbridging oxygen contribution on the spectra, in particular for the high-frequency band, above 800cm−1. The decomposition of the high-frequency Raman feature into vibrations of the depolymerized tetrahedra (i.e., Qn units) has revealed spectral shapes of the partial contributions that cannot be accounted for by simple Gaussians as frequently assumed in the treatment of experimentally obtained Raman spectra

NACHOS, a CubeSat-Based High-Resolution UV-Visible Hyperspectral Imager for Remote Sensing of Trace Gases: System Overview, Science Objectives, and Preliminary Results

The Nano-satellite Atmospheric Chemistry Hyperspectral Observation System (NACHOS) is a high-throughput (f/2.9), high spectral resolution (1.3 nm optical, 0.57 nm sampling) hyperspectral imager covering the 300-500 nm spectral region with 350 spectral bands. The combined 1.5U instrument payload and 1.5U spacecraft bus comprise a 3U CubeSat. Spectroscopically similar to NASA’s Ozone Monitoring Instrument (OMI), which provides wide-field coverage at ~20 km spatial resolution, NACHOS offers complementary targeted measurements at far higher spatial resolution of ~0.4 km/pixel from 500 km altitude over its 15 ̊ across-track field of view. NACHOS incorporates highly streamlined onboard gas-retrieval algorithms, alleviating the need to routinely downlink massive hyperspectral data cubes. This paper discusses the instrument design, requirements leading to it, preliminary results, and science goals, including monitoring NO2 as a proxy for anthropogenic greenhouse gases, low-level degassing of SO2 and halogen oxides at pre-eruptive volcanoes, and formaldehyde from wildfires. Aiming for an eventual many-satellite constellation providing both high spatial resolution and frequent target revisits, the current NACHOS project is launching two CubeSats, the first already launched to the International Space Station aboard the NG-17 Cygnus vehicle on February 19, 2022 and awaiting deployment to its final orbit in June, and the second launching June 29, 2022

Inter-tetrahedra bond angle of permanently densified silicas extracted from their Raman spectra

Relative Raman scattering intensities are obtained in three samples of vitreous silica of increasing density. The variation of the intensity upon densification is very different for bending and stretching modes. For the former we find a Raman coupling-to-light coefficient C-B proportional to omega(2). A comparative intensity and frequency dependence of the Raman spectral lines in the three glasses is performed. Provided the Raman spectra are normalized by C-B, there exists a simple relation between the Si-O-Si bond angle and the frequency of all O-bending motions, including those of fourfold (n = 4) and threefold ( n = 3) rings. For 20% densification we find a reduction of similar to 5.7 degrees of the maximum of the network angle distribution, a value in very close agreement with previous NMR experiments. The threefold and fourfold rings are weakly perturbed by the densification, with a bond angle reduction of similar to 0.5 degrees for the former

Small ring statistics and inter-tetrahedra angle in permanently densified silicas: a femtosecond and Raman spectroscopy study

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Vibrations des verres d'oxydes observées par diffusion hyper-Raman

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Non network-former cations in oxide glasses spotted by Raman scattering

International audienceThe depolarized Raman spectra can be used as a probe to reveal the presence of non-network formers in oxide glasses. Two spectral responses involving the cations are observed below 400 cm−1 in more than 30 compositions of binary and ternary aluminosilicates. One of the two bands arises solely from cations close to non-bridging oxygen providing thereby a simple test for qualifying the polymerization state of the glass. The second feature involves all cations whatever their role in the glass and is found to be twofold: one contribution arises from cations charge compensating (AlO4)− tetrahedra and the other one from network modifier cations. These results confirm the net vibrational contrast of cations depending on their structural surrounding