From the new infrared (IR) reflectivity and time-domain terahertz spectra combined with available highfrequencydielectric data above the megahertz range in a broad temperature range of 10 to 900 K, a fullpicture of the soft- and central-mode behavior in the classical relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN) issuggested. A detailed comparison is made with the recent hyper-Raman spectroscopy data [Hehlen et al., Phys.Rev. Lett. 117, 155501 (2016)] and also with other available experiments based on inelastic light and neutronscattering. It is revealed that each type of experiment provides slightly different data. The closest agreementis with the hyper-Raman data: both techniques yield the same number of soft-mode components and the samehigh-temperature softening towards the temperature T∗ ≈ 400 K. In addition to evaluation of the IR-terahertz datausing fitting with the standard factorized form of the dielectric function, we performed a successful fitting of thesame data using the effective medium approach (EMA), originally based on the assumption that the mesoscopicstructure of PMNconsists of randomly oriented uniaxially anisotropic polar nanodomains (PNDs) with somewhatharder transverse optical polar modes in the direction along the local PND dipole [Hlinka et al., Phys. Rev. Lett.96, 027601 (2006)]. Evaluation using Bruggeman EMA modeling has been successfully applied in the entireinvestigated temperature range. These results suggest that the response perpendicular to the local dipole moment,at high temperatures induced by random fields rather than PNDs, undergoes a classical softening from hightemperatures with permittivity obeying the Curie-Weiss law, ε⊥ = C/(T − TC), where C = 1.7 × 105K andTC = 380 K, whereas the response parallel to it shows no softening. Below the Burns temperature, ∼620 K, agigahertz relaxation ascribed to flipping of the PNDs emerges from the soft-mode response, slows down, andbroadens, remaining quite strong towards the cryogenic temperatures, where it can be assigned to fluctuations ofthe PND boundaries.

Hyper-Raman scattering (HRS) is a non-linear spectroscopy sensitive to all polar excitations, in particular the soft modes inactive in Raman. HRS has been applied to nano-ceramics of strontium titanate (SrTiO3) of controlled grain size. Contrary to infrared absorption which sees an average medium, the vibrational responses of the core and the shell(s) are split in HRS, allowing thereby probing the structural and dielectric properties of the two subsystems. The resulting structural model successfully reproduces the effective dielectric permittivity measured by dielectric experiments [1]. These results confirm the strong, but still under-exploited, potentialities of HRS for the investigation of polar materials [2].We also demonstrate that a new ground state can be obtained by tailoring the core-shell structure of the particles. High energy X-rays at synchrotron combined to neutron diffraction and HRS revealed a lowering of the ferroelastic ground state towards a new antiferrodistortive phase, accompanied with strong shift of the critical temperature [3,4]. This new phase is discussed within the Landau theory, and the crucial competition between particle shape anisotropy, surface tension, and shear strain is analyzed as well. This shows that controlling the core-shell structure provides an easy way to stabilize new phases that cannot exist in bulk material, just like film deposition on a substrate.

Density functional theory is used to calculate the vibrational properties of pure silica and sodo-silicateglasses with 20, 25, and 33 mol% of Na2O. The infrared and Raman spectra are calculated and the full responses aredecomposed into principal structural components (PSC). Those are for example the SiO4n--tetrahedra with n nonbrigdingoxygens defining the Qn-species at the origin of the structured feature at high frequency, and the Si-O-Sibridges leading the broad Raman R-band at intermediate frequencies. Our results confirm that Si-O-Si bending inbridges with large angle vibrate preferentially at lower frequencies than those with low angle. In addition, the spectralresponse of the Q2-species is bimodal and overlaps with that of the Q3, while the Q4 response covers almost all ofthe spectral range of the Qn-band. The ab-initio individual spectral responses of the PSC are used to reconstruct theexperimental Raman responses. Contrary to the commonly used multi-Gaussian decomposition, this approachprovides unambiguous band-assignments and hence a more accurate way to probe the structural and chemicalproperties of glasses from their spectroscopic signature.

Polarized Raman scattering is performed in a series of sodo-silicate glasses. The Si–O–Si inter-tetrahedral angle and its distribution are extracted using a model developed previously for densified silica, and the result are compared to ab initio atomistic calculations. The two techniques reveal a reduction of the most probable angle of about 0.30°/mol% of Na2O, but the shape of the angular distributions are different. The results suggest that Raman scattering enhances a specific angular distribution of Si–O–Si bridges, likely those close to sodium atoms, highlighting local angular heterogeneities.

This paper presents a survey of soft modes and their relationship to structural phase transitions. After introducing the concept of a soft mode, the origin of softening is considered from a lattice-dynamical point. The Landau theory approach to structural transitions is then discussed, followed by a generalization of the soft-mode concept through the use of the dynamic order-parameter susceptibility. The hyper-Raman spectra in the cubic phase of (Ba0.8Sr0.2)Ti-0.95(Zn1/3Nb2/3)(0.05)O-3 are studied with special emphasis on the lowest-frequency phonon mode. The spectral structure is found to change above the Curie temperature (303-873 K). Soft modes were put forward by Cochran and Anderson about 50 years ago as an explanation of structural phase transitions. Extending Landau's theory of phase transitions, their prediction was that the square of the frequency of the soft mode was proportional omega(0)(T)= C root T-T-C where T-c is the transition temperature.

High-pressure polarized Raman spectra of vitreous silica are measured up to 8 GPa in a diamond-anvil cell at room temperature. The combined use of either a non-penetrating pressurizing medium, argon, or a penetrating one, helium, allows to separate density from stress effects on the Raman frequencies. In the framework of a simple central force model, the results emphasize the distinct role played by the shrinkage of the inter-tetrahedral angle Si-O-Si and the force-constant stiffening during the compression. The polarization analysis further reveals the existence of an additional isotropic component in the high frequency wing of the Boson peak. The pressure dependence of the genuine Boson peak frequency is found to be much weaker than previously reported and even goes through a minimum around 2 GPa in a remarkable coincidence with the anomalous compressibility maximum of silica.

Raman spectroscopy is one of the key experimental methods for the structuralanalysis of glasses, thanks to its simple application and its sensitivity to variations ofcomposition or atomic-scale order. However, the inherent disorder of the glassystructure results to broad and overlapping peaks in the spectra, rendering theirquantitative analysis a challenging task and resulting in a mostly phenomenologicalinterpretation. This drawback can be addressed by the use of modern and accuratetheoretical tools, such as ab-initio vibrational spectroscopies. In this work we havestudied three simple sodosilicate glasses, prepared using a combined classical/ab-initio approach. For these structures, we have used density functional theory (DFT) tocalculate their vibrational density of states, as well as their Raman and IR spectra.The main advantage of the DFT calculations is that the resulting spectra can bedirectly correlated with the structural models and, most importantly, the vibrationalspectrum can be decomposed into exact contributions arising from individualstructural units. In the case of the sodosilicate glasses, we have identifiedcorrelations between spectral characteristics and stoichiometry, the effect of thepresence of bridging and non-bridging oxygens, the contributions of smaller or largerSi-O-Si angles, as well as the ones arising from the presence of the so-called Q-species. These results open the way for the study of even more complex glasses andaid in the interpretation of experimental spectra.