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.

We investigate vitreous silica pressurized under the noble gases, He, Ne and Ar by both spectroscopic techniques and numerical simulations. We find an unexpectedhuge solubility of He and Ne at high pressure together with a large decrease of the static compressibility. We show that this relates to the open and flexible structure of the glass allowing gas atoms to distend the network, the same way a porous material deform upon fluid adsorption. This behavior can be rationalized using generalized poromechanical constitutive equations. Direct optical microscopy observations highlight the swelling of the silica network upon fluid adsorption. Very sharp diffusion front of the fluid into the silica the sample is further observed indicating non trivial kinetics of the diffusing atoms. In the Ne case, adsorption-desorption kinetics canbe tracked by Brillouin spectroscopy.

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.

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Structural and physical consequences of downsizing to form nanopowders or nanoceramics are very important both from applicative and fundamental point of view. We will discuss, using ab‐initio‐based calculation and Landau theory the origins and the consequences on structural properties when synthesizing nano‐objects with BaTiO3 and SrTiO3 , and show how to use nanostructuration to optimize properties such as electrocaloric effects or energy storage. For instance calculations in BaTiO3 show the possibility to form new phases (fig.1) in which vortices of polarization compete with homogeneous polarization or in which (in nanotorus) homogeneous hypertoroidization of nanotori coexists with axialhomogeneous toroidization. In SrTiO3 a new low temperature ground state, evidenced by synchrotron and neutron diffraction is discussed within the Landau theory and compared with the situation in thin films and during pressure experiments. The crucial competition between particles shape anisotropy, surface tension and shear strain is analyzed. Inducing a specific core‐shell structure is therefore a new and easy way to tailor structural properties and to stabilize new phases that cannot exist in bulk material, just like film deposition on substrate.