Application of elastostatic Green function tensor technique to electrostriction in cubic, hexagonal and orthorhombic crystals

The elastostatic Green function tensor approach, which was recently used to treat electrostriction in numerical simulation of domain structure formation in cubic ferroelectrics, is reviewed and extended to the crystals of hexagonal and orthorhombic symmetry. The tensorial kernels appearing in the expressions for effective nonlocal interaction of electrostrictive origin are derived explicitly and their physical meaning is illustrated on simple examples. It is argued that the bilinear coupling between the polarization gradients and elastic strain should be systematically included in the Ginzburg-Landau free energy expansion of electrostrictive materials.Comment: 4 page

The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica

Spectroscopic results on low frequency excitations of densified silica are presented and related to characteristic thermal properties of glasses. The end of the longitudinal acoustic branch is marked by a rapid increase of the Brillouin linewidth with the scattering vector. This rapid growth saturates at a crossover frequency Omega_co which nearly coincides with the center of the boson peak. The latter is clearly due to additional optic-like excitations related to nearly rigid SiO_4 librations as indicated by hyper-Raman scattering. Whether the onset of strong scattering is best described by hybridization of acoustic modes with these librations, by their elastic scattering (Rayleigh scattering) on the local excitations, or by soft potentials remains to be settled.Comment: 14 pages, 6 figures, to be published in a special issue of J. Phys. Condens. Matte

Dauphiné twin observation in quartz using piezo or electro-optic effects

The influence of twinning in quartz on the piezooptic and the electrooptic tensors is examined from the point of view of symmetry. Since some tensor coefflcients change their sign with twinning one can make observations of twins using thèse effects. Several experimental dispositions using uniaxial stress or an electric field are discussed and experimental results are presented for Dauphiné twins, which are not visible by ordinary optical methods

X-ray diffraction and reflectivity study of porous silicon

National audienc

X-ray diffraction investigation of n type porous silicon

International audienceHigh resolution x-ray diffraction has been used to study the effect of doping level and illumination on the formation of n-type porous silicon. For highly doped n+ n+- type porous layers prepared in darkness, an unusual increase of the lattice mismatch with the formation time is observed. When the samples are prepared under illumination, a degradation of the crystal quality, increasing with a decrease of the material doping level, is observed. The possible origins of these effects are discussed

Birefringence study of the irreversible behavior of the incommensurate phase of quartz: effects of chemical impurities

A systematic study of the variations of the irreversible properties of the incommensurate phase of quartz has been performed by birefringence measurements in 9 samples (either synthetic or natural) of various compositions determined by chemical analysis and infrared spectroscopy. The shift of the α-β transition temperature for large impurity concentrations has been verified. The properties of the incommensurate phase are very sensitive to small changes of impurity contents : a global hysteresis going from 10 to 220 mK has been correlated to the OH infrared absorption. Time dependent pinning steps and memory effects have been observed showing the importance of mobile impurities. Finally a regime of discontinuous variation of physical properties appears for slow rate of temperature variations. These results have been discussed in relation to various theoretical models and in particular to the theory of the Defect Density Wave, induced by the diffusion of mobile impurities.Une étude systématique des propriétés irréversibles de la phase incommensurable du quartz a été faite par des mesures de biréfringence sur 9 échantillons (synthétiques ou naturels) de différentes compositions, déterminées par analyse chimique et spectroscopie infrarouge. Les décalages de la température de la transition α-β, produits par des concentrations importantes d'impuretés ont été mesurés. Les propriétés de la phase incommensurable sont très sensibles à la présence de faibles taux d'impuretés: des hystérésis globales variant de 10 à 220 mK, ont été mesurées et une corrélation avec l'absorption infrarouge due aux liaisons OH a été trouvée. Des effets dépendant du temps tels que des marches d'accrochages et des effets mémoires ont été observés, montrant l'importance des défauts mobiles. Enfin, un régime de variations discontinues des propriétés physiques apparaît pour les petites vitesses de variation de la température. Les résultats de ces expériences ont été comparés à diverses prédictions théoriques, telles que celles de la théorie des Ondes de Densité des Défauts dues à la diffusion des impuretés mobiles

CALCULATION OF THE ELASTIC AND ELECTROSTATIC ENERGIES OF AN INCLUSION IN A FERROELECTRIC OR A PIEZOELECTRIC CRYSTAL

Previous optical observations have shown that the first order structural phase transitions of some ferroelectric (e.g. KD2PO4) or piezoelectric (e.g. quartz) crystals present some similarity with martensitic transformations : two phases with different lattice parameters coexist over a small temperature range and the low temperature phase can be twinned ; (this twinning corresponds to alternate shears and electric polarization in the case of KD2PO4). In contrast with martensites both elastic and electric properties are important in determining the morphology of the heterophase structure. In particular the habit plane separating the two phases is no longer a plane without long range strains when electric energies are not negligible. In this case one has to determine the configuration for which the sum of the elastic and electric energies is minimum : this can be done for an inclusion of the new phase in an infinite matrix of the parent phase by extending Khachaturyan's theory to take into account the electric and piezeoelectric properties. We present an outline of this theory in the case of crystals of any symmetry. The effect of a uniform applied stress or of an electric field is also discussed