Pérovskite (structure) | EPFL Graph Search

La pérovskite, du nom du minéralogiste russe L. A. Perovski, est une structure cristalline commune à de nombreux oxydes. Ce nom a d'abord désigné le titanate de calcium de formule CaTiO, avant d'être étendu à l'ensemble des oxydes de formule générale ABO présentant la même structure. Les pérovskites présentent un grand intérêt en raison de la très grande variété de propriétés que présentent ces matériaux selon le choix des éléments A et B : ferroélasticité (par exemple ), ferroélectricité (par exemple ), antiferroélectricité (par exemple PbZrO), ferromagnétisme (par exemple YTiO), antiferromagnétisme (LaTiO) Pérovskite comme structure cristalline Pérovskite idéale cubique La structure pérovskite de plus haute symétrie est une structure de symétrie cubique. C'est par exemple la structure du titanate de baryum BaTiO à haute température (voir figure ci-contre). Dans la structure pérovskite cubique, les anions (ici O2−) forment un réseau d'octaèdres liés par l

Siedlung Halen à Berne. Sauvegarde et extension

Chloé Coninckx

En 1955, cinq dessinateurs fascinés par le Corbusier fondent l'Atelier 5 pour mener à bien leur premier projet communautaire dans une clairière proche de Berne: la Siedlung Halen. S'inspirant de la vielle-ville de Berne, mais dans un langage résolument moderne, ils conçoivent des maisonnettes en bande, comme autant de variations d'assemblages combinatoires, dans l'objectif d'assurer une structure collective efficace. Son langage architectural accompli, la régularité de l'agencement typologique, la structure “simple” et rationnelle en font un ensemble d'une originalité indéniable. Concrètement, des éléments structurants tels que les murs transversaux entre les propriétés, les allées couvertes, les seuils, les ruelles et les places contribuent à l'identité de Halen. La Siedlung est aujourd'hui considérée comme un témoignage majeur de la pratique engagée et novatrice de l'Atelier 5; sa valeur intrinsèque en fait un monument de l'architecture moderne en Suisse. Son état actuel est globalement satisfaisant. Il s'agit malgré tout d'assurer la pérennité de l'ensemble en effectuant quelques interventions ciblées qui répondent aux nouveaux impératifs énergétiques. Dans le but de redynamiser la vie collective, le projet intègre l'extension en toiture des logements existants, l'ajout de nouvelles unités au Sud, un pavillon d'accueil pour les visiteurs, une station vélo (réparation et parking) et l'installation d'une coopérative maraîchère et d'un dépôt vente.

2013

Origins of the macroscopic symmetry breaking in centrosymmetric phases of perovskite oxides

Sina Hashemi Zadeh

Many perovskite materials experience a temperature-driven phase transition at the Curie temperature from a non-centrosymmetric polar ferroelectric phase to a paraelectric phase, where polarization is lost. The paraelectric phase is usually centrosymmetric and therefore non-piezoelectric. However, ferroelectrics still show some forms of electro-mechanical coupling in their paraelectric phase like flexoelectricity, which is not symmetry limited and is practically strong in ferroelectrics. It has been shown that the flexoelectric coefficient of perovskite (Ba0.67,Sr0.33)TiO3 in its centrosymmetric paraelectric state at room temperature is up to three to four orders of magnitude larger than its theoretically predicted values. Such enormous discrepancy demonstrates that either the theory is unable to fully explain the flexoelectric response, or that another electro-mechanical mechanism is playing a role in the apparently large flexoelectric response of the material. It was found that Ba0.60Sr0.40TiO3 (BST6040) samples with apparently large flexoelectric coefficient exhibit symmetry breaking and consequently exhibit forbidden piezoelectric- and pyroelectric-like behavior in its paraelectric phase. In this thesis work, our interest lies in describing and interpreting the conditions where breaking of the symmetry is not associated with the external field nor with the flexoelectricity, but is proper to the material. Origins and mechanism of symmetry breaking were investigated in two different cases: local breaking of centrosymmetry; and macroscopic symmetry breaking. The local breaking of the centrosymmetry may refer to the existence of microscopic polar entities and/or disordered displacement of ions in the unit cell. The global symmetry breaking could be associated with a non-uniform distribution of charged defects and/or the coupling of polar nanoâregions with the strain gradient in the sample. We compared dielectric, elastic, and pyroelectric properties of the Ba1-xSrxTiO3 (BST) family, pure BaTiO3 in perovskite (p-BTO) and hexagonal (h-BTO) forms, and SrTiO3 to study the governing mechanisms of symmetry breaking. In addition, the atomic structure of BST6040 was studied by high resolution scanning transmission electron microscopy for direct evidence of polar regions and to understand their exact nature and origin. According to experimental data, the paraelectric phase of p-BTO shows local and global breaking of nominal centrosymmetricity and possesses microscopic and macroscopic polarizations, which are associated with precursors of the ferroelectric phase ordered by the strain gradient in the sample. On the other hand, h-BTO does not exhibit macroscopic pyroelectric response because the precursors are paraelectric-ferroelastic at room temperature, and therefore non-polar. Studying the material response to dynamic electric field showed that the Rayleigh-like relation, which describes dynamics of domain walls in a random pinning environment, cannot describe the dynamics of polar entities in these materials. Results of our high-resolution atomic studies showed distinct static polar clusters with the average size of few nanometers where A-site, B-site and O ions are displaced from their ideal cubic positions. We believe that all presented experimental data can be consistently explained by the presence of microscopic precursors of the ferroelectric phase and their interaction with the strain gradient in the material.

EPFL2017

Piezoelectric anisotropy and free energy instability in classic perovskites

Marko Budimir

Several members of the perovskite family are investigated: the temperature dependence of dielectric, elastic, piezoelectric and coupling coefficients of KNbO3 in its orthorhombic ferroelectric phase is determined experimentally; the values of dielectric stiffness coefficients at constant stress, α, are estimated for the 6th order Landau-Ginzburg-Devonshire phenomenological description of KNbO3; the origins of the enhanced piezoelectric responses along non polar directions in perovskites are investigated by studying phenomenologically the temperature evolution of the piezoelectric anisotropy in a material with a sequence of ferroelectric-ferroelectric phase transitions (BaTiO3) and in a material with only one ferroelectric phase (PbTiO3); the influence of external bias fields on piezoelectric response and its anisotropy in ferroelectric perovskites is discussed phenomenologically by studying tetragonal BaTiO3, PbTiO3 and Pb(Zr,Ti)O3 under electric bias field applied anti-parallel to the spontaneous polarization and uniaxial compressive bias stress along the polarization direction; a discussion about a common underlying thermodynamic process able to generally describe the enhancement of the piezoelectric response and its anisotropy is given by investigating the Gibbs free energy flattening in tetragonal BaTiO3, PbTiO3 and Pb(Zr,Ti)O3 upon changes of temperature, bias fields and composition. The main conclusions resulting from this work are: a comparison of the results of electromechanical properties measurements on KNbO3 obtained in this work, the ones found in the literature, and estimates using LGD phenomenology from this work gives discrepancies that suggest that published measurements should be redone; in the absence of bias fields, the intrinsic origin of the enhanced piezoelectric responses in perovskites is the anticipation of a phase transition, no matter what is the cause of that transition (temperature, composition); in the presence of sufficiently high bias fields, an enhanced piezoelectric response along non-polar direction can be predicted in some materials (PbTiO3 under high uniaxial compressive stress along the polar direction); the influence of the external bias fields on electromechanical properties of perovskites may generally be of significant importance: if the electric fields applied anti-parallel to the spontaneous polarization or the uniaxial compressive bias stresses applied along the polarization axis are high enough, the perovskite system can be strongly dielectrically softened (metastable state), increasing the values of dielectric permittivities and piezoelectric coefficients; in the vicinity of a coercive field or a phase transition these electromechanical coefficients can increase by several orders of magnitude; the flattening of the Gibbs free energy profile of each of examined perovskite systems, regardless of whether this flattening is caused by temperature or composition variation, or by applying compressive pressure or antiparellel electric field bias, leads to enhancements of dielectric susceptibilities and of the piezoelectric response; the anisotropy of the free energy flattening is the origin of the anisotropic enhancement of the piezoelectric response, which can occur either by polarization rotation or by polarization contraction.

EPFL2006