High-pressure phase transitions in BiFeO3: hydrostatic vs. non-hydrostatic conditions

We report high-pressure x-ray diffraction experiments on BiFeO3 (BFO) single crystals in diamond-anvil cells up to 14 GPa. Two data sets are compared, one in hydrostatic conditions, with helium used as pressure-transmitting medium, and the other in non-hydrostatic conditions, with silicon oil as pressure-transmitting medium. It is shown that the crystal undergoes different phase transitions in the two cases, highlighting the high sensitivity of BFO to non-hydrostatic stress. Consequences for the interpretation of high-pressure structural studies are discussed.Comment: 6 pages, 4 figure

Multiple high-pressure phase transitions in BiFeO3

We investigate the high-pressure phase transitions in BiFeO3 by single crystal and powder x-ray diffraction, as well as single crystal Raman spectroscopy. Six phase transitions are reported in the 0-60 GPa range. At low pressures, up to 15 GPa, 4 transitions are evidenced at 4, 5, 7 and 11 GPa. In this range, the crystals display large unit cells and complex domain structures, which suggests a competition between complex tilt systems and possibly off-center cation displacements. The non polar Pnma phase remains stable over a large pressure range between 11 and 38 GPa, where the distortion (tilt angles) changes only little with pressure. The two high-pressure phase transitions at 38 and 48 GPa are marked by the occurence of larger unit cells and an increase of the distorsion away from the cubic parent perovskite cell. We find no evidence for a cubic phase at high pressure, nor indications that the structure tends to become cubic. The previously reported insulator-to-metal transition at 50 GPa appears to be symmetry breaking.Comment: 11 pages, 8 figure

Experimental evidence that a high electric field acts as an efficient external parameter during crystalline growth of bulk oxide

International audienceA new crystal growth device, in which a high static external electric voltage (up to 14 kV) is added to a floating zone method, is described. Our first experiments show that the application of such an electric field acts like an external force, introducing a pressure effect which is in direct competition with temperature in the solid/liquid thermodynamic equilibrium. High electric fields could therefore be an additional parameter in crystal growth, opening original routes to the synthesis of new materials

High-pressure investigations of CaTiO3 up to 60 GPa using X-ray diffraction and Raman spectroscopy

In this work, we investigate calcium titanate (CaTiO3 - CTO) using X-ray diffraction and Raman spectroscopy up to 60 and 55 GPa respectively. Both experiments show that the orthorhombic Pnma structure remains stable up to the highest pressures measured, in contradiction to ab-initio predictions. A fit of the compression data with a second-order Birch-Murnaghan equation of state yields a bulk modulus K0 of 181.0(6) GPa. The orthorhombic distortion is found to increase slightly with pressure, in agreement with previous experiments at lower pressures and the general rules for the evolution of perovskites under pressure. High-pressure polarized Raman spectra also enable us to clarify the Raman mode assignment of CTO and identify the modes corresponding to rigid rotation of the octahedra, A-cation shifts and Ti-O bond stretching. The Raman signature is then discussed in terms of compression mechanisms.Comment: 11 pages, 6 figures, 4 table

Rotation de la polarisation dans les systèmes morphotropiques (Cas de Pb(Sc1/2Nb1/2)O3-PbTiO3)

CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Les matériaux multiferroïques

L’origine du couplage de différentes propriétés physiques au sein d’un même matériau est un sujet central de la physique de la matière condensée et interpelle les scientifiques depuis des siècles. Nous nous intéressons ici à une classe de matériaux qui possèdent simultanément plusieurs propriétés dites ferroïques (ferromagnétisme, ferroélectricité et/ou ferroélasticité), dont le couplage dépend de la mise en forme, de la structure cristallographique et de l’arrangement des spins magnétiques, et constitue un sujet qui combine un fascinant défi scientifique avec un grand potentiel d’applications

Observation of rotation of polarization in thin films of Pb(Sc1/2Nb1/2)0-3-PbTi03 via a monoclinic phase

Using a simple methodology based on the classical Bragg–Brentano X-ray geometry we have evidenced at room temperature a pseudo-cubic phase and a monoclinic (Pm) phase in thin films of PbSc1/2Nb1/2O3 (PSN) and (PbSc1/2Nb1/2O3)0.57–(PbTiO3)0.43 (PSN–PT) respectively, the latter compound corresponding to a morphotropic phase boundary (MPB) composition. Depending on the substrate and the thickness, different domains configuration are obtained. Competition between epitaxial and thermal stress induces a complex structural evolution with temperature which is different from that of ceramic samples. Indeed PSN films transform from a pseudo-cubic (triclinic) phase towards a cubic phase whereas in PSN–PT, a succession of transformations from a pure Pm phase toward a tetragonal (possibly non polar) phase is observed, a cubic phase never being reached up to 950 K