Mael Guennou: Polarity in (inorganic) bismuth-based perovskites
|Wanneer:||do 23-06-2016 11:00 - 12:00|
The peculiar crystal chemistry of bismuth-based perovskites has attracted a considerable attention in the field of functional oxides, in relation to important scientific and technological issues, such as the elimination of lead from piezoelectric devices, and the design of intrinsic multiferroic materials. The key element is the presence of an electron “lone pair” 6s2, which imparts to the Bi3+ ion a tendency to off-center in its coordination polyhedron, and, from a macroscopic point of view, induce ferroelectric and piezoelectric properties. Yet, not all BiMO3 are ferroelectric; but ferroelectricity has to compete with other structural distortions of the perovskite structure, notably the tilts of the oxygen octahedra, and cooperative Jahn-Teller distortion. The delicate balance between these competing – or sometimes collaborating – instabilities can be further tuned by external parameters, such as electric field, pressure, or epitaxial strain in thin films.
Over the past few years, we have investigated phase diagrams of Bi-based perovskites using high hydrostatic pressure, which allows modifying the interatomic distances and, thus the interactions, to a much larger extent than any other parameter. Our complementary use of Raman scattering and synchrotron X-ray diffraction has revealed a variety of phase transitions and exciting phenomena in significant Bi-based perovskites, notably: (i) a remarkable sequence of transitions in the model multiferroic BiFeO3 interpreted as the presence of “nano-twins”; (ii) an unexpected polar phase with giant distortion in BiMnO3 at very high pressure, associated to a reduction of the cooperative Jahn-Teller distortion; (iii) a comparatively puzzling stability of the orthorhombic phase up to unprecedented strain states in BiCrO3; and (iv) insulator-to-metal transitions in the 50 GPa range in all cases. This talk will summarize our investigations of high-pressure phase diagrams, with emphasis on the most recent results.More recently, following the renewal in the field of photoferroelectrics – i.e. studies of interaction of light and ferroelectric materials, Bi-based perovskites have also attracted attention due to their band-gap in the visible range and the variations of their optical properties with temperature, pressure and electric field. Yet the details of their electronic structures and electronics levels remain so far unclear. We will show this can be tackled using resonant Raman spectroscopy on the model perovskite BiFeO3