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UCL - Dr Eric BOUSQUET - Geometric Ferroelectricity and multiferroism in Fluoro-Perovskites

Le Jeudi 18 Juin 2015 de 11h00 à 12h00
Dr Eric BOUSQUET
Physique Théorique des Matériaux, Université de Liège
will give a seminar entitled
Geometric Ferroelectricity and multiferroism in Fluoro-Perovskites
on Thursday 18 June 2015, 11:00 a.m.,
seminar room of building SC17 (SST/IMCN/NAPS,
Chemin des Etoiles, 8 bte L7.03.01) – LLN
For more information please contact the secretariat of the division: vinciane.gandibleux@uclouvain.be

Because of its large polarizability, the metal-oxygen bond in perovskite oxides ABO3 is
particularly favorable for promoting a transition-metal off-centering in the MO6 octahedra.
This property has been identified to be the key mechanism for ferroelectricity in ABO3
perovskites such as BaTiO3. Ferroelectrics also exist, in several other material chemistries
that do not contain oxygen, such as fluorine-based compounds, including polymers and
ceramics and thus in many crystal classes and stoichiometries. Given the low polarizability of
M-F bond, we can expect the mechanism for ferroelectricity in fluorine-based crystals to be
distinct from the one found in oxides. For example, it has been observed that molecular
reorientations are the main mechanism of ferroelectricity in polymers. These alternative
mechanisms are of particular interest because, unlike the oxides, they are not contraindicated
by partially filled transition metal d-orbitals, and so they have a high potential in
having simultaneous ferroelectric and magnetic ordering (multiferroicity). However, none of
the known fluorides crystals with the perovskite structure has been reported to be
ferroelectric.
Here we present a first-principles study to investigate the possible existence of a
ferroelectric instability in magnetic ABF3 fluoro-perovskites. Interestingly, we find that
several fluoro-perovskites have a sizeable ferroelectric instability in their high symmetry
cubic structure while their ground state is non polar. In contrast to the oxides, however, the
fluorides have close to nominal Born effective charges, indicating a different mechanism for
the ferroelectric instability. We will show that the ferroelectric instability in ABF3 actually
originates from a geometric-like effect rather than a charge transfer, which drives unusual
responses with respect to the oxides.
Going beyond, we will also show that in fluoro-perovskites coherent epitaxial strain
allows to destabilize the non-polar bulk Pnma ground sate structure toward a ferroelectric
ground state, and thus making them good multiferroics due to the presence of large noncollinear
ferromagnetic spin canting. The induced polarization is not large (though of several
μC/cm-1), as expected from nominal Born effective charges, but we found a purely non-linear
polarization/strain response that does not follow the usual linear dependence observed in
ferroelectric ABO3 perovskites. This unusual behaviour drives large non-linear peizoelectric
and magnetoelectric responses. All of that makes the fluorides of large interest in the
understanding of the physics of ferroelectrics as well as in their possible technological
applications.

Dernière mise à jour par EDT MAIN Mardi 12 Mai 2015