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Oxyborate compounds for new multifunctional materials – BORA-BORA

BORA-BORA: OxyBORAte compounds for new multifunctional materials

Large spontaneous polarization is a known property of iron ludwigite, but vast possibilities remain to identify and optimize new ferroelectrics/multiferroics in the ludwigite borates family, while getting a deeper understanding of the fundamentals aspects of multiferroicity.

Main objectives and challenges

Although a giant magnetoelectric effect has been observed in warwickite Fe2BO4, and a spontaneous polarization reaching up to 20 µC/m2 has been measured in mixed valence ludwigite Fe3BO5, the potential of oxyborates as multifunctional materials remain vastly unexplored. Playing with substitution on the many crystallographic sites of the ludwigite structure offer almost infinite possibilities to induce interesting magnetoelectric properties. The objectives of the project were therefore three-fold: <br />- the synthesis of new oxyborate compounds with a ludwigite crystal structure, either in poly- or single-crystal forms, using divalent or trivalent transition metals. <br />- the characterization and identification of new ferroelectrics/new multiferroics. <br />- the understanding of the mechanisms behind ferroelectricity in oxyborates, to disentangle the ingredients necessary to induce multiferroicity, which can have different origins, such as exchange-striction, spin-current, metal-ligand d-p hybridization or polar charge-ordering.

Beside solid state chemistry research for the synthesis of new oxyborates compositions, a combination of experimental techniques has been used: principally X-ray and neutron diffraction, for the studies of crystal and magnetic structures, along with X-ray absorption spectroscopies, to investigate atomic electronic and magnetic configurations, in addition to macroscopic physical measurements including electric polarization.

Amongst the new ludwigite compounds which have been synthesized, the most promising one, with respect to multiferroic properties, is Cu2CrBO5, in which spin driven multiferroicity is arguably at the origin of the spontaneous polarization of 35 µC.m-2 which is observed below magnetic ordering at TN = 120 K.
The whole phase diagram Fe3-xMnxBO5 has been investigated, with compositions being synthesized and studied for the first time ; changes in the magnetic couplings as Mn2+ is introduced, and competing easy-axis anisotropy and magnetic exchanges in the decoupled sub-lattice regime (x < 1.5) are amongst the parameters controlling the properties of this system.
In parallel, the potential of ludwigite oxyborate as electrode materials with high energy density and safety has been studied ; oxyborate represent an interesting new class of conversion-type electrode materials for lithium batteries, as exemplified by electrochemical performances Cu2MBO5 (M = Fe, Mn and Cr) for instance.

Only a fraction of existing boron oxides has been studied in this project, the results obtained on ludwigites can be readily used for further studies on other original oxyborate crystal frameworks .

Those results have led to the publication of three articles in peer reviewed scientific journals ; they have also been presented repeatedly at international conferences.
The Bora-Bora project is a fundamental research project managed by Françoise Damay (Laboratoire Léon Brillouin), in collaboration with Christine Martin (CRISMAT) and Lucie Nataf (SOLEIL synchrotron). This 36 month project started in October 2016, and was supported by an ANR grant of 401 k€.

Electronic ferroelectrics remain up to now largely overlooked and deserve more attention, both on the solid state chemistry side, to find new suitable candidates, and on a fundamental one, to establish the origin and parameters necessary to this type of multiferroicity.
Based on the discovery of a giant magnetoelectric effect in charge-ordered Fe2BO4, the aim of BORA-BORA is to synthesize and characterize in details compounds belonging to two systems of borates, namely the warwickite and the ludwigite. In the search for new ferroelectrics, both structures provide two key features : mixed valence, and low dimensional network, of transition metal atoms, two known ingredients for electronic multiferroicity. The ludwigite structure offers in addition the possibility to play both on the nature of the substituting transition metal cation and on the site on which it will be substituted preferentially.
Synthesis of new compounds and combination of local and larger scale structural characterization techniques with physical properties measurements will be at the core of the project, whose aim is not only to identify new electronic ferroelectrics, but also to shed light on the mechanisms at play in these materials, an issue that is still open up to now. The partnership involves researchers from the Léon Brillouin and the CRISMAT laboratory, as well as the SOLEIL synchrotron. With regards to structural characterizations, X-ray, electron and neutron diffraction techniques will be used to study crystal structures, charge-ordering superstructures phenomena, and non-centrosymmetry issues that are essential in ferroelectrics. XAFS and XANES spectroscopies will give additional information on the valence states of the different cations, their surrounding crystal fields, and if relevant, their spin states. Dielectric constant and pyroelectric polarization measurements will be an important part of the physical characterizations, and will be complemented with neutron diffraction experiments, the knowledge of magnetic structures being necessary to identify the possible magnetoelectric coupling mechanisms. Magnetic circular dichroism experiments will be performed to correlate valence and the local magnetic moment.

Project coordinator

Madame Françoise DAMAY (Laboratoire Léon Brillouin)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

LLB Laboratoire Léon Brillouin
CRISMAT UMR6508
SOLEIL Synchrotron SOLEIL

Help of the ANR 400,895 euros
Beginning and duration of the scientific project: September 2016 - 36 Months

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