Photomagnetism and structural distortions — The secret relationship unravelled by XMCD in the hard X-ray range
KEY-WORDS : XMCD at the K-edge of transition metal ; Local structure ; Magnetism ; Photoswitching properties ;Hard x-ray spectroscopy ; Methodological developments
Photoswitchable molecular compounds containing transition metals (TM) are appealing candidates for the future of electronic and high-density data storage. Among them, Prussian Blue Analogs (PBAs) of general formula CxA4[B(CN)6](8+x)/3.nH2O (C= alkali cation; x=0-4; A,B = TM of the first row) present photoswitching properties depending on the TM and the stoichiometry; however these properties are usually observed at low temperature (below 150K). Our research aims at overcoming this working temperature issue that limits their applications. According to literature and our previous results, a promising solution seems a gentle modification of the distribution of the electronic density along the A-NC-B linkage via a slight structural distortion around the photoactive species.
The goal of this MagDiDi project is therefore to completely elucidate the relationship between slight structural distortions and photoswitching properties, which will allow adjusting the working temperature. However, a tool able to quantify the slight structural distortions involved in the photoswitching properties of PBAs is still missing. So we propose here to develop a new methodology based on the X-ray Magnetic Circular Dichroism (XMCD) at the TM K-edge, which is a derived technique from X-ray Absorption Spectroscopy (XAS). Our first investigation under pressure (0-7 GPa) of the non-photoswitchable NiFe PBA family indeed demonstrated that XMCD is very sensitive to small structural distortions considered in the adjustment the photoswitching properties.
Our first task is (i) to synthesize and characterize model PBAs, and (ii) to establish from a pivotal PBA a scale that related the variations of the XMCD signal to small structural distortions of the bimetallic cyanide network and to the lifetime of photoswitchable compounds; this scale will then be used to chemically adjust the working temperature of the latter. The second task consists in (i) identifying and disentangling the physical effects originating TM K-edge XMCD, which are still poorly understood, and (ii) establish the quantitative relationship between XMCD signals and structural distortions of the A-NC-B linkage, and consequently between structural distortions and photoswitching properties. These two tasks, which will especially enable to develop a new spectroscopic tool, will be achieved by a systematic study of PBAs from task 1 by XMCD at the K-edge of both TM present in the PBA; these PBAs will be chosen to independently vary the electronic and structural parameters that can have an effect on the XMCD signal. To go further, this experimental part will be coupled to a theoretical study of the XMCD signals for the most relevant PBAs using a DFT-based approach, already used to finely interpret x-ray absorption spectra. Once these two tasks are finished, the last task will be to apply our new tool to other families of compounds and/or compounds of smaller size (oxides, alloys, PBA nanoparticles…) in order to extend the use of the new spectroscopic tool to other scientific concerns.
The results of this project, mainly fundamental, are at the interface between physics and chemistry. They will be a major step in our understanding of (i) the relationship between structure and photoswitching properties in PBAs as well as their derivatives (nanomaterials, molecules), and (ii) the physical processes behind the TM K-edge XMCD. We will thus improve our general knowledge on magnetism and on the interaction of X-ray with matter. Finally, the new methodology will be a priceless tool for all the communities interested in magnetic properties in relationship with the fine structure of materials, whatever the type of compound and its size.
Coordinator : Amélie Bordage
Other involved people :
- Anne Bleuzen : Scientific collaborator, PhD advisor of the ANR-funded PhD
- Giulia Fornasieri : Scientific collaborator for nanomaterials
- Eric Rivière : Technical collaborator for SQUID magnetometry
Partner: Institut de Chimie Moléculaire et des Matériaux d'Orsay
Duration : 4 ans (January 1st 2018 - May 31st 2022)
Grant : 212 247 €
PhD and internships in the framework of the ANR project
PhD (Starting on 2018/10/01)
Advisor : Anne Bleuzen ; Co-advisor : Amélie Bordage (Doctorate school 571 "Chemical Sciences : Molecules, materials, instrumentation and biology")
The increasing demands for improved data storage eventually require a technological mutation. Photoswitchable molecular materials offer appealing perspectives and the goal of this thesis project is to transform this attractive idea into a realistic alternative. Metal-metal charge-transfer inorganic compounds are very interesting candidates but their photoswitching properties are observed at too low temperature to be used in any application. One way to adjust their property is to modulate the fine distortions of their structure. The objective of this interdisciplinary (physics and chemistry) thesis is therefore to elucidate the relationship between slight structural distortions and photoswitching properties in model compounds: Prussian Blue Analogs (PBAs). To reach this goal we will develop a new tool based on the X-ray magnetic circular dichroism (XMCD) at the K-edge of transition metals. PBAs will be first used as model-compounds so that chemistry helps solving physical problems; they will enable to establish a methodology for the quantification of slight structural distortions from TM K-edge XMCD and to deeply understand physical processes originating TM K-edge XMCD. PBAs will then be considered as solutions to the problems currently meet in the area of data storage, this time the physical tools allowing chemistry to answer today’s technological challenges.
The subject described in details is available HERE .
- A multi-beamlines study of bimetallic Prussian Blue analogs for methodological developments in transition metal K-edge XMCD. A. N'Diaye, A. Bordage, A. Bleuzen (14th SOLEIL USers'meeting, Gif-sur-Yvette (France), January 17th-18th 2019)
- Investigation of the structure-property relationship in Prussian Blue Analogs by a coupled laboratory/multi-beamlines synchrotron approach. A. N'Diaye, A. Bordage, A. Bleuzen (GECOM CONCOORD, Erquy (France), May 19th-24th 2019)
- Investigation of bimetallic Prussian Blue Analogs properties for the methodological developments in transition metal K-edge XMCD. A. N'Diaye, A. Bordage, A. Bleuzen (Journées Scientifiques GDR MCM2, Dourdan (France), November 25th-27th 2019)
- Towards the quantification of small structural distortions in bimetallic Prussian Blue analogs by XMCD at the transition metal K-edge. A. N'Diaye, A. Bordage, A. Bleuzen (15th SOLEIL Users'meeting, Gif-sur -Yvette (France), January 16th-17th 2020)
1. Investigation of the photomagnetism origin in Prussian Blue analogues — L3 (13th Jan - 31st July) => Data treatment of X-ray absorption spectra
1. Investigation of the photomagnetism origin in Prussian Blue analogues — M2 (5th Feb - 13th July) => Preparation of model compounds, experiment on the ODE beamline (SOLEIL synchrotron), data treatment of X-ray absorption spectra
2. Syntheses and characterizations of CoFe and NiCr Prussian Blue analogues — IUT (23rd Ap - 3rd July) => Preparation of model compounds
3. Syntheses and characterizations of CuFe and MnFe Prussian Blue analogues — IUT (23rd Ap - 3rd July) => Preparation of model compounds
4. Investigation by X-ray spectroscopy of Prussian Blue Analogs — Mag1 (28th May - 20th July) => Data treatment of X-ray absorption spectra