Institut de Chimie Moléculaire et des Matériaux d'Orsay

 

Photomagnetism and structural distortions — The secret relationship unravelled by XMCD in the hard X-ray range

ANR JCJC "MagDiDi" no. ANR-17-CE29-0011

 

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 1^st 2018 - December 31^st 2021) — Prolongated till April 30^th 2023

Grant : 212 247 €

 

1. A. N'Diaye, A. Bordage, L. Nataf, F. Baudelet, T. Moreno, A. Bleuzen. A cookbook for the investigation of coordination polymers by transition metal K-edge XMCD (2021) Journal of Synchrotron Radiation 28:1127-1136

2. A. N'Diaye, A. Bordage, L. Nataf, F. Baudelet, E. Rivière, A. Bleuzen. Towards quantitative magnetic information from transition metal K-edge XMCD of Prussian blue analogs (2022) Inorganic Chemisry 61:6326-6336

3. A. N'Diaye, A. Bordage, L. Nataf, F. Baudelet, E. Rivière, A. Bleuzen. Interplay between transition metal K-edge XMCD and magnetism in Prussian Blue analogs (2022) ACS Omega 7:36366-36378

4. A. Bordage, A. N'DIaye, A. Bleuzen. Prussian Blue analogs and transition metal K-edge XMCD: A longstanding friendship (2022) Comptes Rendus Chimie 25:281-288

 

PhD (Defended on Feb. 11^th 2022) : XMCD at the K-edge of transition metals and Prussian Blue Analogs : A new approach for the understanding of the signals

Advisor : Anne Bleuzen ; Co-advisor : Amélie Bordage (Doctorate school 571 "Chemical Sciences : Molecules, materials, instrumentation and biology")

The subject described in details is available HERE .

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Last modifications on May 30^th 2023