Enlightening the Alkali Ion Role in the Photomagnetic Effect of FeCo Prussian Blue Analogues
Résumé
Fe-Co Prussian blue analogues of general formula A x Co y [Fe(CN) 6 ] z are responsive, non-stoichiometric materials whose magnetic and optical properties can be reversibly switched by light irradiation. However, elucidating the critical influence of the inserted alkali ion, A + , on the material's properties remains complicated due to their complex local structure. Here, by investigating soluble A[Fe 4-Co 4 ] cyanido cubes (A = K, Rb and Cs), both accurate structural and electronic information could be obtained. Firstly, XRD analyses reveal distinct interactions between the inserted A + ions and the {Fe 4-Co 4 } box, which impact the structural distortion in the cubic framework. These distortions vanish and a displacement of the small K + ion from a corner toward the center is observed as a cobalt corner Co II HS is oxidized to Co III LS. Secondly, cyclic voltammetry experiments performed at variable temperatures show distinct splitting of the Co II HS Co III LS peak potentials for the different A + cations, which can be qualitatively linked to different thermodynamic (standard potentials) and kinetic (energy barriers) parameters associated with the structural reorganization accompanying this redox coupled spin state change. Moreover, for the first time, photo-magnetism was investigated in frozen solution to avoid effects of intermolecular interactions. The results show that the metastable state is stabilized following the trend K > Rb > Cs. The outcome of these studies suggests that the interaction of the inserted alkali ions with the cyanide cage and the structural changes accompanying the electron transfer impact the stability of the photo-induced state and the relaxation temperature: the smaller the cation, the higher the structural reorganization and the associated energy barrier, and the more stable the metastable state.
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