Anne Bleuzen

Molecules to build solids: high TC molecule-based magnets by design and recent revival of cyano complexes chemistry

Abstract This paper was presented as a session lecture at the XXXIII ICCC in Florence (29 August–4 September 1998). It intends to point out some recent achievements in the chemistry and physics of transition metal polycyanides in the field of molecular magnetism. Prussian blue is sometimes considered as the first coordination compound and the paper shows how it is possible to obtain brand new results with Prussian blue analogues when looking at these antique systems with fresh eyes. Hexacyanometalates revealed in the last few years as very flexible molecular precursors to build three-dimensional molecule-based magnets with tunable and high Curie temperatures or to grow high nuclearity clusters with tunable high spins and anisotropy. The use of a localized electron orbital model allowed the authors’ team to push the Curie temperatures from 5.6 K in the Prussian blue itself to above room temperature in a vanadium–chromium Prussian blue analogue. Several groups confirmed the result and are improving it. In the same way, high spin molecules with ground spin states ranging from S=3/2 to 27/2 were obtained. The paper reviews some of the steps which lead to these spectacular findings and some of the prospects opened in molecular materials by this revival of polycyanide chemistry.

Generic features of a photo-induced magnetic structure, exhibited by the Rb0.52Co[Fe(CN)6]0.84, 2.31H2O Prussian Blue analogue

Appealing generic features of photo-induced magnetic structures are illustrated on the example of the Prussian Blue analogue Rb0.52Co[Fe(CN)6]0.84, 2.31H2O: (i) the ability of controlling by light the concentration in magnetic moments. This is conveniently achieved by combining an initial complete photo-transformation of the material, and various subsequent thermal annealings in the range 80–100 K. The dilution phase diagram of the ferrimagnetic system is, for the first time, derived. (ii) The magnetic metastability of the ‘raw photo-induced’ (RPI) state obtained at low temperatures (below Tc) and in low magnetic fields (below Hc). The RPI state is characterised by a non-relaxed domain structure, due to the lack of thermal fluctuations during the building process of the ferrimagnetic domains. It evolves irreversibly under the effect of temperature or magnetic field. The thermal aging properties of the RPI state are characterised.

Photo-Induced Electron Transfer and Magnetic Switching in CoFe Cyanides: Study of the Metastable State

Abstract Photo-induced magnetisation has been recently evidenced by Hashimoto et al. in CoFe cyanide[1]. We synthesized a diamagnetic RbFeCo cyanide able to present the same photomagnetic effect. The magnetic properties of the excited state, the electronic structure and the local structure of the ground and the excited states have been investigated. The conditions required to observe the phenomenon and the mechanism of the electron transfer are discussed.

Photomagnetism in CxCo4[Fe(CN)6](8+x)/3·n H2O Prussian blue analogues: looking for the maximum photo-efficiency

Abstract A family of CoFe Prussian blue analogues CxCo4[Fe(CN)6](8+x/3)□(4–x)3 (xxa0=xa0amount of alkali cation inserted per conventional cell, Cxa0=xa0Na, K, Rb, Cs; □xa0=xa0[Fe(CN)6] vacancy) have been synthesized and characterized. Their photomagnetic properties have been investigated by magnetic measurements before and after irradiation and X-ray diffraction under continuous irradiation. We show that the photo-induced magnetism depends on several parameters: (i) the amount of CoIII–FeII diamagnetic excitable pairs per cell; (ii) the amount of [Fe(CN)6] vacancies, and (iii) the amount and nature of the alkali cations per cell. We evidence a discontinuity in the properties change when the amount of alkali cation x varies, around xxa0=xa01. For x

Magnetic Metastability of the Photo-Induced State of CIxCo[Fe(CN)6]y Prussian Blue Analogues (CI=Rb, Cs)

A generic feature of photo-induced magnetic structures is reported, on the example of the photo-magnetic Prussian Blue analogues CxCo[Fe(CN)6]y, zH2O (C=Rb, Cs): the magnetic metastability of the “raw photo induced” (RPI) state obtained at low temperatures (below TC) and in low magnetic fields (below HC). The RPI state is characterised by a non-relaxed domain structure, due to the lack of thermal fluctuations during the building process of the ferrimagnetic domains. It evolves irreversibly under the effect of temperature or magnetic field. Some insight is given into the building mechanism of the RPI state, by consideration of the progressive increase of the Curie temperature.