Crossover phenomena

Bousseksou A, Varret F, Goiran M, Boukheddaden K, Tuchagues J-P (2004) The Spin Crossover Phenomenon Under High Magnetic Field. 235 65-84 Bousseksou A, see Tuchagues J-P (2004) 235 85-103 Bowers MT, see Wyttenbach T (2003) 225 201-226... 

Tuchagues J-P,BousseksouA,Molnkr G, McGarvey JJ,VarretF (2004) The Role of Molecular Vibrations in the Spin Crossover Phenomenon. 235 85-103 Tuchagues J-P, see Bousseksou A (2004) 235 65-84 Tuherm T, see Samoson A (2005) 246 15-31... 

The Spin Crossover Phenomenon under High Magnetic Field... 

Over the past few decades, a large variety of ligand systems have been tested with the aim of obtaining novel iron(II) spin crossover systems which could possibly be utilised in electronic devices [1]. In most cases an Fe(II)N6 chro-mophore is required in order to generate the spin crossover phenomenon [2]. A large majority of the ligands used are represented by heterocyclic systems, in which the lone electron pair on the nitrogen atom coordinates to the Fe(II) ion. 

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

The Discovery of the Spin Crossover Phenomenon for Iron(III) Compounds 261... 

Despite these differences, the similarities predominate and virtually all the features noted for spin crossover in iron(II) are also found for iron(III). Because of the great emphasis on the cooperative aspects of the spin crossover phenomenon, iron(II) systems have tended to dominate more recent research. However, there are very striking examples among the iron(III) systems which are of strong relevance to these aspects and there is certainly scope for future work in this area. This is evident in much of the very recent work where it can be seen that specific strategies to increase the cooperativity have been successful and have led, for example, to solid iron(III) systems which display the LIESST effect [137, 138]. The generation of polymeric species as a means of increasing cooperativity, an approach which has been widely adopted for iron(II), has received relatively little attention for iro-n(III) and this is an area which can be expected to be exploited further. 

De Gennes applied his method to study the chain behaviour similar to that used by Wilson (1971) to study magnetic phenomena. He pointed out that at a certain concentration the behaviour of a polymer chain is analogous to the magnetic critical and tricritical phenomena. De Gennes classifies the concentration c into three categories the dilute solution d, the semi-dilute solution c and the concentrated solution c". The concentration c is equivalent to the critical point where the crossover phenomenon occurs from randomness... 

The major problem associated with the operation of DMFCs is the gradual diffusion of methanol through the membrane - known as methanol crossover - that leads to the establishment of a mixed potential at the cathode and, consequently, to a decrease of the working voltage of the cell. Because of the methanol crossover phenomenon, the maximum methanol concentration used in DMFCs is about 2M and membranes as thick as 175 pm are used. 

Utilising the Spin Crossover Phenomenon - From Rational Design to Functional Molecular Materials... 

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