Opposite effects of interactions and disorder on the switching properties of the spin transition compound [Fe^II(L)₂][ClO₄]₂ · C₇H₈

We report on a detailed analysis of the thermal transition hysteresis loop and the relaxation curves of the recently investigated compound [FeII(L)2][ClO4]2 · C7H8 [V. Mishra et al., Inorg. Chem. (2008)]. We developed a variant of a previous kinetic cooperative model, modified for including short-range and long-range Gaussian distributions of energy parameters. The resulting set of coupled master equations was solved numerically for the dynamic and the quasi-static case. We have elucidated the various correlations between the distribution widths and the long-range interaction parameter, which basically have opposite effects. It is shown that presence of a steep hysteresis loop may hide a sizable short-range distribution, while the stretched character of the relaxation curves may also hide the presence of strong interactions. We also show that the coherent analysis of the set of relaxation curves at different temperatures does not allow to disentangle properly all the relevant parameters. We discuss these data with respect to the other photomagnetic experiments, T(LIESST) curve and light-induced thermal hysteresis (LITH) loop, and finally we try to address the question of the real nature of the cooperative relaxation process

Iron(II) spin transition 1,2,4-triazole chain compounds with novel inorganic fluorinated counteranions

New one-dimensional spin transition coordination polymers of formula [Fe(NH(2)trZ)(3)](AF(6)) center dot nH(2)O (A = Ti, Zr, Sn; NH(2)trz = 4-amino-1,2,4-triazole) have been synthesized in MeOH and EtOH media. These materials display an abrupt and hysteretic spin transition around 200 K as well as a reversible thermochromic effect on, cooling. A preliminary evaluation of the lattice dynamics in the high-spin and low-spin states is presented. (c) 2006 Elsevier Ltd. All rights reserved

Light-induced phase separation in the [Fe(ptz)6](BF4)2 spin-crossover single crystal

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Spin-transition in nearly cubic site in [Fe<SUP>II</SUP>(L)<SUB>3</SUB>][PF<SUB>6</SUB>]<SUB>2</SUB>

The spin-transition (1A1&#8596;5T2) behaviour of a new mononuclear iron(II) compound [FeII(L)3][PF6]2[L = 2-[3-(2'-pyridyl)pyrazole-1-ylmethyl]pyridine] has been investigated by 57Fe Mossbauer spectroscopy. Analysis of the Mossbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mossbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom

Thermal, magnetic and electrochemical properties of polymeric copper complexes of 2-hydroxy-l,4-naphthoquinone and its methyl derivative

2563-2567The reaction of 2-hydroxy-l, 4-naphthoquinone (Lawsone;Lw) and 3-methyl-2-hydroxy-l, 4-naphthoquinone (Phthiocol;Phth) with anhydrous CuCl2 results in [Cu(Lw)2(NH3)2] (M-l) and [Cu(Phth)2(NH3)].2H2O (M-2) complexes respectively. Thermal, magnetostructural and electrochemical studies indicate the polymeric nature of both the complexes. The energy of activations calculated from the thermogramms is found to be 65 kJ/Lw, 23.75 kJ/NH3 in M-1 and 178.90 kJ/H2O, 221.5 kJ/NH3, 566.2 kJ/Phth and 28.19 kJ/Phth in M-2. The increasing and decreasing nature of χm.T with decrease in temperature in M-l and M-2 reveals the ferro and antiferromagnetic interactions in M-l and M-2 respectively. The cyclic voltammetry studies indicate occurrence of effective metal ligand interactions in solution for both the complexes

Nonexponential relaxation of the metastable state of the spin-crossover system [Fe(L)<SUB>2</SUB>](ClO<SUB>4</SUB>)<SUB>2</SUB>&#183;H<SUB>2</SUB>O [L = 2,6-Bis(pyrazol-1-ylmethyl)pyridine]

The relaxation of the metastable state of the spin-crossover compound [Fe(L)2](ClO4)2&#183;H2O, with L = 2,6-bis(pyrazol-1-ylmethyl)pyridine, populated by the LIESST (light induced excited spin state trapping) effect, has been investigated by magnetic measurements. The time dependence of the relaxation curve at several temperatures, starting from different initial states, is in the shape of stretched exponentials, and the thermal variation of the photostationary state under constant photoexcitation is progressive and reversible. These features are satisfactorily modeled by considering noninteracting two-level systems with a distribution of activation energies. A suggested origin for the distribution is the conformational flexibility of the nonplanar heterocyclic ligands. The effect of the intensity distribution during the LIESST process is also accounted for in a simple way

The propagation of the thermal spin transition of [Fe(btr)2(NCS)2].H2Osingle crystals, observed by optical microscopy

International audienceWe observed by optical microscopy the propagation of the thermal spin transition in single crystals of the archetype compound [Fe(btr)2(NCS)2]*H2O (btr = 4,4′-bis-1,2,4-triazole). In fresh crystals we could follow well-defined transformation fronts of various shapes. The propagation conditions revealed to depend very sensitively on the structural quality of the crystals. The best observations were obtained on cooling selected fresh crystals, rather thin, protected by an oil layer, under isothermal conditions. They evidenced the progression of a well-defined transformation front through the entire crystal. This is typical for a nucleation and growth process governed by inhomogeneous nucleation. The front shape was simple (circular or linear) and progression was regular in the best cases. Crystals of lesser quality (or already cycled through the transition) exhibited a more complex behaviour, assigned to the impact of internal stresses, either genuine or generated by the previous transformation of the crystals. These observations provide new insights into the physics of like-spin domains, which are briefly discussed