Properties of the low-spin high-spin interface during the relaxation of spin-crossover materials, investigated through an electro-elastic model.

The present work is devoted to the spatio-temporal investigations of spin-crossover lattices during their thermal relaxation from high- to low-spin state. The analysis is performed using Monte Carlo simulations on a distortable 2D lattice the sites of which are occupied by high-spin (HS) or low-spin (LS) atoms. The lattice is circular in shape and the HS to LS transformation results in single domain nucleation followed by growth and propagation processes. The evolution of the LS:HS interface is monitored during the relaxation process, through the mapping of spin states, displacement fields, local stresses, and elastic energy. The results show a curved interface, the curvature of which is reversed at the mid-transformation. The local stresses and elastic energy peak at the vicinity of the HS:LS interface, with sizeable dependence upon the position along the front line which evidences the edge effects.

Magnetic and Mössbauer investigation of the photomagnetic Prussian blue analogue Na(0.32)Co[Fe(CN)6](0.74).3.4H2O: cooperative relaxation of the thermally quenched state.

Thanks to thermal quenching we investigated the relaxation of the metastable state of Na(0.32)Co[Fe(CN)6](0.74).3.4H2O at low temperature. A self-accelerated process has been observed in agreement with the cooperative character of the system, responsible for the large thermal hysteresis of the charge-transfer-induced spin transition. The mean-field analysis of the relaxation is discussed with respect to the equilibrium properties. A sizable deviation from mean-field behavior is observed at the beginning of the relaxation process, which might be attributed to a preliminary structural relaxation of the quenched state.

Photoinduced ferrimagnetic systems in Prussian blue analogues C(I)xCo4[Fe(CN)6]y (C(I) = alkali cation). 3. Control of the photo- and thermally induced electron transfer by the [Fe(CN)6] vacancies in cesium derivatives.

We synthesized a series of CoFe Prussian blue analogues along which we tuned the amount of cesium cations inserted in the tetrahedral sites of the structure. Structure and electronic structure have been investigated, combining XANES, infrared spectroscopy, powder X-ray diffraction experiments, and magnetization measurements. The change of the magnetization induced by light along the series shows that the efficiency of the photoinduced magnetization, evidenced a few years ago in similar compounds by Hashimoto et al. (Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996, 272, 704-705; Sato, O.; Einaga, Y.; Iyoda, T.; Fujishima, A.; Hashimoto, K. J. Electrochem. Soc. 1997, 144, L11-L13; Sato, O.; Einaga, Y.; Iyoda, T.; Fujishima, A.; Hashimoto, K. J. Phys. Chem. B 1997, 101, 3903-3905; Einaga, Y.; Ohkoshi, S.-I.; Sato, O.; Fujishima, A.; Hashimoto, K. Chem. Lett. 1998, 585-586; and Sato, O.; Einaga, Y.; Fujishima, A.; Hashimoto, K. Inorg. Chem. 1999, 38, 4405-4412), depends on a compromise between the number of excitable diamagnetic pairs and the amount of [Fe(CN)6] vacancies giving the network flexibility. Besides the efficiency of the photoinduced process, the amount of [Fe(CN)6] vacancies also controls a thermally induced electron transfer.

Metastable states of an Ising-like thermally bistable system.

The lifetimes of the metastable states are investigated in an Ising-like model associated with thermally bistable systems. A discrete mesoscopic Markovian dynamic is established using an optimized version of the previously presented Monte Carlo entropic sampling method. This is well suited to an extensive study of the role of the physical parameters: temperature, interaction parameter, electronic energy gap. By combining a discrete Markovian mesoscopic dynamic and the absorbing Markov chain technique, we obtain an analytical access to the average lifetime of the metastable state. One-variable and two-variable approximations for the original microscopic master equation are presented and discussed. A typical difference in the thermal dependence of the lifetime of the low- and the high-temperature metastable states is found, and explained as a consequence of the temperature-dependent field associated with the Ising-like model. The validity, the advantages, and the limits of the method are discussed, as well as the possible consequences on the behavior of spin transition systems. A prospective for a possible phenomenological finite-size scaling is presented.

Spin crossover in a catenane supramolecular system.

The compound [Fe(tvp)(2)(NCS)(2)] . CH(3)OH, where tvp is 1,2-di-(4-pyridyl)-ethylene, has been synthesized and characterized by x-ray single-crystal diffraction. It consists of two perpendicular, two-dimensional networks organized in parallel stacks of sheets made up of edge-shared [Fe(II)](4) rhombuses. The fully interlocked networks define large square channels in the [001] direction. Variable-temperature magnetic susceptibility measurements and Mössbauer studies reveal that this compound shows low-spin to high-spin crossover behavior in the temperature range from 100 to 250 kelvin. The combined structural and magnetic characterization of this kind of compound is fundamental for the interpretation of the mechanism leading to the spin crossover, which is important in the development of electronic devices such as molecular switches.