Elastic Frustration in 1D Spin-Crossover Chains: Evidence of Multi-Step Transitions and Self-Organizations of the Spin States

We consider a 1D elastic spin-crossover (SCO) chain in which each site may be in the low-spin or in the high-spin (HS) state. The sites interact elastically through a harmonic coupling, and the local equilibrium distances depend on the spin states of the interacting sites. The Hamiltonian of the system is solved by the Monte Carlo method running on the spin states and the atomic displacements. By considering the existence of an elastic frustration between the equilibrium distances of the nearest-neighbors and the next-nearest-neighbors, we succeeded to highlight a number of original behaviors of the thermal dependence of the high-spin fraction, like multistep transitions, incomplete spin transitions, emergence of self-organized structures, and re-entrant spin transitions, by adjusting only one control parameter. The obtained results allow understanding several experimental data of 1D spin-crossover materials which seem to be model systems for elastic frustration

Quantitative Contact Pressure Sensor Based on Spin Crossover Mechanism for Civil Security Applications

Detailed studies of impacts on the new 1D spin transition polymer [Fe­(hyetrz)₃]­I₂·0.5EtOH have been performed under several controlled contact pressures, showing for high energy values a color change of the compound and allowing a visual detection of the spin transition from high-spin to low-spin states. By performing detailed investigations on freshly impacted samples, using spectroscopic diffuse optical reflectivity, we could follow the variation of the optical spectra as a function of the energy of the impact. The meticulous analysis of the obtained spectra allowed us to establish an absorption peak at 550 nm whose intensity and position well correlate to the energy of the impact. This concept provides a reliable method of measuring the energy of a chock even if the sample does not change its color so much in the visible range. This might be of high importance in several civil security applications, like transportation of artwork or other fragile valuable objects or even in the evaluation of the degree of alteration of a material after a collision