Visualization and quantitative analysis of spatiotemporal behavior in a first-order thermal spin transition: A stress-driven multiscale process

Abstract

We investigated by optical microscopy the propagation of the thermal spin transition in [Fe(btr) 2 (NCS) 2 ]·H 2 O·(btr = bis-triazole) single crystals. Using high-quality fresh crystals embedded in oil we could follow the transformation front of the on-cooling transition. We observed in detail a linear front propagating through the entire crystal at extremely slow velocity, ∼2.5 μm/s on average. The analysis of the temporal dependences of the optical densities (OD) at given positions within a Kolmogorov-Johnson-Mehl-Avrami (KJMA) law led to an exponent ∼0.6, typical for a process governed by planar diffusion. It is inferred that the spin transition is a multiscale process. The agreement with a previous structural investigation, analyzed in terms of an average KJMA law, is shown. We conclude that the stresses generated by the volume change associated with the spin transformation are the driving force for the propagation of the nucleation and growth mechanism. The role of defects is discussed and optical effects specific of the experiment are evidenced.

Multistep Relaxations in a Spin-Crossover Lattice with Defect: A Spatiotemporal Study of the Domain Propagation

“…Snapshot C, shows the growing interface towards the center of the crystal with a net distortion of the interface region that faces the hole position. This behavior is reminiscent of experimental observations [19,20] obtained by optical microscopy, where we have seen that near the defects the interface is deformed and trapped for a while. At point D of Figure 6, the HS/LS interface, given in the corresponding snapshot D (Figure 7), is pinned by the defect.…”

“…We consider an electro-elastic model which mimics the spin-crossover transition, for which we have already discussed some of its thermodynamic properties in recent works [19,21]. This model is based on two-states fictitious spins, S i "`1 and S i "´1, respectively associated with the HS and LS states of the molecule.…”

“…At variance from these electronic models, the continuous medium model developed by Spiering [7] showed that an elastic interaction could as well give rise to the observed first-order transitions. More recently, discrete models based on deformable lattices [8][9][10][11][12][13][14][15][16] were introduced so as to mimic the spatio-temporal features revealed by optical microscopy investigations [17][18][19] for the nucleation and growth of the spin state phases at the thermal transition of SCO single crystals. They conciliate the continuous medium and discrete lattice approaches, with purely elastic interactions, so as to reproduce both lattice and spin transformations of the system.…”

“…Their relevance is established through their ability to reproduce, at least qualitatively, the said spatio-temporal effects, summarized as follows in the well-documented case of the HS Ñ LS transition of fresh single crystals: (i) the transition usually starts from a corner or/and edge of the crystal (depending on its shape); (ii) the spin state spreads over the whole crystal with a well-defined front-line, the shape of which depends on the interplay with the edges of the crystal; (iii) the front-line propagates at a very slow velocity («2-10 µm/s); (iv) the presence of large mechanical stresses may lead to an irreversible damage; and (v) the presence of stresses (after thermal cycling) strongly impact the subsequent transition temperatures. By proceeding to a local analysis of the kinetics of the transformation we deduced that the nucleation and growth process at the thermal transition is a multi-scale process driven by the propagation of mechanical stresses ahead of the transformation front-line [19]. These results pointed out the importance of the coupling between the spin state change and the local deformation of the lattice, leading to a very interesting theoretical problem in which electronic (spin) and structural (lattice parameter) degrees of freedom are strongly interlinked.…”

Spatiotemporal dynamics of the spin transition in[Fe(HB(tz)3)2]single crystals

“…The spatiotemporal characteristics of these phase transitions are indeed essential to interpret the ensuing phenomena as they are directly related to the mechanistic details of the switching of their physical properties. In the case of strongly cooperative SCO systems, singular phenomena and dynamics come into play during the transition within the thermal hysteresis region including nucleation and growth processes [4][5][6][7], reversible photo-control of the LS/HS phase boundary motion [8][9][10], (bidirectional) photo-switching induced by pulsed laser excitations [11][12][13] and subsequent "cascade" phenomena [14]. In all these processes, the non-equilibrium structural domain evolution turns out to be one of the key aspects of these phase transitions.…”