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

Lattice Defects in Sub-Micrometer Spin-Crossover Crystals Studied by Electron Diffraction

Spin-crossover particles of [Fe(Htrz)2trz](BF4) with sizes of some hundred nanometers are studied by in situ electron microscopy. Despite their high radiation sensitivity, it was possible to analyze the particles by imaging and diffraction so that a detailed analysis of crystallographic defects in individual particles became possible. The presence of one or several tilt boundaries, where the tilt axis is the direction of the polymer chains, is detected in each particle. An in situ exposure of the particles to temperature variations or short laser pulses to induce the spin crossover shows that the defect structure only changes after a high number of transformations between the low-spin and high-spin phases. The observations are explained by the anisotropy of the atomic architecture within the crystals, which facilitates defects between weakly linked crystallographic planes.

Elastic Modeling of Two-Step Transitions in Sterically Frustrated 1D Binuclear Spin-Crossover Chains

Among the large family of spin-crossover materials, binuclear systems play an important role due to their specific molecular configurations, allowing the presence of multi-step transitions and elastic frustration. Although this issue benefited from a significant number of spin-based theories, there is almost no elastic description of the spin transition phenomenon in binuclear systems. To overcome this deficiency, in this work we develop the first elastic modeling of thermal properties of binuclear spin-crossover solids. At this end, we investigated a finite spin-crossover open chain constituted of elastically coupled binuclear (A = B) blocks, ⋯A=B−A=B−A=B⋯, in which the considered equivalent A and B sites may occupy two configurations, namely low-spin (LS) and high-spin (HS) states. The sites of the binuclear unit interact via an intramolecular spring and couple to the neighboring binuclear units via other springs. The model also includes the change of length inside and between the binuclear units subsequent to the spin state changes. When injecting an elastic frustration inside the binuclear unit in the LS state, competing interactions between the intra- and the inter-binuclear couplings emerge. The latter shows that according to the intra- and inter-binuclear elastic constants and the strength of the frustration, multi-step transitions are derived, for which a specific self-organization of type (HS = HS)-(LS-LS)-(HS = HS)⋯ is revealed and discussed. Finally, we have also studied the relaxation of the metastable photoinduced HS states at low temperature, in which two relaxation regimes with transient self-organized states were identified when monitoring the elastic frustration rate or the ratio of intra- and intermolecular elastic interactions. These behaviors are reminiscent of the thermal dependence of the order parameters of the system. The present model opens several possibilities of extensions of elastic frustrations acting in polynuclear spin-crossover systems, which may lead to other types of spin-state self-organizations and relaxation dynamics.