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We model the thermal effects in different 2D spin-crossover (SCO) square lattices within the frame of the Ising-like model using Monte Carlo entropic sampling (MCES) method to enhance the scan of macrostates beyond the most probable thermal ones. In fact, MCES allows access to the metastable states, and it is then well adapted to study thermal hysteresis properties. In this contribution, we distinguish, for the first time, the interaction between molecules located in bulk at the surface and those connecting the bulk and surface regions of an SCO lattice. In addition, an extra ligand field contribution is assigned to surface molecules through an interaction parameter L. In the absence of environmental effects on surface nanoparticles, a single thermal hysteresis loop increasing with the lattice size is simulated with a unique bulk and surface equilibrium temperature T e q = T e q b u l k =   T e q s u r f . When environmental effects are accounted for, a two-step behavior associated with two hysteresis loops of widths ΔTS (for the surface) and ΔTB (for the bulk) with an intermediate plateau 14 K wide is obtained in the thermal dependence of the high-spin (HS) fraction for the 6 × 6 lattice. The surface and bulk equilibrium temperatures are then different, both decreasing towards lower values, and the L parameter controls the three states' behavior as well as the hysteresis loop interval. Size effects show that the equilibrium temperature is governed by the surface atoms for a small lattice size (5 × 5) and by the bulk atoms for a large lattice size (7 × 7). Moreover, a change in the size of the lattice results in a variation of the order–disorder (or Curie) temperature, TO.D., and the surface equilibrium temperature, Teq, while only TO.D. changes in bulk. [ABSTRACT FROM AUTHOR]

The properties of spin crossover (SCO) nanoparticles were studied for five 2D hexagonal lattice structures of increasing sizes embedded in a matrix, thus affecting the thermal properties of the SCO region. These effects were modeled using the Ising-like model in the framework of local mean field approximation (LMFA). The systematic combined effect of the different types of couplings, consisting of (i) bulk short- and long-range interactions and (ii) edge and corner interactions at the surface mediated by the matrix environment, were investigated by using parameter values typical of SCO complexes. Gradual two and three hysteretic transition curves from the LS to HS states were obtained. The results were interpreted in terms of the competition between the structure-dependent order and disorder temperatures (TO.D.) of internal coupling origin and the ligand field-dependent equilibrium temperatures (Teq) of external origin. [ABSTRACT FROM AUTHOR]

By using a weak modulated laser intensity we have succeeded in reversibly controlling the dynamics of the spin-crossover (SC) single crystal [{Fe(NCSe)(py)2}2( m-bpypz)] inside the thermal hysteresis. The experiment could be repeated several times with a reproducible response of the high-spin low-spin interface and without crystal damage. In-depth investigations as a function of the amplitude and frequency of the excitation brought to light the existence of a cut-off frequency ca. 1.5 Hz. The results not only document the applicability of SC materials as actuators, memory devices, or switches, but also open a new avenue for the reversible photo-control of the spin transition inside the thermal hysteresis. [ABSTRACT FROM AUTHOR]

The orientation of the high-spin (HS)-low-spin (LS) macroscopic interface at the thermal transition of thin [{Fe(NCSe)(py)2}2(m-bpypz)] crystals is explained by considering the possible vanishing of the structural mismatch between the coexisting phases. The structural property which allows mismatch-free interfaces is characterized. The observed orientations of the interface and the tilt angle between the HS and LS domains are accurately reproduced by a two-dimensional continuous medium model, based on the structural data. Simulations using an atomistic electro-elastic model meet the predictions of the macroscopic analysis and provide information on the distribution of the elastic energy density in the biphasic state. The presence of mismatch-free domain structures can explain the exceptional resilience of these crystals upon repeated switching. [ABSTRACT FROM AUTHOR]

Abstract: We study static and dynamical properties of the two-step spin-crossover solids. A Monte Carlo method with the Arrhenius transition probability is applied to a two-sublattice model with an intrasublattice interaction and an intersublattice interaction. The intrasublattice interaction favors the uniform order of the low spin state or high spin state in each sublattice. The intersublattice interaction favors the opposite orders in the two sublattices. The present model describes well the two-step phase transition. There, an intermediate-temperature ‘antiferro’ phase, where the sublattices have different states, appears between low-temperature phase (LS phase) and high-temperature phase (HS phase), as observed in the experiments. We focus on the relaxation from the saturated HS state which is realized by photo excitation. Two-step relaxation is found: a fast relaxation with sigmoidal shape and following slow relaxation. The former is due to the Arrhenius transition rate and the latter is a result of stochastic relaxation from the metastable antiferro state, which reproduces well the relaxation observed in the experiments. [Copyright &y& Elsevier]

Two-dimensional (2D) square, rectangular and hexagonal lattices and 3D parallelepipedic lattices of spin crossover (SCO) compounds which represent typical examples of first order phase transitions compounds are studied in terms of their size, shape and model through an Ising-like Hamiltonian in which the fictitious spin states are coupled via the respective short and long-range interaction parameters J, and G. Furthermore, an environmental L parameter accounting for surface effects is also introduced. The wealth of SCO transition properties between its bi-stable low spin (LS) and high spin (HS) states are simulated using Monte Carlo Entropic Sampling (MCES) method which favors the scanning of macro states of weak probability occurrences. For given J and G, the focus is on surface effects through parameter L. It is shown that the combined first-order phase transition effects of the parameters of the Hamiltonian can be highlighted through two typical temperatures, TO.D., the critical order-disorder temperature and Teq the equilibrium temperature that is fixed at zero effective ligand field. The relative positions of TO.D. and Teq control the nature of the transition and mediate the width and position of the thermal hysteresis curves with size and shape. When surface effects are negligible (L = 0), the equilibrium transition temperature, Teq. becomes constant, while the thermal hysteresis' width increases with size. When surface effects are considered, L ≠ 0, Teq. increases with size and the first order transition vanishes in favor of a gradual transition until reaching a threshold size, below which a reentrance phenomenon occurs and the thermal hysteresis reappears again, as shown for hexagonal configuration. [ABSTRACT FROM AUTHOR]

The present work is devoted to the study of the non-equilibrium properties of 3D switchable spin-crossover (SCO) multi-layers by accounting for lattice frustration effects and elastic anisotropy. The investigations are performed using an extended version of the original electro-elastic model which includes the lattice volume change at the spin transition between the low-spin (LS) and the high-spin (HS) states. The numerical simulations were made using Monte Carlo (MC) Metropolis technique for the spin state subsystem, combined with a semi-analytical method of mechanical relaxation of the lattice positions, which revealed to be very efficient in terms of computational time. To enhance the quality of the results, the scattered lattice positions were interpolated using the radial basis functions method. The 3D character of the simulations allows the molecules to leave the plane of the 2D layers, thus leading to corrugations and buckling which considerably relax the elastic strain. Various configurations of elastic interactions are then studied, among which, those accounting for anisotropic elastic constants and others including an intrinsic elastic frustration in the LS state. As a result, multi-stepped low-temperature relaxation processes are reproduced for the temporal evolution of the metastable HS fraction, displaying the emergence of complex self-organizations of the spin states and structure in the plateaus regions. In addition to the usual checkerboard antiferro-like HS–LS patterns, spontaneous spatial self-organization into alternated HS and LS layers are also obtained, recalling experimental data of Fe-picolylamine and other compounds of literature, which remained an open theoretical problem to date. [Display omitted] • Development of a very efficient method for the mechanical relaxation • Anisotropic elastic constants inducing multi-stepped transient self-organization • Intrinsic elastic frustration stabilizes the spin states self-organization. • Alternating HS and LS layers, similar to experimental data, are obtained. • Predicted of a 3D antiferro-like coupled HS–LS stripes self-organization. [ABSTRACT FROM AUTHOR]

Optical microscopy technique is used to investigate the thermal and the spatio-temporal properties of the spin-crossover single crystal [Fe(2-pytrz) 2 {Pt(CN) 4 }]·3H 2 O, which exhibits a first-order spin transition from a full high-spin (HS) state at high temperature to an intermediate, high-spin low-spin (HS-LS) state, below 153 K, where only one of the two crystallographic Fe(II) centers switches from the HS to HS-LS state. In comparison with crystals undergoing a complete spin transition, the present transformation involves smaller volume changes at the transition, which helps to preserving the crystal's integrity. By analyzing the spatio-temporal properties of this spin transition, we evidenced a direct correlation between the orientation and shape of HS/HS-LS domain wall with the crystal's shape. Thanks to the small volume change accompanying this spin transition, the analysis of the experimental data by an anisotropic reaction-diffusion model becomes very relevant and leads to an excellent agreement with the experimental observations. [ABSTRACT FROM AUTHOR]