Abstract
The jungle of experimental behaviors of spin-crossover materials contains a tremendous number of unexpected behaviors, among which, the unsymmetrical hysteresis loops having different shapes on heating and cooling, that we often encounter in literature. Excluding an extra effect of crystallographic phase transitions, we study here these phenomena from the point of view of elastic modeling and we demonstrate that a simple model accounting for the bond lengths misfits between the high-spin and low-spin states is sufficient to describe the situation of unsymmetrical hysteresis showing plateaus at the transition only on cooling or on heating branches. The idea behind this effect relates to the existence of a discriminant elastic frustration in the lattice, which expresses only along the high-spin to low-spin transition or in the opposite side. The obtained two-step transitions showed characteristics of self-organization of the spin states under the form of stripes, which we explain as an emergence process of antagonist directional elastic interactions inside the lattice. The analysis of the spin state transformation inside the plateau on cooling in terms of two sublattices demonstrated that the elastic-driven self-organization of the spin states is accompanied with a symmetry breaking.
Highlights
The spin-crossover transition (SCO) is a phenomenon that occurs in transition metal coordination compounds based on Fe (II), Fe (III), Mn (II), Mn (III), Co (III), Cr (II) [1,2,3,4,5] which change their spin state due to external stimuli such as a variation of temperature [6,7,8], pressure [9,10,11], light irradiation [12,13,14,15,16], electrical or magnetic fields [17,18] etc
Many experimental studies [27,28,29,30,31,32] were devoted to this phenomenon, which appears in organometallic complexes, due to the rich variety of collective thermodynamic behaviors such as: (i) gradual spin-transitions [33] of independent SCO molecules, described by a simple Boltzmann distribution over all energy levels involved in weak cooperative systems, (ii) first-order transitions [21] with large thermal hysteresis, (iii) incomplete spin transitions with significant fractions of molecules blocked in the HS state at low-temperature [34] (iv) as well as two-step or multi-step transitions [35,36] which often express the existence of antagonist interactions inside the lattice
We have investigated the physical conditions allowing the emergence of unsymmetrical thermal hysteresis in spin-crossover materials, using a simple elastic model including the interactions between the molecules’ spin states and the lattice
Summary
The spin-crossover transition (SCO) is a phenomenon that occurs in transition metal coordination compounds based on Fe (II), Fe (III), Mn (II), Mn (III), Co (III), Cr (II) [1,2,3,4,5] which change their spin state due to external stimuli such as a variation of temperature [6,7,8], pressure [9,10,11], light irradiation [12,13,14,15,16], electrical or magnetic fields [17,18] etc. Many experimental studies [27,28,29,30,31,32] were devoted to this phenomenon, which appears in organometallic complexes, due to the rich variety of collective thermodynamic behaviors such as: (i) gradual spin-transitions [33] of independent SCO molecules, described by a simple Boltzmann distribution over all energy levels involved in weak cooperative systems, (ii) first-order transitions [21] with large thermal hysteresis, (iii) incomplete spin transitions with significant fractions of molecules blocked in the HS state at low-temperature [34] (iv) as well as two-step or multi-step transitions [35,36] which often express the existence of antagonist interactions inside the lattice.
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