Elastic Frustration Causing Two-Step and Multistep Transitions in Spin-Crossover Solids: Emergence of Complex Antiferroelastic Structures

Abstract

Two-step and multistep spin transitions are frequently observed in switchable cooperative molecular solids. They present the advantage to open the way for three- or several-bit electronics. Despite extensive experimental studies, their theoretical description was to date only phenomenological, based on Ising models including competing ferro- and antiferro-magnetic interactions, even though it is recognized that the elastic interactions are at the heart of the spin transition phenomenon, due to the volume change between the low- and high-temperature phases. To remedy this shortcoming, we designed the first consistent elastic model, taking into account both volume change upon spin transition and elastic frustration. This ingredient was revealed to be powerful, since it was able to obtain all observed experimental configurations in a consistent way. Thus, according to the strength of the elastic frustration, the system may undergo first-order transition with hysteresis, gradual, hysteretic two-step or multistep transitions, and incomplete transitions. Furthermore, the analysis of the spatial organization of the HS and LS species in the plateau regions revealed the emergence of complex antiferro-elastic patterns going from simple antiferro-magnetic-like order to long-range spatial modulations of the high-spin fraction. These results enabled us to identify the elastic frustration as the fundamental mechanism at the origin of the very recent experimental observations showing the existence of organized spatial modulations of the high-spin fraction inside the plateau of two-step spin transitions.

Halogen Substitution Effects on N2O Schiff Base Ligands in Unprecedented Abrupt FeII Spin Crossover Complexes

“…Intermolecular and lattice effects are known to significantly influence the spin‐state energetics and the predicted Δ E (HS−LS) gaps are small for all complexes . Recently, an elastic model with elastic frustration in the lattice has been reported and claimed to be a good description for two‐step spin crossover . Incorporation of such effects might improve the present computed results.…”

“…This reduces the strong s-antibonding interactions thus leading to lower HS-LS gaps. The presence of p-donorh alogen atoms in the qsal ligandsa re found to influence the energies of d xz /d yz orbitals leading to variation in the T 1/2 values for complexes 2-4.T he computedp arameters are in general agreement with the experimental observations, although there are discrepancies in fine detailss uch as ground-state energies, possibly due to al ack of inclusion of an elastic model,r eferred to above, [36] and/or inclusion of intermolecular interactions such as Madelungp otential fields, recently used in calculations on solid-stateF e II spin crossover systems. [37] Experimental Section General All reagents and solvents were purchased from Sigma-Aldrich and used as received.…”

“…[5,35] Recently,a ne lastic model with elastic frustrationi nt he lattice has been reported and claimed to be ag ood description for two-step spin crossover. [36] Incorporation of such effectsm ight improve the present computed results. The intermediate-spin (IS) states lie much higher in energy for all complexes.T his is essentially due to the fact that to achieve the IS spin state, the (d xy ) 2 (d yz ) 2 (d xz ) 1 The presence of halogen atoms in the qsal ligand promote stronger delocalization and particularly perturbs the energies of the d xz and d yz orbitals as we move along the -Cl to -I substitution.A s-Cl is as tronger p donorl igand compared to -Br or -I, the d xz and d yz orbitals in the case of -Cl are strongly destabilized compared to -Br and -I leading to as maller t 2g -e g gap.…”

Hidden Hysteretic Behavior of a Paramagnetic Iron(II) Network Revealed by Light Irradiation

“…This strategy has led to the observation of light‐induced phase collapse for Prussian blue analogs . The second strategy is based on elastic frustration, which results from competition between ferroelastic‐like (cooperativity–rigidity) and antiferroelastic‐like (flexibility) interactions and can inhibit the SCO partially or completely . Through such a strategy, the photoswitching efficiency can be improved, and symmetry breaking can be promoted .…”

“…[25] The second strategy is based on elastic frustration, which results from competition between ferroelastic-like (cooperativity-rigidity) and antiferroelastic-like (flexibility) interactions and can inhibit the SCO partially or completely. [26] Through such a strategy, the photoswitching efficiency can be improved, and symmetry breaking can be promoted. [27] In this context, as part of our and others ongoing work on cooperative polynuclear or extended systems based on functionalized triazole ligands, [28][29][30] we recently reported an original bidirectional photoswitching of a frustrated state.…”