Pressure Tuning of Coupled Structural and Spin State Transitions in the Molecular Complex [Fe(H2B(pz)2)2(phen)]

The large volume change, which accompanies the molecular spin crossover (SCO) phenomenon in some transition metal complexes, prompts frequently the coupling of the SCO with other instabilities. Understanding the driving mechanism(s) of such coupled phase transitions is not only important for fundamental reasons but also provides scope for the development of multifunctional materials. The general theoretical expectation is that the coupling has elastic origin, and the sequence of transitions can be tuned by an externally applied pressure, but dedicated experiments remain scarce. Here, we used high-pressure and low-temperature single-crystal X-ray diffraction to investigate the high-spin (HS) to low-spin (LS) transitions in the molecular complexes [Fe^II(H₂B(pz)₂)₂(bipy)] and [Fe^II(H₂B(pz)₂)₂(phen)]. In the bipyridine complex, the SCO is continuous and isostructural over the whole T, P-range (100-300 K, 0-2 GPa). In the phenanthroline derivative, however, the SCO is concomitant with a symmetry-breaking transition (C2/c to P1̅). Structural analysis reveals that the coupling between the two phenomena can be tuned by external pressure from a virtually simultaneous HS_C2/c-LS_P1̅ transition to the sequence of HS_C2/c-LS_C2/c-LS_P1̅ transitions. The correlation of spontaneous strain and order parameter behaviors highlights that the "separated" transitions remain still connected via strain coupling, whereas the "simultaneous" transitions are partially split.

Pressure-induced multi-step and self-organized spin states in an electro-elastic model for spin-crossover solids

Spin transition materials are known to exhibit a rich variety of behaviors under several stimuli, among which pressure leads to major changes in their electronic and elastic properties. From an experimental point of view, thermal spin transitions under isotropic pressure showed transformations from (i) hysteretic to continuous transformations where the hysteresis width vanishes beyond some threshold pressure value; this is the conventional case. In several other cases very pathological and unexpected behaviours emerged, like (ii) persistent hysteresis under pressure; (iii) non-uniform behavior of the thermal hysteresis width which first increases with pressure and then decreases and vanishes at higher pressures; (iv) furthermore, double step transitions induced by pressure are also often obtained, where the pressure triggers the appearance of a plateau during the thermal transition, leading to two-step transitions, and finally (v) other non-conventional re-entrant transitions, where the thermal hysteresis vanishes at some pressure and then reappears at higher pressure values are also observed. In the present theoretical study, we investigate this problem with an electro-elastic description of the spin-crossover phenomenon by solving the Hamiltonian using a Monte Carlo technique. The pressure effect is here introduced directly in the lattice parameters, the elastic constants and ligand field energy. By considering spin state-dependent compressibility, we demonstrate that a large panel of experimental observations can be qualitatively described with this model. Among them, we quote (i) the conventional pressure effect decreasing the hysteresis width, (ii) the unconventional cases with pressure causing a non-monotonous behavior of the hysteresis width, (iii) re-entrant, as well as (iv) double step transitions accompanied with various types of spin state self-organization in the plateau regions.

Mixed-valence {FeII2FeIII4} hexanuclear complexes with thermally induced Fe(III) spin crossover behavior

Cyano-bridged mixed-valence {Fe6} hexanuclear complexes {[Tp4-MeFeII(CN)3]2[FeIII(Tpa)]2[FeIII(OR)(Tpa)]2}·6ClO4·S {Tp4-Me = tri(4-methyl-pyrazol-1-yl)borate, Tpa = Tris(2-pyridylmethyl)amine; R = -CH3 S = 8MeOH 1, R = -C2H5 S = 6EtOH 2 } have been obtained...

Temperature dependence of the cooperative out-of-equilibrium elastic switching in a spin-crossover material

We explore the key parameters that influence the efficiency of the cooperative low-spin to high-spin conversion through long range elastic intermolecular interactions during the so-called elastic step, triggered by instantaneous photo-induced conversion.

[Fe(abpt)2(NCSe)2] polymorph A: structural studies into the spin crossover behaviour

Crystallographic and UV-Vis analysis of the thermal spin-transition in [Fe(abpt)₂(NCSe)₂], polymorph A, is presented alongside LIESST and high pressure structures.