First Step Towards a Devil's Staircase in Spin‐Crossover Materials

Abstract

The unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)3)2](4,4′‐bipy)2}n exhibits a thermal high‐spin (HS)↔low‐spin (LS) staircase‐like conversion characterized by a multi‐step dependence of the HS molar fraction γHS. Between the fully HS (γHS=1) and LS (γHS=0) phases, two steps associated with different ordering appear in terms of spin‐state concentration waves (SSCW). On the γHS≈0.5 step, a periodic SSCW forms with a HS‐LS‐HS‐LS sequence. On the γHS≈0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS‐LS sequence incommensurate with the molecular lattice. The formation of these different long‐range spatially ordered structures of LS and HS states during the multi‐step spin‐crossover is discussed within the framework of “Devil's staircase”‐type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case.

Isomorphism between the electro-elastic modeling of the spin transition and Ising-like model with competing interactions: Elastic generation of self-organized spin states

“…The last points concern (iii) incomplete transitions and (iv) two-step or multi-step transitions [76][77][78][79][80][81][82] . The latter, take place with the emergence of ordered intermediate phases, which are stabilized leading to the appearance of one or several plateaus in the thermal-dependence of the order parameters, as reported in several experimental studies, [83][84][85][86] which demonstrated the existence of self-organization of the spin states in these regions. The spatial organizations of the spin states were found under the form of antiferro-like (checkerboard) structure 87,88 or may have more complex structures, like ordered alternate HS and LS sheets as in [Fe(2-pic)3]Cl2.EtOH (pic=2-picolylamine) 83 or modulated HS-LS patterns, where the modulation can be commensurate or incommensurate with the lattice parameter 84,85,86 as obtained re multiple transitions are observed during the thermal transition.…”

Spin Crossover in a Series of Non-Hofmann-Type Fe(II) Coordination Polymers Based on [Hg(SeCN)3]− or [Hg(SeCN)4]2– Building Blocks

“…In addition, these dinuclear species in turn act as linkers between linear [Fe(μ-L)] n 2n+ chains, thus resulting in highly crystalline 2D sheetlike coordination polymers generically formulated as {Fe[μ-(Hg(SeCN) 3 ) 2 ](μ-L) 2 }. Both compounds are isostructural with those obtained from the homologous [Hg(SCN) 4 ] 2− precursor when equivalent L bis-monodentate ligands are used, including 4,4′-bipy 24,25 and tvp. 26 On the basis of the limited number of examples reported so far, this in situ transformation from [Hg(XCN) 4 ] 2− to {[Hg II (XCN) 3 ] 2 (μ-L)} 2− (X = S, Se) seems to be much less favored for X = Se.…”

“…Furthermore, the equilibrium temperature for 1, T 1/2 = 170 K, is 55 K higher than the average T 1/2 value shown by its X = S homologue. 24,25 This suggests that the increase in ligand field strength around the Fe II induced by the XCN − (X = S, Se) group does not depend substantially on whether it belongs to the dimeric bridging unit {[Hg II (XCN) 3 ] 2 (μ-L)} 2− or acts as a simple coordinating terminal anion. It is worth noting that differences of similar magnitude in T 1/2 have been found for monomeric 35−44 and polymeric 1D 45−47 and 2D 48−51 [Fe-(L) 2 (NCX) 2 ] SCO complexes.…”

“…Compounds 1 (at 100 and 293 K) and 3 (at 119 K) crystallize in the triclinic space group P 1̅ ( Z = 1). Structurally, both complexes belong to the two-dimensional (2D) heterobimetallic spin crossover coordination polymers generically formulated as {Fe II [(Hg II (SeCN) 3 ) 2 ]­(L) x }·(solvent) previously reported by us and, consequently, will be described together. − The coordination environment of the metal ions and the stacking of the 2D layers for complex 1 are shown in Figure , while those for complex 3 are given in Figure S6. Selected bond lengths and the corresponding average angular distortion parameters ∑ Fe and ∑ Hg defined as the sum of deviations from the ideal octahedron/tetrahedron of the 12 and 6 “ cis ” bond angles, ∑ 12 i = 1 |θ i – 90°| and ∑ 6 i = 1 |θ i – 109.5°|, respectively, are given in Table .…”

“…We also showed previously that the lability of the [Hg­(SCN) 4 ] 2– anions could afford a series of novel bimetallic 2D SCO non-Hofmann-type coordination polymers {Fe­(L′)­[(Hg­(SCN) 3 ) 2 ]} n · x (solvent) (L′ = linear bis-monodentate pyridyl ligand). Indeed, the [Hg­(SCN) 4 ] 2– ion in the presence of suitable bis-monodentate pyridine-like ligands L′ transforms in situ into the robust dinuclear building units {[Hg­(SCN) 3 ] 2 (μ-L′)} 2– , which determine the formation of 2D coordination polymers with interesting temperature- and light-induced SCO behavior. − …”