Theory of the two-step spin conversion induced by the cooperative molecular distortions in spin-crossover compounds

Calorimetric investigations of phase transitions in which electrons are directly involved

“…This complex exhibited two heat capacity peaks due to phase transitions at T = 114.04 K and T = 122.21 K. As the peak temperatures agree with the spin transition temperature estimated from the Mössbauer spectroscopy, these two peaks are closely related to the spin crossover event. The observation of double peaks accords with the reinvestigation of Mössbauer spectroscopy and magnetic susceptibility measurements by Gütlich and his collaborators, (45) showing that a two-step spin conversion in the crossover region occurs at T = 114.0 K and T = 120.7 K. Sasaki and Kambara (46) suggested a theoretical model for the two-step spin crossover assuming a sublattice structure for the HS and LS complexes, between which a quasi "antiferromagnetic" interaction is assumed. Real et al (47) also supported the short-range (44) antiferromagnetic interactions as the source of step-like spin crossover.…”

First Dicyanamide‐Bridged Spin‐Crossover Coordination Polymer: Synthesis, Structural, Magnetic, and Spectroscopic Studies

“…The spin transition in a solid containing two sublattices was studied within the framework of different theoretical models. [61][62][63] If one ignores the microscopic mechanisms of cooperativity, all of these approaches can be reduced to a two-sublattice Ising model. [63] The temperature behaviour of this model was studied in detail, supposing that the two sublattices differ in their cooperativity but have the same energy gap between the LS and HS states.…”

Symmetry Breaking in Iron(II) Spin-Crossover Molecular Crystals

“…The polymorph A of the related complex [Fe(4-epp) 2 ](BF 4 ) 2 (4-epp = 2,6-di{4-methylpyrazol-1-yl}pyridine, see Appendix), reported by Ruben et al, undergoes an abrupt SCO centered at T c = 341 K featuring a hysteresis 8 K wide and a reversible crystallographic phase transition between the space groups P2 1 2 1 2 1 (HS state) and Pbcn (LS state) [48]. The complex [Fe(3-bpp) 2 ][Fe(CN) 5 (NO)] (3-bpp = 2,6-di(1H-pyrazol-3-yl)pyridine, see Appendix), reported by Goodwin et al, undergoes a cooperative SCO centered at 182 K accompanied by a change from tetragonal, space group P4/ncc, in the HS state to the orthorhombic, space group Pnca, in the LS state [49].…”

“…and determine the cooperative character of the SCO, namely steepness, hysteresis and steps. Theoretically explained as the result of a competition between antagonistic short-range and long-range elastic interactions [5][6][7][8][9], observation of steps has been considered for many years an intriguing and relatively rare event. However, the exponential growth experienced by the field in the last decade has uncovered an increasing number of stepwise SCO events featuring ordered intermediate phases (IP) well characterized from a structural viewpoint.…”