Lattice relaxation and order in the low-spin to high-spin transitions in molecular crystals

Abstract

rnA series of the d 6 iron(I1) complexes with bulky organic ligands (like [ Fe(bzpy),(NCS),]) can exist in two spin forms: in the low-spin ( S = 0) form at low temperature and in the high-spin ( S = 2) form at high temperature. In the crystal phase, the transition between these two forms may be either smooth or abrupt. Recently, the abrupt spin transitions were identified with the first-order transitions between different ordered phases occurring in the binary mixtures of the two spin forms of the complex. Here, we apply the method widely used in the field of binary metal alloys to the analysis of the spin transitions. The molecules undergoing the spin transition are modeled by octahedra of variable size which interact when they are immediate neighbors in the crystal lattice. We show that some simple assumptions concerning the intermolecular interaction and crystal geometry relaxation allows one to get the desired first-order phase transitions together with a satisfactory description for the crystal compressibility as a function of temperature.

Modeling molecular crystals formed by spin-active metal complexes by atom–atom potentials

“…This allows us to go on to estimate the contributions of the van der Waals intermolecular forces to the energy (enthalpy) of the spin transitions, which cannot be extracted from experimental data. The results are given in the last columns of Tables III-V. First of all, one can see that this contribution may be either positive or negative, which corroborates the theoretical conclusion of [12]. Another important point is that the lattice contribution to the enthalpy of the spin transition is comparable with its total value.…”

“…Though the estimates obtained with the Lennard-Jones and Buckingham potentials are somewhat different, the general picture is the same. For example, in the case of the Fe(phen) 2 (NCS) 2 crystal we found this component to be equal to +1.81 kcal/mol (6)(7)(8)(9)(10)(11)(12) or +0.57 kcal/mol (6-exp) or +1.54 kcal/mol (6-exp modified), while the total experimental enthalpy (from the calorimetrical data) is +2.05 kcal/mol [9]. It means that one cannot neglect intermolecular interactions in calculating thermodynamical characteristics of the spin transitions in molecular crystals.…”

“…At the same time, within the Slichter-Drickamer model, the type of behavior is tightly related to the sign and magnitude of Γ, so that a smooth transition requires Γ > 0, an abrupt transition occurs at Γ < 0 and hysteresis is possible only if Γ < 0 is less than some critical threshold, which in its turn depends on the transition temperature [8]. It has been shown that if the relaxation of the lattice is not allowed, then under very natural assumptions Γ is positive [11], but the lattice relaxation can lead to Γ of either sign [12].…”

Low‐ and high‐spin iron (II) complexes studied by effective crystal field method combined with molecular mechanics

“…The free energy δ G of a spin crossover crystal (per unit cell) can be written analogously to the expression taken from ref. 53 where δ h and δ s are intramolecular spin crossover enthalpy and entropy change, respectively, T is the temperature, x is the fraction of the high‐spin form (0 ≤ x ≤ 1), α is the Madelung constant, which is on the order of unity, c is a parameter of the intermolecular interaction potential, and r is the distance between anion and cation charge centers. The moieties forming the crystal are in fact bulk organic molecules and thus the exchange repulsion between the component molecules in fact depends on the distance y between the surfaces of the spheres representing them in the model, rather than on the distance between their centers.…”

Photoinduced magnetic bound state in an itinerant correlated electron system with a spin-state degree of freedom

“…[12][13][14][15] Here, photons bring about a charge transfer from the neighboring Fe ions to Co ions, associated with the spin-state change in Co ions from the low-spin (LS) state to the high-spin (HS) one. A main mechanism of the cooperative spinstate transition in a series of materials is supposed to be the elastic interaction; [16][17][18][19] a local volume change of a metal-ligand cluster propagates over a crystal lattice.…”