📝 Original Info
- Title: Modeling molecular crystals formed by spin-active metal complexes by atom-atom potentials
- ArXiv ID: 0904.2742
- Date: 2009-04-17
- Authors: Researchers from original ArXiv paper
📝 Abstract
We apply the atom-atom potentials to molecular crystals of iron (II) complexes with bulky organic ligands. The crystals under study are formed by low-spin or high-spin molecules of Fe(phen)$_{2}$(NCS)$_{2}$ (phen = 1,10-phenanthroline), Fe(btz)$_{2}$(NCS)$_{2}$ (btz = 5,5$^{\prime }$,6,6$^{\prime}$-tetrahydro-4\textit{H},4$^{\prime}$\textit{H}-2,2$^{\prime }$-bi-1,3-thiazine), and Fe(bpz)$_{2}$(bipy) (bpz = dihydrobis(1-pyrazolil)borate, and bipy = 2,2$^{\prime}$-bipyridine). All molecular geometries are taken from the X-ray experimental data and assumed to be frozen. The unit cell dimensions and angles, positions of the centers of masses of molecules, and the orientations of molecules corresponding to the minimum energy at 1 atm and 1 GPa are calculated. The optimized crystal structures are in a good agreement with the experimental data. Sources of the residual discrepancies between the calculated and experimental structures are discussed. The intermolecular contributions to the enthalpy of the spin transitions are found to be comparable with its total experimental values. It demonstrates that the method of atom-atom potentials is very useful for modeling organometalic crystals undergoing the spin transitions.
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We apply the atom-atom potentials to molecular crystals of iron (II) complexes with bulky organic ligands. The crystals under study are formed by low-spin or high-spin molecules of Fe(phen)$_{2}$(NCS)$_{2}$ (phen = 1,10-phenanthroline), Fe(btz)$_{2}$(NCS)$_{2}$ (btz = 5,5$^{\prime }$,6,6$^{\prime}$-tetrahydro-4\textit{H},4$^{\prime}$\textit{H}-2,2$^{\prime }$-bi-1,3-thiazine), and Fe(bpz)$_{2}$(bipy) (bpz = dihydrobis(1-pyrazolil)borate, and bipy = 2,2$^{\prime}$-bipyridine). All molecular geometries are taken from the X-ray experimental data and assumed to be frozen. The unit cell dimensions and angles, positions of the centers of masses of molecules, and the orientations of molecules corresponding to the minimum energy at 1 atm and 1 GPa are calculated. The optimized crystal structures are in a good agreement with the experimental data. Sources of the residual discrepancies between the calculated and experimental structures are discussed. The intermolecular contributions to the entha
📄 Full Content
The Crystal Field Theory (CFT), proposed in [1] and known to majority of chemists through [2], suggests that coordination compounds of d-elements with electronic configurations d 4 , d 5 , d 6 or d 7 can exist either in highspin (HS) or low spin (LS) forms (sometimes intermediate values of the total spin are also possible). In the case of strong-field ligands the d-level splitting measured by the average crystal field parameter 10Dq exceeds the average Coulomb interaction energy of d-electrons P and the ground state is LS. In the case of weak-field ligands with 10Dq ≪ P , the ground state is bound to be HS. If, however, 10Dq ∼ = P , the LS and HS forms of the complex may coexist in equilibrium, and the fraction of either spin form depends on temperature, pressure, and/or other macroscopic thermodynamic parameters. The process when the fraction of molecules of different total spin changes due to external conditions is called a spin crossover (SC) transition. For the first time this phenomenon was reported in 1931 [3]. Nevertheless, extensive studies of SC started only in 1960s-70s. Nowadays, dozens of complexes capable to undergo spin transitions (spin-active complexes) are known, and most of them are those of Fe(II). A general review of the field can be found in [4].
A wealth of potential practical applications like displays and data storage devices (see a detailed review in [5]) is one of the reasons for research activity in this area. Industrial applications pose strict demands on the characteristics of the materials to be used. As a consequence, the problem of predicting SC transition characteristics (whether it is smooth or abrupt, the transition temperature, the width of the hysteresis loop, the influence of additives [6]) is of paramount importance. Theoretical description of spin transitions is a great challenge by itself, and until now a coherent theory allowing to relate the composition of the materials with the characteristics of the transition has not been developed. Discussion of these issues and an overview of the existing theories are given in [7].
In general, the SC modeling includes two aspects: (i) that of the interactions within one molecule of a spinactive complex, and (ii) that of the interactions between these molecules. The latter is crucially important for understanding of specific features of the SC transitions in solids because the SC manifesting itself as a first-order phase transition is controlled by intermolecular interactions. These ideas are built in the simplest model capable of describing spin transitions in solids proposed by Slichter and Drickamer [8]. This model considers the solid as a regular solution of molecules in the LS and HS states. The model predicts, in agreement with the experiments, that the spin transition may be either smooth or abrupt or may exhibit hysteresis, and its character is de-termined by a phenomenological intermolecular parameter Γ, specific for each material. However, the experimental data on the heat capacity and the X-ray diffraction contradict to this model.
The thermal dependence of the heat capacity of the Fe(phen) 2 (NCS) 2 crystal is better explained by an alternative domain model [9]. Diffraction patterns of spin transition crystals, measured at intermediate temperatures, simultaneously contain the Bragg peaks corresponding to the pure LS and HS phases, while no peaks for intermediate lattice of a solution were observed [10]. Another problem is that the parameter Γ is phenomenological one, and it cannot be sequentially derived in terms of microscopic characteristics of the constituent molecules or their interactions. 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].
Significant progress in the understanding of the spin transitions in crystals is attributed to the Ising-like models of intermolecular interactions in spin-active materials [13]. Adaptations of the initial Ising model to the spin transitions include corrections for intramolecular vibrations, domain formation, parameters distribution, elastic distortions, presence of two metal atoms in a spin-active molecule, etc. [7,14]. These models do not have analytical solutions and they are solved either in a mean field approximation which leads to results analogous to (or even coinciding with) the Slichter-Drickamer model [7] or numerically.
In spite of the diversity of the models used in the literature, the theoretical description of the spin transitions is not yet satisfactory. First, the existing theo
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