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We present a detailed analysis of the results of our numerical study of the crystal and electronic structure of the room temperature organometallic ferrimagnet of general composition V(TCNE)$_{x}$ with $% xapprox 2$. The results of the LSDA+$U$ study show that the experimentally determined structure complies with the magnetic measurements and thus can serve as a prototype structure for the entire family of the M(TCNE)$_{2}$ organometallic magnets. The results of the numerical study and of the magnetic experiments are interpreted using model Hamiltonians proposed here. This allowed us to obtain estimates of the critical temperature in three- and two-dimensional regimes and to give an explanation of the differences in behavior of probably isostructural V(TCNE)$_{2}$ and Fe(TCNE)$_{2}$ species.
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-4textit{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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