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Calculs ab initio des intégrales de saut et d'échange dans des composés de métaux de transition

Abstract : This work deals with the evaluation of coupling elements between magnetic centres in transition metal complexes. First, we discuss about the validity of a Heisenberg Hamiltonian to represent systems based on two unpaired electrons and two magnetic orbitals per centre and show the avantages of a new Hamiltonian we proposed for this kind of system. We then present the results of ab initio calculations that allow a better understanding of the magnetic behavior of two interesting transition complexes. The first one is a molecular organic complex based on a Ni2+ ion (d8, two unpaired electrons in two magnetic orbitals) getting two nitroxide radicals (one unpaired electron each one) in its coordination sphere. Two very different crystalline forms of this complex were synthezised. In the alpha phase, the two nitroxide radicals are equivalent and in trans positions with respect to the centre of symmetry occupied by the Ni2+ ion. In the beta phase, the two nitroxide radicals are not equivalent and the "nitroxide-Ni-nitroxide" angle is close to be 90 degrees. Whereas the alpha phase exhibits antiferromagnetic couplings between the Ni2+ ion and the radicals, the Ni2+ ion in the beta phase exhibits an antiferromagnetic coupling with one of the radical and a ferromagnetic coupling with the other. The second complex studied is a mixed valence compound, NaV2O5, one of the only two inorganic compounds that exhibit a Spin-Peierls transition. Whereas this transition is well known theoretically and was observed in organic compounds whose magnetism is one dimensional, it is not the case in inorganic compounds such as CuGeO3 and NaV2O5. We study the the high temperature (above the transition temperature) and low temperature structures so that the comparison of the values of the couplings in these phases allows a better understanding of the nature of the transition.
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https://tel.archives-ouvertes.fr/tel-00007277
Contributor : Nicolas Suaud <>
Submitted on : Tuesday, November 2, 2004 - 6:08:34 PM
Last modification on : Friday, February 28, 2020 - 1:57:38 PM
Long-term archiving on: : Thursday, September 13, 2012 - 12:35:16 PM

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Nicolas Suaud. Calculs ab initio des intégrales de saut et d'échange dans des composés de métaux de transition. Matière Condensée [cond-mat]. Université Paul Sabatier - Toulouse III, 2000. Français. ⟨tel-00007277⟩

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