StatusThe thesis was presented on the 29 June, 2007
Approved by NCAA on the 18 October, 2007
Abstract– 0.60 Mb / in romanian
– 0.49 Mb / in english
4.52 Mb /
The present Thesis is devoted to the study of polynuclear mixed valence systems and clusters containing paramagnetic ions with unquenched orbital angular momenta.
General microscopic approach to the problem of electronic interactions in such type systems is elaborated. The analytical expressions for the matrix elements of double exchange, two-electron transfer and exchange-transfer are obtained for mixed valence systems containing arbitrary numbers of spin-cores, localized electrons and itinerant electrons. The concept of two kinds of exchange-transfer (kinetic and potential) is introduced and different mechanisms of the exchange-transfer are analyzed. On the basis of the developed approach the magnetic properties of tetrahedral and distorted tetranuclear iron-sulfur clusters [Fe4S4]+, [Fe4S4]3+ and [Fe4S4]2+, hexanuclear octahedral clusters [Fe6(µ3-X)8(PEt3)6]+ (X=S, Se), two-electron-reduced polyoxoanion with Keggin structure and diphtalocyanine based MV chains [YPc2]•CH2Cl2 and [ScPc2]•CH2Cl2 are explained.
The generalization of the vibronic Piepho model to the case of many-electron mixed valence dimers is performed, and on this basis the localization – delocalization phenomenon in these systems are investigated as well as their “vibronic” magnetic properties. Refinement of the conventional Robin and Day classification of mixed valence compounds is proposed. The analytical expressions for the effective “vibronic” hyperfine parameters are deduced. The localization-delocalization phenomenon in mixed valence trimers and tetramers is studied as well.
The isotropic Lines model is extended to the case of heteronuclear clusters containing orbitally degenerate Co(II) ions octahedrally coordinated by the ligands. On this basis the magnetic behavior of heteronuclear iron-cobalt complex [Fe2CoO(CH3COO)6(3-Cl-Py)3] is interpreted. The Lines model is also generalized to the case of axially and rhombically distorted octahedral surroundings of ions with unquenched orbital angular momenta (anisotropic Lines model). The microscopic approach to the problem of exchange interaction between Co(II) ions in the ground Kramers doublet states is elaborated on the basis of the anisotropic Lines model. The magnetic behavior of trigonal bipyramidal cyano-bridged single molecule magnet [MnIII(CN)6]2[MnII(tmphen)2]3 is explained.
General microscopic approach to the problem of the orbitally-dependent kinetic exchange is developed. With the aid of the developed approach the magnetic behavior of [Ti2Cl9]3- cluster is explained, the main factors governing the magnetic anisotropy in Co(II)-dimers are elucidated and the conditions for the applicability of the isotropic Lines model are found. The role of the orbitally-dependent exchange in the formation of the magnetization reversal barrier in linear cyano-bridged manganese clusters (models of single molecule magnets) is revealed. An enigmatic isotropy of exchange interaction in Cs3Yb2Cl9 and Cs3Yb2Br9 crystals is explained on the basis of the microscopic theory of the orbitally-dependent kinetic exchange.
The theory of the orbitally-dependent double exchange in mixed valence dimers containing metal ions with unquenched orbital angular momenta is developed and the magnetic anisotropy of such type systems is investigated. The combined effect of anisotropic double exchange and vibronic coupling on the magnetic anisotropy of mixed valence dimers is studied in the framework of generalized Piepho model.