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Peer reviewe

Intramolecular charge disproportionation and the band structure of A(3)C(60) superconductors

Realistic electronic model calculations provide convincing evidence for intramolecular charge disproportionation of the LUMO (lowest unoccupied molecular orbitals) band structure in the family of A(3)C(60) (A = K, Rb, Cs) superconductors. The conventional LUMO band splits into three similar subbands showing an overall occupied bandwidth of 1 eV and a steep decrease of the density of states at the Fermi level when the band population deviates from half filling. The disproportionation effect results also in nonzero Jahn-Teller distortions, inhomogeneous charge distribution among carbon atoms, and low frequency reorientation modes of Jahn-Teller distortions on each fullerene sites.status: publishe

On the simple determination of the self-consistent atomic charges and Madelung potentials in ionic crystals

A simple method for the self-consistent calculation of the atomic charges and Madelung potentials in ionic crystals is suggested. The method is based on electronegativity equalization and permits the obtaining of good charge distributions. Test calculations are carried out for a number of structures like cubic perovskites, high-T-c, copper oxides and mixed valence crystals. (C) 1997 Published by Elsevier Science B.V.status: publishe

Structure and magnetism in Fe-Gd based dinuclear and chain systems: the interplay of weak exchange coupling and zero field splitting effects

The synthesis and characterization of two Fe-Gd systems based on bpca- (Hbpca = bis(2-pyridilcarbonyl)amine) as bridging ligand is presented, taking the systems as a case study for structure-property correlations. Compound 1, [FeLSII(μ-bpca)2Gd(NO3)2(H2O)]NO3·2CH3NO2, is a zigzag polymer, incorporating the diamagnetic low spin FeLS(II) ion. The magnetism of 1 is entirely determined by the weak zero field splitting (ZFS) effect on the Gd(III) ion. Compound 2 is a Fe(III)-Gd(III) dinuclear compound, [FeLSIII(bpca)(μ-bpca)Gd(NO3)4]·4CH3NO2·CH3OH, its magnetism being interpreted as due to the antiferromagnetic coupling between the SFe = 1/2 and SGd = 7/2 spins, interplayed with the local ZFS on the lanthanide center. In both systems, the d-f assembly is determined by the bridging capabilities of the ambidentate bpca- ligand, which binds the d ion by a tridentate moiety with nitrogen donors and the f center by the diketonate side. We propose a spin delocalization and polarization mechanism that rationalizes the factors leading to the antiferromagnetic d-f coupling. Although conceived for compound 2, the scheme can be proposed as a general mechanism. The rationalization of the weak ZFS effects on Gd(III) by multiconfiguration and spin-orbit ab initio calculations allowed us to determine the details of the small but still significant anisotropy of Gd(III) ion in the coordination sites of compounds 1 and 2. The outlined methodologies and generalized conclusions shed new light on the field of gadolinium coordination magnetochemistry