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New BEDT-TTF/[Fe(C5O5)3]3- Hybrid System:  Synthesis, Crystal Structure, and Physical Properties of a Chirality-Induced α Phase and a Novel Magnetic Molecular Metal

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posted on 28.05.2007, 00:00 by Eugenio Coronado, Simona Curreli, Carlos Giménez-Saiz, Carlos J. Gómez-García, Paola Deplano, Maria Laura Mercuri, Angela Serpe, Luca Pilia, Christophe Faulmann, Enric Canadell
The paramagnetic and chiral anion [Fe(C5O5)3]3- (C5O52- = croconate) has been combined with the organic donor BEDT-TTF (=ET = bis(ethylenedithio)tetrathiafulvalene) to synthesize a novel paramagnetic semiconductor with the first chirality-induced α phase, α-(BEDT-TTF)5[Fe(C5O5)3]·5H2O (1), and one of the few known paramagnetic molecular metals, β-(BEDT-TTF)5[Fe(C5O5)3]·C6H5CN (2). Both compounds present layers of BEDT-TTF molecules, with the α or β packing modes, alternating with layers containing the high-spin S = 5/2 Fe(III) anions and solvent molecules. In the α phase, the alternation of the chiral [Fe(C5O5)3]3- anions along the direction perpendicular to the BEDT-TTF chains induces an alternation of the tilt angle of the BEDT-TTF molecules, giving rise to the observed α phase. The α phase presents a semiconductor behavior with a high room-temperature conductivity (6 S·cm-1) and an activation energy of 116 meV. The β phase presents a metallic behavior down to ca. 120 K, where a charge localization takes place with a reentrance to the metallic state below ca. 20 K followed by a metal−semiconductor transition at ca. 10 K. The magnetic properties are dominated by the paramagnetic S = 5/2 [Fe(C5O5)3]3- anion with an extra Pauli-type paramagnetism in the metallic β phase. The ESR spectra confirm the presence of the high-spin Fe(III)-containing anion and show a progressive localization in the organic sublattice along with an antiferromagnetic coupling below ca. 120 K that, in the metallic β phase, could be at the origin of the transition from the metallic to the activated conductivity regime. The correlation between crystal structure and conductivity behavior has been studied by means of tight-binding band structure calculations which provide a rationalization of the charge distribution and conductivity results.