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Structural Anomalies Associated with Antiferromagnetic Transition of Single-Component Molecular Metal [Au(tmdt)2]

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posted on 02.11.2009, 00:00 by Biao Zhou, Akiko Kobayashi, Yoshinori Okano, HengBo Cui, David Graf, James S. Brooks, Takeshi Nakashima, Shinobu Aoyagi, Eiji Nishibori, Makoto Sakata, Hayao Kobayashi
The crystal structure of the single-component molecular metal [Au(tmdt)2] was examined by performing powder X-ray diffraction experiments in the temperature range of 9−300 K using a synchrotron radiation source installed at SPring-8. The structural anomalies associated with antiferromagnetic transition were observed around the transition temperature (TN = 110 K). The continuous temperature dependence of the unit cell volume and the discontinuous change in the thermal expansion coefficient at TN suggested that the antiferromagnetic transition of [Au(tmdt)2] is a second-order transition. Au(tmdt)2 molecules are closely packed in the (021̅) plane with two-dimensional lattice vectors of a and l (= 2a + b + 2c). The shortest intermolecular S···S distance along the a axis shows a sharp decrease at around TN, while the temperature dependence of l exhibits a characteristic peak in the same temperature region. A distinct structure anomaly was not observed along the direction perpendicular to the (021̅) plane. These results suggest that the molecular arrangement in only the (021̅) plane changes significantly at TN. Thus, the intermolecular spacing shows anomalous temperature dependence at around TN only along that direction where the neighboring tmdt ligands have opposite spins in the antiferromagnetic spin structure model recently derived from ab initio band structure calculations. The results of single-crystal four-probe resistance measurements on extremely small crystals (∼25 μm) did not show a distinct resistance anomaly at TN. The resistance anomaly associated with antiferromagnetic transition, if at all present, is very small. The Au−S bond length decreases sharply at around 110 K; this is consistent with the proposed antiferromagnetic spin distribution model, where the left and right ligands of the same molecule possess opposite spin polarizations. The tendency of the Au−S bond to elongate with decreasing temperature is ascribed to the small energy gap between the pdσ(−) (or SOMO + 1) and the asym-Lπ(d) (or SOMO) states of the Au(tmdt)2 molecule.