Cd 2 Os 2 O 7 crystallizes in the pyrochlore structure and undergoes a metal-insulator transition ͑MIT͒ near 226 K. We have characterized the MIT in Cd 2 Os 2 O 7 using x-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. Cd 2 Os 2 O 7 orders magnetically at the MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on Cd 2 Os 2 O 7. These calculations indicate that Cd 2 Os 2 O 7 is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition-unlike a Mott transition-is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below T MIT .

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Metal Carbonyl Syntheses, XX. – Simple Method for the Preparation of Tc(CO)3 Complexes: Synthesis, Structures, and Reactivity of the Novel Cubane‐Type Cluster Na[Tc3(CO)9(OCH3)4] Carbonyltechnetium complexes of type LTc(CO)3 are accessible along a simple route from Na[Tc3(CO)9(OCH3)4] (2) which represents a novel alkoxy complex of cubane‐type geometry (single‐crystal X‐ray diffraction study). 2 is an isolable product of the incomplete carbonylation of sodium pertechnetate (1) conducted in methanol. Under carbon monoxide pressure, 2 yields the binary metal carbonyl Tc2(CO)10 (3), pentamethyl‐cyclopentadiene gives the halfsandwich complex (η5‐C5Me5)Tc(CO)3 (4), and with benzene/hydrochloric acid the ionic complex [(η6‐C6H6)Tc(CO)3]Cl (5) is formed.

SummarySynthesis and characterization of the first organometallic Tc(VII) complex CH3TCO3 is described. It behaves differently to CHßReOs conccrning rcactions with alkenes and with aromatic amine bases. Furthermore we present the synthesis and structure of the first organometallic düster of Tc(I) Na[Tc3(CO)9(/i3-OCH3)(/i2-OCH3)3] with a cuban-like structure. The düster act as a crown ether with high stability towards sodium. In the crystal the difierent duster units are linked via an isocarbonyl bond. The düster proved to bc a useful educt for subsequent Substitution rcactions with tridentate ligands. Carbonylation of TBA[Tc04] under 1 atm CO fmally gave in quantitative yield, the very versatile educt mer-TcCl(CO)3[P(C6H5)3]2 which readily undergoes substitutions with several tridentate ligands.