Sample records for PALLADIUM 120 (palladium 120)
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3

Ternary Transition Metal Acetylides AI2M0C2 (AI = K, Rb; M0 = Pd, Pt): Neutron Diffraction Studies and Electronic Propertiesm1, Z = 1) were refined from time of flight powder neutron diffraction data (Polaris, ISIS/RAL) at room temperature and 4.5 K. The resulting C C distances within the C22 anion are in the range 124.4 124.7 pm (295 K). This is distinctly longer than the expected distance for a C C triple bond (120 pm). At low temperatures slightly longer C C distances are found (125.0 126.7 pm), which is due to an increasing wobbling of the C2 dumbbells around their center of gravity with increasing temperature. This is also reflected in the temperature dependence of the lattice parameters a and c. Lattice parameter c, which is parallel to the C2 dumbbells, decreases with increasing temperature, whereas a shows a typical positive thermal expansion. Differing to previous work ternary palladium acetylides A2PdC2 were obtained as red (A = Na) and yellow powders (A = K, Rb) by using a slightly improved synthesis. By means of diffuse reflectivity direct bandgaps were determined to 2.09 eV (A = Na), 2.55 eV (A = K), and 2.77 eV (A = Rb). This is in good agreement with direct bandgaps obtained from band structure calculations. These band structure calculations indicate very small indirect bandgaps of only a few tenth of an eV. Dedicated to Professor Bernd Harbrecht on the Occasion of His 60th Birthday


2010-01-01

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