Ternary Transition Metal Acetylides A(I)2M(0)C2 (A(I) = K, Rb; M(0) = Pd, Pt): Neutron Diffraction Studies and Electronic Properties
Résumé
The crystal structures of K2PdC2, K2PtC2, and Rb2PdC2 ( P-3m1, 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 (C2)2- anion are in the range 124.4 - 124.7 pm (295 K). This is distinctively 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 centre 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 dumbbell, 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 band gaps 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 band gaps obtained from band structure calculations. These band structure calculations indicate very small indirect band gaps of only a few tenth of an eV.
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