Ternary transition metal acetylides A _2~I M ~0C_2 (A~I = K, Rb; M~0 = Pd, Pt): Neutron diffraction studies and electronic properties

摘要

The crystal structures of K_2PdC_2, K _2PtC_2, and Rb_2PdC_2 (P3?m1, 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 C~(22-) 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 C_2 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, whichis parallel to the C_2 dumbbells, decreases with increasing temperature, whereas a shows a typical positive thermal expansion. Differing to previous work ternary palladium acetylides A_2PdC_2 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.