Ternary Transition Metal Acetylides A I 2 M 0 C 2 ( A I = K, Rb; M 0 = Pd, Pt): Neutron Diffraction Studies and Electronic Properties

The crystal structures of K 2 PdC 2 , K 2 PtC 2 , and Rb 2 PdC 2 ( P $\bar3$ m 1, 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 2 2– 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 , which is 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 2 PdC 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.