Na2C2 and K2C2: Synthesis, crystal structure, and spectroscopic properties

By the reaction of sodium or potassium salved in liquid ammonia with acetyl ene and subsequent heating in high vacuum Na2C2 and K2C2 could be synthesis ed as single phase products. The crystal structures described by Foppl coul d be confirmed by X-ray and neutron diffraction experiments (K2C2) on powde red samples. Both compounds crystallise in a tetragonal structure (I4(1)/ac d, no. 142, Z = 8) which can be described as a distorted variant of the ant ifluorite-structure type. At temperatures above room temperature (Na2C2: 58 0 K, K2C2: 420 K) a reversible phase transition (1(st) order transition) to a cubic modification (Fm(3) over bar m, no. 225, Z = 4) has been observed, analogous to the alkaline earth metal acetylides. This high temperature mo dification represents an undistorted antifluorite structure with disordered C-2(2-) dumbbells. The results of raman- and C-13-MAS-NMR-spectroscopic in vestigations are in agreement with acetylide dumbbells in the title compoun ds and allow a comparison to the respective monoalkalimetal and alkaline ea rth metal acetylides.