Synthesis of Mg2C: a magnesium methanide.

Carbides, which have been intensively studied for more than half a century, still remain a major center of scientific and technological attention. A large number of new promising phases have been predicted to exhibit exceptional structural and electronic properties, as well as high-temperature superconductivity. In particular, magnesium compounds containing Mg C and C C bonds are quite fascinating from both fundamental science and synthesis perspectives. The properties of such compounds are determined by the nature of the chemical bonds present, allowing a variety of different materials to be suggested, such as ionic semiconductors, superhard sp and/or sp carbon networks intercalated with Mg, 9] and novel polymeric carbides. Furthermore, the intrinsic nature of Mg C chemical bonding is of great importance to polar organometallic compounds and to understanding the covalent/ionic nature of carbanions. The ambient-pressure chemistry of the Mg C system was studied quite thoroughly in the past. Magnesium forms an acetylide-type carbide, MgC2, [13] similar to all other alkalineearth metals. Mg also forms Mg2C3, [14] a derivative of propadiene (H2C=C=CH2), which is unique for the alkalineearth metals and is one of only a handful of examples that contain the rare [C=C=C] group. Herein, we present the formation of a third carbide of magnesium, namely Mg2C. This compound is stabilized at pressures above 15 GPa, but is fully recoverable to ambient conditions and contains the very unusual C methanide anion. 15] Both in situ and ex situ X-ray diffraction experiments revealed the formation of magnesium carbide, Mg2C, directly from a stoichiometric mixture of the elements at pressures between 15–30 GPa and temperatures of 1775–2275 K (Figure 1). Samples were recovered in powder form, which have a brown color, and Rietveld analysis indicates that the compound takes on the antifluorite structure (Li2O) in the cubic crystal system with space group Fm3̄m (No. 225) with lattice parameter a = 5.4480(4) . A comparison of local bonding environments (structural coordination) for Mg2C is presented in Figure 1b. Contrary to Mg2C3 and MgC2, Mg2C does not contain covalent C C bonds. According to our structural data, the ambient-pressure Mg C distance in Mg2C (2.36 ) is larger than the minimal Mg C distances in both Mg2C3 (2.21 ) and MgC2 (2.17 ) and smaller than the Mg C distance in Al2MgC2 (2.487), where Mg has octahedral coordination. Carbon within Mg2C is coordinated eightfold by magnesium, whereas carbon coordination within Mg2C3 and MgC2 is much more sophisticated. If the whole carbon anions are considered as structural units, Mg2C3 and Mg2C have the same coordination number 8, but in the first case they form a distorted and elongated dodecahedron, while in the second case the coordination polyhedron is a regular cube. In MgC2 the C2 dumbbell coordination number is 6 (elongated octahedron). Among the Group 2 elements, beryllium forms the only known methanide-type carbide. Be2C, as well as a second known methanide, Al4C3, are quite hard, low-compressibility compounds with a large degree of covalent bonding character (the ionic/covalent nature is described below). The minority phase synthesis of Li4C was reported previously, but minimal yields (0–10%) have precluded definitive characterization. Although never experimentally observed until now, the isostructural magnesium analogue of Be2C, namely Mg2C, was first suggested by Corkill and Cohen about twenty years Figure 1. a) X-ray diffraction data with MoKa radiation (*), Rietveld refinement (c), and difference (at bottom). Tick marks are shown for Mg2C (top) and MgO impurity (bottom). b) Carbon and magnesium coordination in Mg2C. c) NMR spectrum of Mg2 C (99% of isotope purity).