Two C28 Clathrates

Although carbon fullerenic clathrates, characterized by eclipsed bonds, have not been synthesized yet, their structure may be reflected in the assembly of tetrahedral diamondoid clusters (hollow diamonds) due to their assembling into the eclipsed configuration. The detection by El Goresy et al. (Lunar Planetary Sci 34:art. No. 1016, 2003a; C R Geosci 335:889–898, 2003b; Meteorit Planet Sci 39:A36, 2004) in highly shocked meteoritic rocks of a cubic diamond-like polymorph with almost 400 atoms per unit cell stimulated the present investigation on hypothetical small carbon clathrates with 4-membered rings on which hollow diamonds can be constructed via a cluster assembly. Two polytypes of a novel C28 clathrate, one body-centered orthorhombic (bco) and the other simple cubic (sc), are proposed, with a detailed ab initio characterization of the electronic and zero-wave-vector vibrational structures of the bco phase. The assembly of C14 diamondoid clusters into the sc-C28 array is briefly discussed in comparison with the above polymorph in order to illustrate a viable method of topological analysis of complex crystalline structures.