I have found a paper which is interesting, it is possible to form exotic calcium carbides under high pressures. For more details see Y.L. Li, S.N. Wang, A.R. Oganov, H.Y. Gou, J.S. Smith and T.A. Strobel, Nature Communications, 2015, 6, 6974. The supporting information with this paper includes details of the unit cells of the exotic carbides. I hope to bring you pictures soon of these.