Traditionally, all superhard carbon phases including diamond are electricinsulators and all conductive carbon phases including graphite are mechanicallysoft. Based on first-principles calculation results, we report a superhard butconductive carbon phase C21-sc which can be obtained through increasing the sp3bonds in the previously proposed soft and conductive phase C20-sc (Phys. Rev. B74, 172101 2006). We also show that further increase of sp3 bonds in C21-scresults in a superhard and insulating phase C22-sc with sp3 bonds only. WithC20-sc, C21-sc, C22-sc and graphite, the X-ray diffraction peaks from theunidentified carbon material synthesized by compressing the mixture oftetracyanoethylene and carbon black (Carbon, 41, 1309, 2003) can be understood.In view of its positive stability, superhard and conductive features, and thestrong possibility of existence in previous experiments, C21-sc is a promisingmulti-functional material with potential applications in extreme conditions.
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