Giant fullerenes (e.g. C_>90) show clear connections with short carbon nanotubes. Herein we report the X-ray structures of eight EMFs containing a La₂C₂ cluster, namely, La₂C₂@C₂(41)-C₉₀, La₂C₂@D₃(85)-C₉₂, La₂C₂@C₁(132)-C₉₄, La₂C₂@C₂(157)-C₉₆, La₂C₂@C₁(175)-C₉₈, La₂C₂@D₅(450)-C₁₀₀, La₂C₂@C_s(574)-C₁₀₂, and La₂C₂@C₂(816)-C₁₀₄. It is evident that the cage length increases roughly along with increasing of the carbon atom number, resulting in an increase of the La-La distance. This is a clear indication that giant fullerenes tend to grow along the long axis to achieve a tubular appearance. Encapsulation of a large La₂C₂ cluster inside D₅(450)-C₁₀₀ induces a 5% axial compression of the cage, as compared with the structure of La₂@D₅(450)-C₁₀₀.^[1] Moreover, it is demonstrated that the defective cage of C₂(816)-C₁₀₄ is a starting point to form the other tubular cages known so far, i.e., D₅(450)-C₁₀₀, C_s(574)-C₁₀₂, and D_3d(822)-C₁₀₄, presenting evidence for the top-down formation mechanism of fullerenes.^[2] Overall, our experimental results have unambiguously confirmed that insertion of a C₂-unit exerts a synergistic effect in stabilizing these giant cages_.^[3]

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