Figure 3: Carbon patterns in the Ca-C system.

(a) The isolated carbon anions in the Pnma structure of Ca₂C. (b) Carbon dimers observed in the C2/m structure of Ca₂C, P4/mbm and C2/c structures of Ca₃C₂, Immm structure of CaC, and C2/m or C2/c structures of CaC₂. (c) The carbon trimer occurs in the C2/m structure of Ca₂C₃ at zero pressure. (d) Zigzag C₄ groups observed in P2₁/c structure of CaC at 20 GPa. (e) Zigzag carbon chains in the Imma structure of CaC at 58 GPa. (f) Carbon chains with two types of carbon-carbon bondings in C2/c structure of Ca₂C₃ at 34.5 GPa. (g) Carbon chains with four types of carbon-carbon bongdings in P-1 structure of Ca₂C₃ at 40 GPa. (h) Armchair carbon chains in the Cmcm structure of CaC₂ at 4 GPa. (i) Carbon stripes in the P-1 structure of CaC₂ at 20 GPa. (j) Carbon ribbons in the Immm structure of CaC₂ at 10 GPa. (k) Carbon ribbons in the Imma structure of Ca₂C₃ at 65 GPa. Bond lengths (in Å) are given. The inequivalent C1, C2, C3, and C4 (occupying different Wyckoff positions in the unit cell, see Supplementary Table 1) are shown by red, blue, yellow, and grey spheres, respectively.