Position of K atoms in doped single-walled carbon nanotube crystals

Recent experiments by Lee et al. [Nature (London) 363, 255 (1997)] show that doping carbon single-wall nanotube (SWNT) ropes with K, Rb, or Br₂ leads to metallic conductivity, but the structure and properties are not known. We used molecular dynamics to predict structures and properties which should help motivate and interpret experiments on SWNT/K. We find the optimum stoichiometry to be KC₁₆ if the K cannot penetrate the tubes and K₁ C₁₀ (K^exo₅ C^endo₃ C₈₀, 3 within the tube) if they can. We predict the optimum structure and the associated powder-diffraction x-ray pattern expected for K_n C₉₀ from n = 0−10 (optimum is n = 5). The Young’s modulus per tube along the tube axis varies from 640 to 525 GPa for n = 0 to 5.