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 K1C10 ( K5exoK3endoC80, 3 within the tube) if they can. We predict the optimum structure and the associated powder-diffraction x-ray pattern expected for KnC80 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.