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 n0ex0ex=0ex0ex0--10 (optimum is n0ex0ex=0ex0ex5). The Young's modulus per tube along the tube axis varies from 640 to 525 GPa for n0ex0ex=0ex0ex0 to 5.