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 ${\mathrm{Br}}_2$ 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 ${\mathrm{KC}}_{16}$ if the K cannot penetrate the tubes and ${\mathrm{K}}_1{\mathrm{C}}_{10}$ ( ${\mathrm{K}}_5^{\mathrm{exo}}{\mathrm{K}}_3^{\mathrm{endo}}{\mathrm{C}}_{80}$, 3 within the tube) if they can. We predict the optimum structure and the associated powder-diffraction x-ray pattern expected for ${\mathrm{K}}_n{\mathrm{C}}_{80}$ from $n=0\mathrm{}–10$ (optimum is $n=5\mathrm{}$). The Young's modulus per tube along the tube axis varies from 640 to 525 GPa for $n=0\mathrm{}$ to 5.