Physica B 323 (2002) 203–205 Electronic structure of single wall carbon nanotube bundles under compression as compared to graphite and hexagonal graphene stacking Marcel H.F. Sluiter a, *, Vijay Kumar a,b , Yoshiyuki Kawazoe a a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japan b Dr. Vijay Kumar Foundation, 45 Bazaar Street, K.K. Nagar (West), Chennai 600 078, India Abstract Single-wall carbon nanotube bundles (SWCNTBs) undergo structural phase transformations under pressure. Examination of the electronic band structure reveals that bundles of SWCNTs with three-fold symmetry have increased quasi band gaps, while bundles of SWCNTs without three-fold symmetry remain metallic under hydrostatic compression. r 2002 Elsevier Science B.V. All rights reserved. PACS: 61.46.þw; 61.50.Ks; 71.15.Nc; 73.63.Fg Keywords: Fullerenes; Nanotubes; Hydrostatic pressure; Phase transition; Electronic structure Recently, conflicting results have been obtained concerning the transformations in SWCNTBs under hydrostatic pressure [1,2]. Ab initio total energy calculations on armchair and zigzag single wall carbon nanotube bundles have shown [3] that the nature of the phase transformation under hydrostatic pressure is determined by the sym- metry of the nanotubes. Bundles consisting of nanotubes with three-fold symmetry undergo a gradual polygonization as the pressure is in- creased. Bundles made from nanotubes without three-fold symmetry may have small deviations from hexagonal symmetry of the lattice even at zero pressure and exhibit a first-order transition to a monoclinic structure concurrent with a deforma- tion of the nanotube cross section into an oval shape as the pressure is increased. This transition cannot be correctly observed if hexagonal symme- try is imposed on the unitcell [4,5] as was realized by Charlier et al. [6]. Study of the electronic structure changes under pressure is important for understanding the interaction of gases such as hydrogen in nanotubes. Under pressure, the contact area between individual nanotubes in SWCNTBs increases and the inter-nanotube distance decreases. This leads to stronger interactions between the nanotubes. Also in cases where the nanotubes are not three-fold symmetric, local AA-type arrangements of neighboring nanotubes occur. LDA calculations of stacked graphene layers show that Bernal graphite AB stacking is favored over AA stacking by about 8 meV=atom: AA stacking also has a larger layer spacing (0:360 nm for AA *Corresponding author. Tel.: +81-22-215-2481; fax: +81- 22-215-2052. E-mail address: marcel@imr.edu (M.H.F. Sluiter). 0921-4526/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(02)00900-6