Theoretical study on the structural phase transformation of BeO and the electronic structure of deformed carbon nanotubes

Abstract

We study the pressure-induced phase transformation of BeO through first-principles pseudopotential calculations within the local-density-functional approximation (LDA). The stable wurtzite structure transforms into a zinc-blende structure at a pressure 62 GPa and then successively into a rocksalt phase at 127 GPa. With the generalized-gradient approximation (GGA) for the exchange-correlation potential, we find the transition pressures to be increased significantly, without altering the transition sequence. The wurtzite structure transforms into the zinc-blende structure at 91 GPa, while the zinc-blende-to-rocksalt transition occurs at 147 GPa. This unusual transition sequence is attributed to the large charge asymmetry and the small bond length of BeO. As compared to other metals or insulators, which undergo structural phase transitions via metallization and/or increase of coordination number, the wurtzite to zinc-blende transition of BeO is unusual, because the packing fraction is invariant. To understand the transport behavior of deformed tubes, it is important to investigate the effect of mechanical deformations on the electronic structure. We perform extensive tight-binding and first-principles pseudopotential calculations within the local-density-functional approximation to investigate the effect of radial deformations on the electronic structures of the carbon nanotubes. Calculations of the electronic structure demonstrate that band gap modification such as opening and closure is easily achieved by radial deformations perpendicular to the tube axis. Deformations open the gap in metallic armchair tubes only if the mirror symmetries are broken, and further distortions enhance greatly the gap up to about 0.1 eV due to new bonds formed between facing layers. These gaps are sufficiently large enough for the tube to be semiconducting. Similar deformations increase the gap in small-gap zigzag tubes, but gap closure occurs due to enhanced $σ^*-π^*$ hybridization ...