The metal-insulator transition of NbO 2 : An embedded Peierls instability

Results of first-principles augmented spherical-wave electronic-structure calculations for niobium dioxide are presented. Both metallic rutile and insulating low-temperature NbO 2 , which crystallizes in a distorted rutile structure, are correctly described within density functional theory and the local density approximation. Metallic conductivity is carried by metal t 2 g orbitals, which fall into the one-dimensional d ∥ band and the isotropically dispersing e g π bands. Hybridization of both types of bands is almost negligible outside narrow rods along the line X - R . In the low-temperature phase splitting of the d ∥ band due to metal-metal dimerization as well as upshift of the e g π bands due to increased p - d overlap remove the Fermi surface and open an optical band gap of about 0.1 eV. The metal-insulator transition arises as a Peierls instability of the d ∥ band in an embedding background of e g π electrons. This basic mechanism should also apply to VO 2 , where, however, electronic correlations are expected to play a greater role due to stronger localization of the 3 d electrons.