Using density functional theory within the local density approximation (LDA) and generalized gradient approximation (GGA), we discuss the relative stability of the orthorhombic (or the $\beta )$ phase and the hexagonal (or the $\delta )$ phase of tantalum pentoxide. We find that, both LDA and GGA pseudopotential calculations favor the $\delta$ phase, whereas spin-orbit all-electron calculations favor the $\beta \left(\delta \right)$ phase within the LDA (GGA); however, the all-electron energy differences are too small to be significant. The spin-orbit coupling has a significant effect on the band gaps and the band gap is indirect (direct) in the $\delta \left(\beta \right)$ phase. The chemical bonding between Ta and oxygen ions is of a mixed type (mainly ionic but partly covalent).

• Received 17 November 2003

DOI:https://doi.org/10.1103/PhysRevB.69.165202