Sc crystallizes in the hcp structure at ambient pressure. The structure of the high-pressure phase Sc-II has been debated for a long time. Most recently, two different solutions for the Sc-II phase were proposed, both involving a composite incommensurate structure consisting of a host and a guest substructure. To explore the Sc-II crystal structure we perform first-principles total-energy calculations and find that the approximant modeling of the structure suggested by McMahon et al. [Phys. Rev. B 73, 134102 (2006)] is the stable structure for pressures above the calculated transition pressure of $20\mathrm{GPa}$. This theoretical value is in perfect agreement with the experimentally measured value of $23\mathrm{GPa}$. Analyses of the band structures provide an explanation why the structure proposed by Fujihisa et al. [Phys. Rev. B 72, 132103 (2005)] is not the correct Sc-II structure.

• Received 22 June 2006

DOI:https://doi.org/10.1103/PhysRevB.74.104119