Atomic reconstruction of niobium (111) surfaces

The {111} surface of niobium single crystals has been investigated by ultra-high vacuum scanning tunneling microscopy. An atomic reconstruction has been experimentally observed and has been characterized by a lattice constant two times larger than that expected for a body-centered cubic structure. Moreover, an upper incipient structure is observed on these (2 × 2) surfaces. The upper unit cell is 23 times larger than the bulk one and is also rotated by 30° with respect to the bulk structure. A model is proposed to explain the competition between a covering of the free surface by a non-dense atomic structure and an island formation of niobium atoms in the dense configuration. The experimental results are finally discussed with the help of ab initio calculations used to determine the surface energies of the associated atomic configurations.