Nb(111) surface reconstruction

PhD thesis started in 2014, Benjamin DOUAT

Context. Niobium is widely used both for its mechanical properties, for instance in aeronautic industry, and superconductivity. However, its high reactivity to oxygen leads to a significant modification of its physical properties. For instance, 1%at. of dissolved oxygen results in a 1K drop of its superconducting transition temperature due to the formation of a thin oxide layer at the free surface. If Nb{100} and {110} surfaces have been extensively studied (laminated Nb sheets are textured in those directions), no data are available concerning the {111} surface.

Results. The {111} surface of niobium single crystals has been investigated by ultra high vacuum scanning tunneling microscopy. An atomic reconstruction has been experimentally evidenced. It is characterized by a lattice constant two times larger than that expected for a body centered cubic structure, called (2×2) reconstruction. Moreover, an upper incipient structure is observed on these (2×2) surfaces. The upper unit cell is (2×31/2by2x31/2) times larger than the bulk one and is also rotated by 30° with respect to the bulk structure. Also suggested in the middle of the 70’s on the basis of LEED experiments, the (2×2) reconstruction had never been finely characterized. A model is proposed to explain these reconstructions in the framework of energetical considerations.

   Nb_reconstruit                            Modele_Nb_reconstruit

Figure 1. Reconstructed niobium(111)  surface (left) and corresponding atomic model (right)



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