Structure, Symmetry and Chirality of bcc and hcp Surfaces

S.J. Jenkins and S.J. Pratt

Department of Chemistry, University of Cambridge, Cambridge, UK

The intrinsic chirality of certain high-index surfaces of fcc metals has been recognised for some time, and has generally been described in terms of the structural details associated with low-coordination kink sites [1, 2, 3].  One major deficiency in this approach, however, is that it is far from clear how surface chirality relates to surface structure when considering non-fcc materials.  Indeed, we recently pointed out [4] that there exists a class of bcc surface that is indisputably chiral despite possessing no kink sites whatsoever.  In order to describe the relationship between the symmetry and structure of general high-index surfaces more appropriately, therefore, we have developed a new conceptual framework that is applicable to the surfaces of any crystal structure [5].  Examining the surfaces of bcc and hcp materials within this framework, we can identify well-defined surface structure and symmetry categories, and make definitive statements about the combinations of these that may arise.  For example, all bcc surfaces fall into one of three structure categories (flat, stepped or kinked) and one of five symmetry categories (chiral, singly-, doubly-, triply- or quadruply-reflexive), but certain combinations can never arise (e.g. chiral and flat, or doubly-reflexive and stepped).  In the more complex hcp case, we identify precisely six distinct structure categories and nine distinct symmetry categories.  Amongst the notable surface types that are permissible for hcp materials, we find a class having intrinsic glide symmetry, another displaying an intrinsic racemic quality, and yet another featuring double-chirality akin to that found in the phenomenon of molecular diastereoisomerism.  Our approach furthermore allows these different surface properties to be deduced directly from knowledge of the Miller indices, rather than painstakingly obtained by inspection of the surface atomic coordinates.

[1] C.F. McFadden, P.S. Cremer and A.J. Gellman, Langmuir 12, 2483 (1996)

[2] A. Ahmadi, G.A. Attard, J. Feliu and A. Rodes, Langmuir 15, 2420 (1999)

[3] G.A. Attard, J. Phys. Chem. B 105, 3158 (2001)

[4] S.J. Pratt and S.J. Jenkins, Surf. Sci. Lett. 585, L159 (2005)

[5] S.J. Jenkins and S.J. Pratt, Surf. Sci. Reports (invited, accepted)