Registry Shifts

Figure 4.1 Schematic vertical cut through a surface (a) when bulk terminated, (b) exhibiting multilayer surface relaxation and (c) a registry shift, respectively.

In many cases, the formation of a surface leads to atomic displacements in the surface selvedge which, however, saves the lateral symmetry resulting by mere bulk truncation. As already mentioned, the only allowed atomic movements for this case are - for the surface of an elemental solid with only a single atom in the layer unit cell - rigid movements of surface-parallel layers (Figure 4.1) either normal to the surface ((multi)layer relaxation) or parallel to it registry shift). For surfaces with more than one atom in the layer unit cell (compounds, as an example), in addition intraunit-cell relaxations are possible without breaking the lateral translational symmetry. 

Investigations of surface-parallel layer relaxations, that is, rigid shifts of surface layers with respect to bulk layers (registry shifts), are rather rare. This is because for many low-index surfaces there is no reason for such shifts because atoms in layer i... 

Of course, the second layer may also be registry shifted with respect to the third layer (and so on). Yet, the corresponding values revealed are either neghgibly small or within the error limits of the structure determination. The same holds for other open surfaces for which a top-layer registry shift has been detected, as Fe(210) [20] or Fe(310) [16],... 

The atomic structure of a surface is usually not a simple tennination of the bulk structure. A classification exists based on the relation of surface to bulk stnicture. A bulk truncated surface has a structure identical to that of the bulk. A relaxed surface has the synnnetry of the bulk stnicture but different interatomic spacings. With respect to the first and second layers, lateral relaxation refers to shifts in layer registry and vertical relaxation refers to shifts in layer spacings. A reconstructed surface has a synnnetry different from that of the bulk synnnetry. The methods of stnictural analysis will be delineated below. 

Compound CAS Registry Number Boiling point, °C Density at 20°C, g/cm Dipole moment, 10-"° Cm Index of refraction, < PR nmr Parameters chemical t shift, ppm ... 

Film CAS Registry Number Growth method Possible dopants Oxidizing color shift References... 

Compound CAS Registry Number Therm ochromic transition Approximate °C Spectral shift References... 

Fig. 14. A grid superimposed on the basal plane of a triangular single-layer M0S2 nanocluster. The intersections of the white lines indicate the sulfur atomic positions on the basal plane. At the edges, the protrusions are observed to be shifted out of registry with the basal plane lattice. An STM line scan across the bright brim of the single-layer M0S2 nanocluster is illustrated on the right. The arrow indicates the direction and position of the scan in the image.

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