Octopolar reconstruction

Figure 32. Schematic drawing of OH-terminated (left), bulk-terminated (middle) and octopolar reconstructed (right) NiO(l 11) surfaces...

Fig. 9. Simple schematic diagrams of the adsorption geometries of (a) NO and (b) CO on the octopolar reconstruction of NiO(l 11), proposed on the basis of NEXAFS results [75],...

Fig. 10 Schematic drawing of the Ni-terminated octopolar reconstruction of NiO(l 11). Small circles stand for Ni atoms and the large ones for O atoms. The first and second planes are respectively 75% and 25% vacant. The arrows indicate the basis vectors of the p(2x2) surface lattice mesh. The symmetry related radial relaxations 5,5 and 5 are respectively represented around apex atoms labeled 1 and 2 by dashed and dotted arrows and applies to the second oxygen and the third nickel layers that are patterned. (Right) Side view of the octopolar reconstruction and of the two first relaxations and Q perpendicular to the surface plane.

Fig. 11 (a) Experimental Patterson map derived from the in-plane scattering from an air annealed NiO(lll) single crystal (b) Calculated Patterson map for a relaxed octopolar reconstruction (c) Comparison between the experimental (right half circles) and calculated (left half circles) structure factors. 

To summarise, the theoretically predicted octopolar reconstruction effectively applies to air annealed NiO(lll) in order to overcome the divergence of the electrical field. Interestingly, the observed relaxations remain very limited with respect to the ideal structure. 

The in-plane scattering has been measured at several temperatures up to 950K (Fig. 12). The structure factors continuously evolve indicating that the internal structure of the reconstruction changes during anneals. The final structure, which is expected to be close to the final transformation state, fortunately yields much larger intensities than the octopolar reconstruction and several reconstruction rods and CTRs could be measured in this case. The out-of-plane CTRs still resemble the octopolar reconstruction showing that the number of planes involved remains close to that of the octopolar reconstmction. 

Fig. 13 (Left) Structural transformation steps needed to obtain the spinel configuration starting from the O-terminated octopolar reconstruction. Large circles (resp. small) stand for O (resp. Ni) atoms. (1) O-terminated Octopolar reconstruction (2) Rotation and centrifuge motion of the Ni atoms (3-4) translation of an O atom on top of a Ni atom (5) global [010]/3 shift of the reconstructed layer with respect to the bulk. The final situation corresponds to a spinel configuration. (Right) Out of plane positions of the atoms.

Obviously, NiO(l 11) is terminated and stabilised by p(2x2) reconstructions with different internal configurations. Importantly the electrostatic criterion is always fulfilled showing that it is very important when polar surfaces are considered. Interestingly, the octopolar reconstruction prediction and the observation of the p(2x2) surface cell were far not enough to understand the polar NiO(lll) surface. Note also that oxide thin films may not exhibit the same surface structure as their bulk counterpart. The studies carried out on thin oxide films may thus only be carefully extrapolated to the bulk surface. 

The recent prediction that a particular p(2x2) octopolar surface reconstruction (Fig. 10), which cancels the divergence of the electric field in the crystal, may stabilise the surface [93] and the existence of (111) facets on small... 

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