Rocksalt surfaces

Other rocksalt surfaces, such as the 110 and the 111 faces, have higher surface energies and are thus less stable. Their surface atoms 

---

Fig. 6. Kink in < 100 > step on 100 surface of rocksalt structure as chemisorption site.

Periclase (MgO) has the rocksalt structure, which is face-centered cubic with each Mg-+ cation surrounded by six 0 anions in a regular octahedral arrangement, and each O " similarly coordinated to six cations at the corners of a regular octahedron. By far the most stable surface for oxides having the rocksalt structure is the (100), illustrated in Fig. 8.13... 

Fig. 8.13. A model of the (100) surface of the rocksalt structure of MgO. Large circles are oxygen anions, small circles are Mg cations. A (100) step to another (100) terrace is shown, as also is a missing anion point defect.

The above considerations are borne out experimentally on most rocksalt ionic compounds. For example, when magnesium metal is burned, the tiny MgO smoke particles that are formed are almost perfect cubes (see Fig. 2.4 in Ref. 1). The need to form a non-polar surface and to maximize the ligand coordination of surface ions makes the (100) surface energy much lower than that of other possible surfaces in the rocksalt structure. This is also manifest in the cubic shape of grains of table salt, NaCl. The (110) surface of MgO, whose ions are only four-fold coordinated, is also much less stable than the (100) surface [24]. 

The rocksalt structure consists in two interpenetrating fee lattices of anions and cations, in which all atoms are in an octahedral environment. It is met in alkaline-earth oxides (MgO, CaO, SrO, BaO) and in some transition metal oxides like TiO, VO, MnO, FeO, CoO, NiO, etc, with cations in a 4-2 oxidation state. The non-polar surfaces of lowest Miller indices are the (100) and (110) surfaces they have neutral layers, with as many cations as oxygen ions, and their outermost atoms are 5- and 4-fold coordinated, respectively. Actually, planar surfaces can only be produced along the (100) orientation. The polar direction of lowest indices is (111) it has an hexagonal 2D unit cell, three-fold coordinated surface atoms and equidistant layers of either metal or oxygen composition. 

There exist three polymorphs of ZnO, with zinc-blende, wurtzite and rocksalt structures. Atoms are tetra-coordinated in the two first forms, while in the latter, which is stabilized at higher pressure, the local environment is octahedral. The most thoroughly studied phase is the wurtzite one. Non polar (1010) and (1120) as well as polar (0001) or (OOOT) planar surfaces can be prepared. 

The SrTiOa (111) [211-214] and (110) polar surfaces [215-217] and the BaTi03(lll) surface [218] have been produced and studied. At variance with rocksalt polar surfaces, many of these investigations suggest that one can obtain non-reconstructed quasi-planar polar surfaces. It should be realized that the perovskite structure is such that there exist ordered configurations of vacancies in the surface layers compatible with (1x1) diffraction patterns. In addition, SrTiO3(110) displays a variety of reconstructions, such as c(2 x 6) [215,217], under reducing conditions. No precise determination of the layer stoichiometry has been performed. 

Fig. 17). This occurs on the rocksalt oxide (111) surfaces (/ = 1/2), the ZnO(OOOl) surface (/ = 1/4), the oxygen termination of corundum oxide (0001) surface (/ = 1.5), etc. These partial fillings were indeed found in the quantum calculations. For some other polar surfaces, / is integer, and the surface can remain insulating. This takes place for example, on the (111) or (110) polar surfaces of SrTiOs (/ = 1). 

Nickel oxide, like MgO, usually adopts the relatively simple rocksalt lattice. The natural cleavage plane of NiO is (100), and studies have shown that the resulting surfaces are of high quality, relaxing only slightly away from the ideal bulk terminated (100) surface (see Fig. 1). Structural determinations of adsorbates have been performed on both this surface and the polar (111) surface. To circumvent surface charging problems almost all of these studies have been performed on highly oriented NiO thin films. 

Several adsorption studies on probe molecules such as CO, NO, CO2, H2O, H2 and O2 have been undertaken. Here we mention the remarkable differences we find for CO adsorption on rocksalt (100) and corundum (0001) surfaces. While CO stands upright - as expected - with the carbon end pointing to the substrate on NiO(lOO) it lies almost flat on Cr203(0001) and V203(0001). We show, that while the adsorption energies are similar in both cases, CO as a probe molecule reacts strongly towards differences in the electric field distributions on the surfaces. The experimental observations are complemented with theoretical... 

Fig. 1 shows the rocksalt lattice [15]. We will discuss MgO and NiO as limiting cases of oxides, one containing a simple metal ion and the other one a transition metal ion. The (100) surface of such a material represents a non-polar surface, the (111) surface represents a polar oxide surface. Since the lattice constants are very similar for both oxides (MgO 4.21 A, NiO 4.17 A) [15], we expect the surface structures to be similar. The non-polar surface exhibits a nearly bulk terminated surface as shown in Fig. 2a and it is very similar for both materials. We have put together information from FEED [16-21] and STM [22-25] analysis. There is very small interlayer relaxation and only a small rumpling of the surface atoms, whereby the larger anions move outwards and the small cations very slightly inward. A completely different situation is encountered for the polar (111) surfaces. Due to the divergent surface potential [13] on an ideally, bulk terminated polar surface, the surfaee reconstructs and exhibits a so... 

---