Quasi-hexagonal structure

Cuesta and Kolb [52] have studied bromide adsorption on Au(lOO) electrodes using in situ STM. Two quasi-hexagonal structures were found, which was in agreement with the previously published SXS data. 

FIGURE 10.1 Unreconstructed (lxl) noble metal surfaces for (a) (100), (b) (111), and (c) (110) planes. Reconstructed (d) quasi-hexagonal structure (only the topmost layer is shown) and (e) (1x2) (110) plane. Black, dark-gray, and light-gray balls denote the upper (top), second, and third lower rows of the crystal lattice, respectively. 

For example, it is possible to find with the help of the STM images that the highly crystalline nature of the oxide film on the nickel surfaces is Ni(100) in the alkaline solutions [46]. At low potentials, a well-ordered rhombic structure is formed, which is resistant to reduction and is assigned to the irreversible Ni(OH)2 formation. At higher potentials, it is possible to see a quasi-hexagonal structure consistent with NiO(lll). In other words, a crystalline oxide is formed of the NiO(lll) order independently of the crystal orientation of nickel. Moreover, it is very useful and practical to obtain a reduced surface by the cathodic treatments, where the hydrogen evolution produces the chemical and electrochemical reductions of nickel. However, this has to be done with careful attention and in light alkaline solutions to avoid nickel dissolution. When this is performed, monoatomic steps mostly oriented in the (100) and (111) directions are observed [47]. 

In general, for high-coverage metal adlayer phases that form at potentials close to the reversible potentials of bulk deposition, incommensurate and hexagonal or quasi-hexagonal structures are common. The interatomic distances are potential-dependent, often having values below the distances in bulk materials. This phenomenon has been termed dectrocompres-sion. Commensurate adlayer structures are less common and usually form with coadsorbed anions. 

Fig. XVI-8. (a) The quasi-hexagonal surface structure of clean Pt(lOO) surface, (b) Adsorption of CO lifts this reconstruction to give the structure corresponding to the termination of (100) planes (from LEED studies). [Reprinted with permission from G. Ertl, Langmuir, 3, 4 (1987) (Ref. 56). Copyright 1987, American Chemical Society.]...

Figure 4.2 Quasi-hexagonal dislocation loop lying on the (111) glide plane of the diamond crystal structure. The <110> Burgers vector is indicated. A segment, displaced by one atomic plane, with a pair of kinks, is shown a the right-hand screw orientation of the loop. As the kinks move apart along the screw dislocation, more of it moves to the right.

The hexaboride crystal structure is related to the CsCl structure so by analogy the glide planes are (100) and the glide directions are (100). At the cores of glide disloca-tions the structure becomes quasi-hexagonal. 

Fig. 4.8 Theoretical model of the (1 x 5) reconstructed surface of Ir (001). Six [110] atomic rows squeeze into 5 row-spacings of the (1 x 1) structure, resulting in a quasi-hexagonal, or pseudo-hexagonal, atomic arrangement with buckled [110]...

An example of such order is shown by the hexagonal symmetry of SBS as revealed by LAXD, electron microscopy and mechanical measurements. In composite materials the choice of phase is at the disposal of the material designer and the phase lattice and phase geometry may be chosen to optimise desired properties of the material. The reinforcing phase is usually regarded elastically as an inclusion in a matrix of the material to be reinforced. In most cases the inclusions do not occupy exactly periodic positions in the host phase so that quasi-hexagonal or quasi-cubic structure is obtained rather than, as in the copolymers, a nearly perfect ordered structure. 

Indeed, in situ STM imaging with lateral atomic resolution of a flame-annealed Au(lOO) substrate in deaerated perchloric or sulphuric acid solutions fi ee of Ag ions shows the existence of a potential-induced surface reconstruction with an undulated quasi-hexagonal ( quasi-hex ) structure at Ew < 240 mV (cf. Figs. 2.5 and 2.7). Reconstructed domains were not observed at higher hE, which indicates that the quasi-hex reconstruction of the Au(lOO) surface is lifted by the applied positive electrode potential. In deaerated perchloric or sulphuric acid solutions containing Ag ions, Ag UPD is clearly indicated in cyclic voltammograms (Fig. 3.20a) as well as in q E) or r E) isotherms at = constant in the underpotential range 0 mV < A < 600 mV (Fig. 3.20b). Electrosorption valency measurements under ITL... 

Cu UPD on Au(lOO) is less studied. In situ STM studies showed that Cu UPD adlayers on Au(lOO) are less ordered than on Au(lll) substrates. A relatively stable and expanded quasi-hexagonal Cu overlayer structure was observed from medium to... 

Adsorption of 9/16 ML Na on Al(lll) at low temperature leads to the formation of a well-ordered (4 x 4) phase. The structure of this phase has been shown by SEXAFS [48] to consist of an epitaxial, quasi-hexagonal Na layer on... 

The characteristic (5x1) reconstruction of the clean surface expresses after careful cleaning [110] and has been explained in a model structure by coverage of an fcc(lOO) surface with a quasi-hexagonal close packed monolayer of the Ir... 

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