Fee positions

FIGURE 6.8 The FeSj ctystal structure. The Fe ions occupy the fee positions the cubic ceii aiso contains four S-S dumbells. 

Fig. 5.2-2 (a) Surface diagrams face-centered cubic (fee) positions given in terms of a/2 [2.2] (b) Surface diagrams body-centered cubic (bcc) positions given in terms of a/2 [2.2] (c) Surface diagrams diamond, GaAs (positions given in terms of a/2) Ga atoms are denoted by unprimed symbols As atoms by primed symbols and shaded circles [2.2]... 

The structure of pyrite, FeS2 can be considered as a modified NaCl structure (cubic, Pa3) Fe atoms occupy fee lattice positions S-S covalently bonded pairs are also centered on fee positions but they alternate in directionality. This alternation destroys the overall face-centered symmetry. Pyrite is stable up to 743 °C at which temperature it breaks down to pyrrhotite and sulfur [387]. The phase relationship between pyrite and the orthorhombic (Pmnn) polymorph, marcasite, is still enigmatic [388]. In marcasite, S-S pairs point in the same direction in each layer. This causes a distortion from cubic to orthorhombic. The b dimension of the marcasite unit cell is almost identical with the lattice parameter of pyrite. In the transformation of marcasite to pyrite an orientation relationship 001 p// 101 m, with <100>p//[010]m is observed [389]. For the reverse transformation of pyrite to marcasite the similar atomic arrangement in pyrite 001 and marcasite 101 planes was mentioned [390]. 

The Ag (100) surface is of special scientific interest, since it reveals an order-disorder phase transition which is predicted to be second order, similar to tire two dimensional Ising model in magnetism [37]. In fact, tire steep intensity increase observed for potentials positive to - 0.76 V against Ag/AgCl for tire (1,0) reflection, which is forbidden by symmetry for tire clean Ag(lOO) surface, can be associated witli tire development of an ordered (V2 x V2)R45°-Br lattice, where tire bromine is located in tire fourfold hollow sites of tire underlying fee (100) surface tills stmcture is depicted in tlie lower right inset in figure C2.10.1 [15]. 

Surprinslngly, we observe an drastic effect of the concentration on the SRO contribution (figure 2) indeed, in PtaV, the maxima are no longer located at a special point of the fee lattice but the (100) intensity is splltted perpendicularly in the (010) direction and presents a saddle point at (100) position. Notice that these two maxima are not located just above Bragg peaks of the ordered state the A B ground state presents Bragg peaks at ( 00) and equivalent positions whereas the SRO maxima peak between ( 00) and (100). 

We have measured the experimental SRO contribution in PtsV and Pt V alloys. The PtsV SRO displays maxima at (100) positions despite a ground state built with the (1 0) concentration wave. For Pt V, the maxima are not located at special points of the fee lattice. 

Cu crystallizes in the fee and its melting point is 1356 K. The experimental data for single-crystal Cu/H20 interfaces are also controversial. 567 570,572 57X The first studies with Cu(l 11), Cu(100), and Cu(l 10) in surface-inactive electrolyte solutions (NaF, Na2S04) show a capacitance minimum at E less negative than the positive limit of ideal polarizability of Cu electrodes (Table 11). depends on the method of surface... 

Figure 12(a) shows graphically the dependence of the pzc on the crystallographic orientation of the surface for Ag, Au, and (tentatively) Cu, all three crystallizing in the same fee system. The plots exhibit a typical pattern, with minima and maxima that fall at the same angle for all three metals, and that are correlated with the density of atoms on the given surface. In particular, the pzc is more positive for dense surfaces and more negative for open surfaces. 

A consistently anomalous (with respect to electrochemical evidence) position of Au has been found by two different groups. According to Kuznetsov etal.,437 the complete neglect of differential overlap (CNDO) method predicts for any given metal a weaker interaction on the more dense surface. Thus the predicted sequence is (111) < (100) < (110) for fee metals such as Cu, Ag, and Au and (0001) < (1100) for hep metals such as Zn and Cd. However, for the most compact surfaces, the calculated sequence is Hg < Ag(l 11) < Cu(l 11) =Zn(0001) < Au(l 11) < Cd(0001). 

Other data support the above picture. Hexanol adsorbs very weakly on Ag(l 10), more weakly than expected, and in any case less than on the (100) face.440 Such a poor adsorption on (110) faces has been explained in terms of steric hindrance caused by the superficial rails of atoms. Consistently, adsorption on the (110) face of Cu is vanishing small.587 Predictions based on a linear regression analysis of the data for pentanol (nine metals) give a value of-12 kJ mol 1 for Cu(l 10) and about -16 kJ mol 1 for Au(110). No data are available for polycrystalline Au, but Au(l 11) is placed in the correct position in the adsorption of hexanol.910 Thus, these data confirm the hydrophilicity sequence Hg < Au < Ag and the crystal face sequence for fee metals (111) < (100) < (110). 

Figure 21. Angular movement of the fee end of a bilayer during the flow of a cathodic current using the conducting polymer as cathode. A platinum sheet (left side of the picture) is used as anode. The reference electrode is observed at the bottom, a to e Movement during the reduction process e to a Movement under flow of an anodic current. The movement is stopped at any intermediate point (a, b, c, d, or e) by stopping the current flow, and this position is maintained for a long time without polarization.

As schematically represented in Fig. 3 the structure can be considered two interpenetrating fee lattices of 8,2(8,2)12 units the 8,2(8,2)12 units of each fee lattice differ only by the 90° rotation of these units. Thus there are eight of these 8,2(8,2)12 units or 1248 8 atoms in the unit cell. The metal atom positions and the location of the remaining 8 atoms in the structure can be pictured in the octant of the cell shown in Fig. 3. Six metal atom sites exist in each octant of the ceil, and these are statistically half-filled. The sites are located 1.27 10 pm (for YB g) inside the cell from the center of each face of an octant one such site is depicted in Fig. 3. The center of each octant is occupied by either a 36- or a 48-8 atom group, which are labeled, respectively, configurations I and II (Fig. 4). Half of the octants contain configuration I, and half contain 11 in a random fashion. ... 

The isomer shift, d, arises from the Coulomb interaction between the positively charged nucleus and the negatively charged s-electrons, and is thus a measure for the s-electron density at the nucleus, yielding useful information on the oxidation state of the iron in the absorber. An example of a single line spectrum is fee iron, as in stainless steel or in many alloys with noble metals. 

The charge distribution determined within clusters by CNDO has been reported for only a few cases. Let us consider only one cluster, the 13-atom fee cluster with only two geometrically different types of atom. There is a center atom with 12 nearest neighbors, and there are 12 surface atoms each with 4 nearest neighbors. At the equilibrium bond length (0.34 nm) the center atom has a net positive charge, but this situation is reversed at the bulk experimental distance (0.288 nm). 

The c(4 x 2)-2CO structure observed20 at Ni(lll) at room temperature has CO occupying both fee and hep threefold hollow adsorption sites with a surface coverage of 0.5 ML. So as to maximise the 0-0 distance, the molecular axis is tilted away from the surface normal towards atop positions. Corrugation of the adlayer is attributed to a CO-induced buckling of the surface nickel atoms, which is manifested by height differences between adjacent CO molecules (Figure 8.6). 

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