Surface order-disorder transition

The ability of XPD and AED to measure the short-range order of materials on a very short time scale opens the door for surface order—disorder transition studies, such as the surface solid-to- liquid transition temperature, as has already been done for Pb and Ge. In the caseofbulkGe, a melting temperature of 1210 K was found. While monitoring core-level XPD photoelectron azimuthal scans as a function of increasing temperature, the surface was found to show an order—disorder temperature 160° below that of the bulk. 

Various types of work in addition to pV work are frequently involved in experimental studies. Research on chemical equilibria for example may involve surfaces or phases at different electric or magnetic potentials [11], We will here look briefly at field-induced transitions, a topic of considerable interest in materials science. Examples are stress-induced formation of piezoelectric phases, electric polarization-induced formation of dielectrica and field-induced order-disorder transitions, such as for environmentally friendly magnetic refrigeration. 

STM has also been used to examine the dynamics of potential-dependent ordering of adsorbed molecules [475-478]. For example, the reversible, charge-induced order-disorder transition of a 2-2 bipyridine mono-layer on Au(lll) has been studied [477]. At positive charges, the planar molecule stands vertically on the surface forming polymeric chains. The chains are randomly oriented at low surface charge but at higher potentials organize into a parallel array of chains, which follow the threefold symmetry of the Au(l 11) substrate as shown in Fig. 34. Similar results were found for uracil adsorption on Au(lll) and Au(lOO) [475,476]. 

Motoo [305] has also been able to show that an order-disorder transition in the overlayer can affect the activity of the surface (this may be related to the homogeneous adatom surface distribution vs cluster, or island formation), and that the elec-trocatalytic activity depends solely on the surface structure and not on bulk properties. The latter point has been demonstrated [306] by noting the similarity in the behavior of Au on Pt and of Pt on Au. Limited synergetic effects have been observed in both cases. 

The lattice gas model is used to elucidate the "diffusion order-disorder transition on catalyst surfaces [92-102]. Finally, as has been mentioned already, this model is important in decoding thermodesorption spectra. 

Fig. 12.4. Order-disorder transition in Cd arachidate multilayers (A) 20°C - chains are normal to the surface (B) 70°C - chains are tilted randomly (C) 95°C - chains acquired gauche defects and distances between head groups increased (D) 110°C - chains are completely disordered and distances between head groups increased further.

Figure 26 shows typical order parameter profiles obtained across the films at various temperatures. Due to the repulsive wall-A interaction the order parameter pA(z)-pB(z) is always depressed towards negative values for z near the walls. From previous studies of the same model in the bulk [190,191] one expects an order-disorder transition to a lamellar phase for T=2 (note AB/kB=l), and consequently one sees for D=20 and D=30 well-developed lamellae oriented parallel to the walls, evidenced by the periodic variation of the order parameter across the film, with an amplitude close to the saturation value unity already. However, for D=14 the system does not develop this type of order, and for D=24 there seems to be well-developed order near the right surface but not near the left surface, which is unexpected since both surfaces are equivalent. 

Region II, 0.38 to 0.27 h Water condenses over the surface elements that interact most weakly with water, to complete the solvent shell. There is a rise and a fall in the heat capacity at the junction of regions II and III, characteristic of an order—disorder transition, which can be understood in terms of the high-coverage phase change described by Hill (1949). 

Tapia and Eklund (1986) carried out a Monte Carlo simulation of the substrate channel of liver alcohol dehydrogenase, based on the X-ray diffraction structure for this enzyme. The addition of substrate and the associated conformation change induce an order—disorder transition for the solvent in the channel. A solvent network, connecting the active-site zinc ion and the protein surface, may provide the basis for a proton relay system. A molecular dynamics simulation of carbonic anhydrase showed two proton relay networks connecting the active-site zinc atom to the surrounding solvent (Vedani et ai, 1989). They remain intact when the substrate, HCOf, is bound. 

FK5.6.I9 Schematic representation of the structural formation and order-disorder transition for photoactive LBK showing (A) the compressed monolayer on the water surface with densely packed chromophore side chains oriented into the gas phase and the polymer backbone facing the water surface, and (B) LBK transfer from the water to a solid support, resulting in well-ordered smetic-tike (bilayered) multilayer assemblies. (C) After phcnoinduced trars to cts isomerization, a largely disordered struaure is obtained and the layered structure is completely lost (reproduced from reference 72 with permission from Wifey-VCH). 

Fig.8. Variation with temperature of the average segregant concentration at the Ll2(100) surface (solid lines) and at the first underlayer (dotted lines) in AB3 model alloy calculated in the FCEM approximation for different segregation/order factors r (marked near the plots). Arrows indicate order-disorder transition temperatures (for r =3.5, Ts=Tb).

In addition to the segregation/order factor, and depending on its magnitude, the crystal structure and surface orientation can strongly affect the surface composition in ordered alloys. For example, unlike the case of the equiatomic bulk truncated composition of Ll2(100), LRO tends to maintain the Ll2(lll) surface with nominal bulk concentration (0.25). Therefore, the two ordered surfaces are expected to exhibit quite different segregation characteristics for the same r value (Fig. 10). Moreover, SRO causes pronounced changes of surface sublattice and average compositions associated with a considerable reduction of the order-disorder transition temperature (especially in fee alloys). 

The surfaces of CU3AU alloy (bulk structure LI2) was studied thoroughly by various techniques and theoretical approaches, especially in relation to the order-disorder transition [52-63]. Recently, medium-energy ion-scattering spectroscopy (MEIS) measurements confirmed the stabilization of bulk-truncated equiatomic termination for this surface at low temperatures. Starting at about 500 K, the Au atoms in the surface layer begin to move to the second... 

All three surface phases of Pb on Cu(lOO) undergo order-disorder transitions at elevated temperatures. The temperatures at which the structures disorder were first reported by Sanchez and co-workers [86, 87] using TEAS. They found that the c(4x4) phase disorders at 545 K. The critical exponent is near zero indicating a first-order transition. Their data also suggests first-order transitions to disordered structures for the c(2x2) and c(5V2xv2)R45° phases at temperatures of 498 K and 490 K, respectively. 

Schweizer E, Persson BNJ, TUshaus M, Hoge D, Bradshaw AM (1989) The potential energy surface, vibrational phase relaxation and the order-disorder transition in the adsorption system Pt lll -CO. Surf Sci 213 49... 

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