Transition, reconstructive

Experiments carried out on monocrystalline Au(lll) and Au(lOO) electrodes in the absence of specific adsorption did not show any fre-quency dispersion. Dispersion was observed, however, in the presence of specific adsorption of halide ions. It was attributed to slow adsorption and diffusion of these ions and phase transitions (reconstructions). In their analysis these authors expressed the electrode impedance as = R, + (jco iJ- where is a complex electrode capacitance. In the case of a simple CPE circuit, this parameter is = T(Jcaif. However, an analysis of the ac impedance spectra in the presence of specific adsorption revealed that the complex plane capacitance plots (C t vs. Cjnt) show the formation of deformed semicircles. Consequently, Pajkossy et al. proposed the electrical equivalent model shown in Fig. 29, in which instead of the CPE there is a double-layer capacitance in parallel with a series connection of the adsorption resistance and capacitance, / ad and Cad, and the semi-infinite Warburg impedance coimected with the diffusion of the adsorbing species. A comparison of the measured and calculated capacitances (using the model in Fig. 29) for Au(lll) in 0.1 M HCIO4 in ths presence of 0.15 mM NaBr is shown in Fig. 30. 

Thus, for years now wheels have been tested with eddy-currents with a special emphasis on finding superficial defects on the wheel s running surface. At the beginning, hand-guided probes were used which the tester slid across the surface that was to be tested. To do so, the wheel was placed on a turn table which the operator had to move manually. A higher test reliability in the transitional radius between the body and the bead wheel was achieved by using special form-fitted probes. Nonetheless, it was not possible to rule out completely that the wheel was not scanned 100 %. This test method did not allow for documentation and a subsequent reconstruction of the test. 

I.P.P.D and its relatives have become standard procedures for the characterization of the structure of both clean surfaces and those having an adsorbed layer. Somoijai and co-workers have tabulated thousands of LEED structures [75], for example. If an adsorbate is present, the substrate surface structure may be altered, or reconstructed, as illustrated in Fig. VIII-9 for the case of H atoms on a Ni(llO) surface. Beginning with the (experimentally) hypothetical case of (100) Ar surfaces. Burton and Jura [76] estimated theoretically the free energy for a surface transition from a (1 x 1) to a C(2x 1) structure as given by... 

Restructuring of a surface may occur as a phase change with a transition temperature as with the Si(OOl) surface [23]. It may occur on chemisorption, as in the case of oxygen atoms on a stepped Cu surface [24]. The reverse effect may occur The surface layer for a Pt(lOO) face is not that of a terminal (100) plane but is reconstructed to hexagonal symmetry. On CO adsorption, the reconstruction is lifted, as shown in Fig. XVI-8. 

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

Surface reconstructions have been observed by STM in many systems, and the teclmique has, indeed, been used to confmn the missing row structure in the 1 x 2 reconstruction of Au(l 10) [28]. As the temperature was increased within 10 K of the transition to the disordered 1 1 phase (700 K), a drastic reduction in domain size to -20-40 A (i.e. less than the coherence width of LEED) was observed. In this way, the STM has been used to help explain and extend many observations previously made by diffraction methods. 

By applying a pulling force at a portion of the solute molecule in a specific direction (see chapters of Eichinger et al. and Schulten in this volume), conformational transitions can be induced in specific directions. In order to reconstruct information about the underlying potential function governing protein motion, the irreversible work performed on the system by these forces must be discounted ([Balsera et al. 1997]). 

Bancroft et al. (1965), and in the case of CaO and the B1 to B2 transition discovered by Jeanloz and Ahrens (1979), complete reversion of the low-pressure phase occurs upon unloading. These latter transitions involve rearrangement of the lattice which can occur via its deformation rather than complete reconstruction. The volume change in the Si02 transition is much larger than in the case of CaO, as seen in Fig. 4.15. In contrast to the pressure-volume plane when plotted in the Ph-u, plane, the occurrence of these transitions is less striking in this representation (Fig. 4.14). 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

It is well known that the catalytic oxidation of CO on certain Pt surfaces exhibits oscillatory behavior, within a restricted range of pressures and temperatures, which are coupled with adsorbate-induced surface phase transitions [16,17]. In fact, in their clean states the reconstructed surfaces of some crystallographic planes, e.g. Pt(lOO) and Pt(llO), are... 

According to the data obtained with SXRS in salt solutions,519 520 at a < 0 the surface of Au(lll) forms a ( 3 x 22) structure as in a vacuum. At a > 0 the reconstruction disappears and the (1 x 1) structure is observed. On the reconstructed Au(l 11) surface there are 4.4% more atoms than on the (1 x 1) structure and on the reconstructed Au( 100) there are 24% more atoms than on the (1 x 1) structure.506,519 This phase transition shifts in the negative direction with the adsorbability of the anion. The adsorption-induced surface reconstruction of Au(l 11) electrodes has been studied in situ by second harmonic generation by Pettinger et al.521... 

Figure 5.58 Reconstructed LC-MS-MS ion chromatograms for selected-reaction monitoring of methoxyfenozide using the m/z 367 to m/z 149 transition from the continual post-column infusion of a standard solution of analyte during the HPLC analysis of a...

From a theoretical point of view, the stability of nanocrystalline diamond was discussed by several authors. Badziag et al. [25] pointed out that, according to semi-empirical quantum chemistry calculations, sufficiently small nanocrystalline diamond (3-5 nm in diameter) may be more stable than graphite by forming C-H bonds at the growing surface. Barnard et al. [26] performed the ab initio calculations on nanocrystalline diamond up to approximately 1 nm in diameter. The results revealed that the surfaces of cubic crystals exhibit reconstruction and relaxations comparable to those of bulk diamond, and the surfaces of the octahedral and cubooctahedral crystals show the transition from sp to sp bonding. 

Further STM and SXS smdies [Wu et al., 1998] concerning this phenomenon indicated that the presence of specifically and nonspecifically adsorbing anions as well as organic molecules (e.g., pyridine, bipyridine, and uracil) may also lift the reconstructed surface by exhibiting a structural transition, and it has been extensively studied and reviewed in [Kolb, 1996]. 

Reconstructive phase transitions Chemical bonds are broken and rejoined the reconstruction involves considerable atomic movements. Such conversions are always first-order transitions. 

The phase diagram for Si02 is shown in Fig. 12.11. The transition between a- and 13-quartz only requires minor rotations of the Si04 tetrahedra, the linkage pattern remaining unaltered (Fig. 18.9, p. 224) this transition takes place rapidly. The other transitions, on the other hand, require a reconstruction of the structure, Si-O bonds being untied and rejoined they proceed slowly and thus render possible the existence of the metastable mod-... 

The coordination numbers of the atoms increase from 6 to 8 in the NaCl —> CsCl type conversion therefore, it is a reconstructive phase transition which can only be a first-order transition. 

Surface reconstruction has been earlier observed and reported in the literature [116]. Sequential reductive and oxidative thermal treatment usually leads to bulk transition from CoOx + La203 to LaCo03, respectively. On the other hand, the restoration of the perovskite structure is not observed under severe conditions at higher temperature. In those temperature conditions, the sintering of Co crystallites leads to irreversible redox cycle with the preferential formation of Co304 under lean conditions. 

The structures of two polymorphs of pleconaril, enantiotropically related with a transition temperature of 35.7°C, have been reported [36], Form I was described as consisting of a network of dimers, while Form III was described as a three-dimensional network of monomers. The two forms contradicted the density rule, and the solid solid transition could occur only through a destructive-reconstructive mechanism. A quantitative differential scanning calorimetry method was also described that enabled the quantitative determination of Form I in bulk Form III to be made at levels as low as 0.1%. 

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