Atom mobility

In solid state chemistry the limited atomic mobility in the solid state controls chemical changes and leads to explicit consideration of the relative location of potential reactants (the configuration) and solid state reactivity as controlled by solid state defects. The same factors dominate shock-induced solid state chemistry. 

That the first stage of ordering (resistivity decrease) is correlated with excess vacancies not being in thermal equilibrium can be seen from measurement during isochronally lowering the temperature from the disordered state (0), which shows that atomic mobility is frozen below 280°C. 

From a comparison of the evolution of hardness of all samples during isochronal armealing it can be concluded that for high deformation in the disordered state and deformation in the ordered state, recovery and recrystallization is prevented up to T, in the sample deformed to 40% reduction in the disordered state recovery and recrystallization processes seem to start as soon as atomic mobility is enabled (260°C). 

With reference to rule (3) (Sec. 7.1.3) regarding metal atom mobility, we note that the Gibbs free energies of formation (AG( ) at 298 K for LaNi s and for LaH2 are about -67 and -171 kJ mol-1 respectively. Thus the following disproportionation reaction is highly favored [29] ... 

Glasses typically are metastable substances. Like crystalline solids they exhibit macroscopic form stability, but because of their structures and some of their physical properties they must be considered as liquids with a very high viscosity. Their transition to a thermodynamically more stable structure can only be achieved by extensive atomic movements, but atom mobility is severely hindered by cross-linking. 

Method (2) can be further subdivided depending on whether the mixed metal vapor is quench-cooled or if a heated substrate is used to encourage atom mobility after condensation. 

The existence of active sites on surfaces has long been postulated, but confidence in the geometric models of kink and step sites has only been attained in recent years by work on high index surfaces. However, even a lattice structure that is unreconstructed will show a number of random defects, such as vacancies and isolated adatoms, purely as a result of statistical considerations. What has been revealed by the modern techniques described in chapter 2 is the extraordinary mobility of surfaces, particularly at the liquid-solid interface. If the metal atoms can be stabilised by coordination, very remarkable atom mobilities across the terraces are found, with reconstruction on Au(100), for example, taking only minutes to complete at room temperature in chloride-containing electrolytes. It is now clear that the... 

In terms of models for reduction of triple bonds, a series of isolated iron carbonyl clusters, which nicely show H-atom mobility, provides a... 

Most chemists were more comfortable with speculations about movements of atoms than with flows of aether squirts. In particular, the idea of hydrogen atom mobility was to become a leading theme in late-nineteenth-century organic chemistry, based in the work of Williamson at midcentury. Williamson s investigations of etherification led him to a theory of the water "type" as well as to experimental proof that water is H20, not HO. Williamson clearly expressed the idea of chemical equilibrium as a balance between two sets of molecules in which some atoms or (uncharged) radicals may exist freely for short periods of time.43 In addition to its uncontestable central role in the "quiet revolution" of the 1850s,44 this was a paper that inspired both chemists and physicists to think about the "degree and kind of motion"45 of atoms within the molecule as well as the motion of the molecule as a whole. 

The bond strength of substrate surface atoms is diminished by anion adsorption, thus causing significant surface relaxation. The bonds may become so weak as to induce mobility in the surface atoms. Surface atom mobility has been studied in detail at atomic resolution or near atomic resolution in model systems. 

The mobility of metal atoms in bare metal clusters and small metallic nanoparticles (NPs) is of fundamental importance to cluster science and nanochemistry. Atomic mobility also has significant implications in the reactivity of catalysts in heterogeneous transformation [6]. Surface restmcturing in bimetallic NP and cluster catalysts is particularly relevant because changes in the local environment of a metal atom can alter its chemical activity [7, 8]. 

Surface-to-subsurface migrations in bimetallic NPs can completely change the surface composition and drastically alter catalytic performance. Despite the importance of atomic mobility in bimetallic systems, little is known about the dynamic processes of NPs and clusters due, in large part, to the lack of suitable experimental techniques. 

Annealing in metals can first lead to stress relaxation in which stored internal strain energy due to plastic deformation is relieved by thermally activated dislocation motion (see Figure 5.18). Because there is enhanced atomic mobility at elevated temperatures, dislocation density can decrease during the recovery process. At still higher temperatures, a process known as recrystallization is possible, in which a new set of... 

---