Interfaces, crystal/vapor structure

Crystal/crystal interfaces possess more degrees of freedom than vapor/crystal or liquid/crystal interfaces. They may also contain line defects in the form of interfacial dislocations, dislocation-ledges, and pure ledges. Therefore, the structures and motions of crystal/crystal interfaces are potentially more complex than those of vapor/crystal and liquid/crystal interfaces. Crystal/crystal interfaces experience many different types of pressures and move by a wide variety of atomic mechanisms, ranging from rapid glissile motion to slower thermally activated motion. An overview of crystal/crystal interface structure is given in Appendix B. 

Interfaces may be sharp or diffuse. A sharp interface possesses a relatively narrow core structure with a width close to an atomic nearest-neighbor separation distance. Examples of sharp crystal/vapor and crystal/crystal interfaces are shown in Figs. B.l and B.2. 

To reduce the free energy contributed by the surface tension term, the molecules at the liquid crystal/vapor interface favor a layer structure. In the smectic phase, the outermost layers favor a better molecular packing than exists in the interior. The enhanced surface order has been reported for various liquid crystal phases, for example the surface SmA order on the bulk isotropic or nematic sample [50] the surface SmI order on a SmA film [47] the surface SmB gx order on a SmA film [45,48] the surface SmI on a SmC film [17,93] the surface B on a SmA film [49] the surface crystal E order on a SmBhex film [100]. Realizing the importance of the surface tension in characterizing the liquid crystal free-standing films, we... 

Early theories of crystal-liquid interfaces were based on phenomenological models of interface structure that were extensions of models for liquid-vapor or crystal-vapor interfaces. At one end are models, such as that due to Jackson, that view the interface as being relatively sharp with a clear distinction between liquid and crystal particles even at the interface. At the other are theories of diffuse structureless interfaces that, while useful for liquid-vapor interfaces, are not ideal for the highly structured crystal-liquid case. 

Difference in the environmental phases. Since the interface roughness will be different for the same crystal species depending on whether the crystal was grown from the melt, solution, or vapor phases, different growth forms are expected for different environmental phases. This implies that the Tracht of the same crystal species will depend on the structure of the environmental phases, the degree of condensation, and the solute-solvent interaction. 

With such low concentrations of components available to form critical nuclei, hydrate formation seems unlikely in the bulk phases. However, at an interface where higher concentrations exist through adsorption (particularly at the vapor-liquid interface where both phases appear in abundance) cluster growth to a supercritical size is a more likely event. High mixing rates may cause interfacial gas + liquid + crystal structures to be dispersed within the liquid, giving the appearance of bulk nucleation from a surface effect. 

Most relevant for the oxygen transport should be the defective crystal structure of both catalyst components. The defective structure and the intimate contact of crystallites of the various phases are direct consequences of the fusion of the catalyst precursor and are features which are inaccessible by conventional wet chemical methods of preparation. Possible alternative strategies for the controlled synthesis of such designed interfaces may be provided by modem chemical vapor deposition (CVD) methods with, however, considerably more chemical control than is required for the fusion of an amorphous alloy. 

Over the last two decades the exploration of microscopic processes at interfaces has advanced at a rapid pace. With the active use of computer simulations and density functional theory the theory of liquid/vapor, liquid/liquid and vacuum/crystal interfaces has progressed from a simple phenomenological treatment to sophisticated ah initio calculations of their electronic, structural and dynamic properties [1], However, for the case of liquid/crystal interfaces progress has been achieved only in understanding the simplest density profiles, while the mechanism of formation of solid/liquid interfaces, emergence of interfacial excess stress and the anisotropy of interfacial free energy are not yet completely established [2],... 

The synthesis of a typical model catalyst used in these studies is shown schematically in Fig. 2. The procedure begins with a refractory metal substrate, such as Mo, Ta, or Re, that has been cleaned by standard procedures and verified clean with surface analytical techniques. The structure of the substrate is chosen specifically to match the particular oxide film to be grown since crystal orientation and the nature of the interface or critical parameters in obtaining a high-quality film. A thin metal oxide film, typically 1-10 nm thick, is then deposited onto the metal substrate by vapor deposition of the parent metal in an O2 environment. Thin films of Si02V ° ... 

The S/L interfaces, in general, have much in common with the surfaces from the structural point of view, in that both the vapor and liquid in contact with a crystal surface are a structureless fluid. However, the presence of a short-range structural order clearly distinguishes a liquid from a vapor. Bernal [72] noted that the short-... 

Although there are now a large number of conductors and superconductors based on BEDT-TTF salts, the majority of these materials have been prepared only as single crystals via electrochemical methods [54J. To make functional systems that can be interfaced readily with high-Tc structures, it is necessary to prepare thin films of these organic conductors. Recently, methods have been developed for the deposition of thin films of (BEDT-TTF)2l3 via vapor processing steps [55-57J. 

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