Crystals 466 INDEX

Lang B, Joyner R W and Somorjai G A 1972 LEED studies of high index crystal surfaces of platinum Surf. Sc 30 440-53... 

Using single crystals it has been shown that different low-index crystal faces see Section 20) exhibit different corrosion rates. However, the relative corrosion rate of the different faces varies with the environment and these structural effects are of little practical significance. On the other hand, the fact that polycrystal grains of different crystallographic orientation may corrode at different rates, is of some importance. 

The temperature behavior of low446,491,503 558 as well as high Miller index crystal faces of Au447,448 has been examined in 0.01 M perchloric acid solutions. For all gold surfaces studied, C, was found to decrease and Ea=Q moved to less negative values with increasing t 446-448 491503-558... 

We now describe a relatively simple MD model of a low-index crystal surface, which was conceived for the purpose of studying the rate of mass transport (8). The effect of temperature on surface transport involves several competing processes. A rough surface structure complicates the trajectories somewhat, and the diffusion of clusters of atoms must be considered. In order to simplify the model as much as possible, but retain the essential dynamics of the mobile atoms, we will consider a model in which the atoms move on a "substrate" represented by an analytic potential energy function that is adjusted to match that of a surface of a (100) face-centered cubic crystal composed of atoms interacting with a Lennard-Jones... 

Henero E, Franaszczuk K, Wieckowski A. 1994. Electrochemistry of methanol at low index crystal planes of platinum An integrated voltammetric and chronoamperometric study. J Phys Chem 98 5074-5083. 

Bischoff and coworkers showed that there is a clear parhcle size dependent shift of the linearly absorbed CO stretching vibrational frequency on Pt supported in a zeolite [144]. They observed shift from 2055 cm" for CO chemisorbed on 1-2 nm particles to 2070 cm" for CO on 4—5nm particles. They correlated the frequency shift with the occurrence of higher indexed crystal planes for the smaller particles. 

Figure 3.16. Some simple defects found on a low-index crystal face 1, the perfect flat face, a terrace 2, an emerging screw dislocation 3, the intersection of an edge dislocation with the terrace 4, an impurity adsorbed atom 5, a monatomic step in the surface, a ledge 6, a vacancy in the ledge 7, a kink, a step in the ledge 8 an adatom of the same type as the bulk atoms 9, a vacancy in the terrace 10, an adatom on the terrace. (From Ref. 12, with permission from Oxford University Press.)...

The Physical Properties are listed next. Under this loose term a wide range of properties, including mechanical, electrical and magnetic properties of elements are presented. Such properties include color, odor, taste, refractive index, crystal structure, allotropic forms (if any), hardness, density, melting point, boiling point, vapor pressure, critical constants (temperature, pressure and vol-ume/density), electrical resistivity, viscosity, surface tension. Young s modulus, shear modulus, Poisson s ratio, magnetic susceptibility and the thermal neutron cross section data for many elements. Also, solubilities in water, acids, alkalies, and salt solutions (in certain cases) are presented in this section. 

II-VI semiconductors, such as CdSe and CdS, normally have the wurtzite structure (see Chapter 1) where each element is tetrahedrally coordinated. Under high pressures (2 GPa), these transform to the six-coordinate NaCl (rock salt) structure. However, if pressure is applied to a CdSe nanocrystal of about 4 nm in diameter, it now takes much more pressure, about 6 GPa, to transform it to the rock salt structure. It is thought that this may be a resistance to the exposure of high-index crystal planes... 

With regard to the attachment and detachment energies, the corners of a crystal or a rough interface that is constructed by kinks alone are sites where the process proceeds most quickly, whereas the low-index crystal faces, corresponding to smooth interfaces, represent the direction with the minimum rate of normal growth and dissolution. As a result, if a single crystalline sphere is dissolved in an isotropic environmental phase, a dissolution form bounded by both flat and curved crystal faces appears. This is called the dissolution form, which is not the same as the growth form. 

Table 3.1. Wood and matrix notation for a variety of superlattices on low Miller index crystal surfaces...

Low Miller index surfaces of metallic single crystals are the most commonly used substrates in LEED investigations. The reasons for their widespread use are that they have the lowest surface free energy and therefore are the most stable, have the highest rotational symmetry and are the most densely packed. Also, in the case of transition metals and semiconductors they are chemically less reactive than the higher Miller index crystal faces. 

II. The Atomic Structure of Surfaces. Structures of Low and High Miller Index Crystal Surfaces. 5... 

Structures of Low and High Miller Index Crystal Surfaces... 

One of the most exciting observations of LEED studies of adsorbed monolayers on low Miller index crystal surfaces is the predominance of ordering within these layers (18). These studies have detected a large number of surface structures formed upon adsorption of different atoms and molecules on a variety of solid surfaces. Conditions range from low temperature, inert gas physisorption to the chemisorption of reactive diatomic gas molecules and hydrocarbons at room temperature and above. A listing of over 200 adsorbed surface structures, mostly of small molecules, adsorbed on low Miller index surfaces can be found in a recent review (/). 

One of the strengths of the approach employed here is that we have freedom over the choice of the transition metal for the tip and also the structure of the tip employed. Usually we use Pt and W tips and represent the tip apex as a pyramid-like cluster epitaxed on a substrate that is orientated along some low Miller index crystal plane (for example, 111, 110, or 100 surface planes). Generally we find that the structure of the tip has quite a big impact on the images obtained. Sharp tips, such as those constructed on Pt 100, Pt 111 or W 100 surfaces tend to yield higher resolution images than those obtained with more blunt tips (for example the 111 surface of (bcc) W as shown in Scheme II). However,... 

E. Herrero, K. Frenasczuk, and A. Wieckowski, J. Phys. Chem. 8 5074 (1994). Methanol eoxidation at low-index crystal planes. 

Fig. 1. Surface structure often found on low-index crystal faces. 1, A terrace perfectly flat crystal face. 2, An emerging screw dislocation. 3, The intersection of an edge dislocation with a terrace. 4, A ledge or monatomic step, 5. A kink a step in a ledge. 6, A vacancy in a ledge. 7, An adsorbed growth unit on a ledge.

Quality control systems usually used for judging the quality of oils and fats or oil blends used in margarine production could evaluate color, color stabihty, flavor, flavor stabihty, free fatty acid, peroxide value, active oxygen method (AOM) stabihty, iodine value, shp melting point, fatty acid composition, refractive index, crystallization rate, and sohd fat/temperature relationship (solid fat index) (5, 91, 112, 113). 

E. Herr and H. Baltes, KOH etching of high-index crystal planes in silicon, Sensors Actuators 31, 283, 1992... 

Hoshi et al. extended the formation of reduced CO2 at Pt single crystal electrode to those of high index crystal orientations. They determined the initial rate of CO2 reduction Vt=Q from the slope of the time course of adsorbed CO formation at t = 0 as shown in Fig. 17. Figure 18 shows dependences of on the electrode potential at a series of electrodes Pt(S)-[ 7(111) X (111)] in 0.1-M HCIO4. The electrode surfaces are composed of n atomic rows of (111) terrace and one atomic height of (111) step. Adsorbed hydrogen is present below 0.3 V vs. re-... 

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