Interlayer distance, contraction

On the basis of the SEXAFS derived bond distances one can attempt to derive the surface plane relaxation in the surface normal direction. By taking as a plausible reference a Co—Cu ideally non relaxed distance of 2.53 A, intermediate between the bulk Co and bulk Cu, our experimental values lead to conclude that there is a contraction of this distance (Fig. 5). The contraction is found to be 4 + 2% for the (111) face and 11 + 5 % for the (110) face. Moreover a small expansion of the second interlayer spacing (5 5%). is measured in the (110) case. The oscillatory damping of the first interlayer distances has been observed before and calculated, in parti-... 

We call this Pt(100) surface reconstructed. Surface reconstruction is defined as the state of the clean surface when its LEED pattern indicates the presence of a surface unit mesh different from the bulklike (1 x 1) unit mesh that is expected from the projection of the bulk X-ray unit cell. Conversely, an unreconstructed surface has a surface structure and a so-called (1 x 1) diffraction pattern that is expected from the projection of the X-ray unit cell for that particular surface. Such a definition of surface reconstruction does not tell us anything about possible changes in the interlayer distances between the first and the second layers of atoms at the surface. Contraction or expansion in the direction perpendicular to the surface can take place without changing the (1 x 1) two-dimensional surface unit cell size or orientation. Indeed, several low Miller index surfaces of clean monatomic and diatomic solids exhibit unreconstructed surfaces, but the surface structure also exhibits contraction or expansion perpendicular to the surface plane in the first layer of atoms (9b). 

There are two major findings of modern surface science that were uncovered during studies of clean, solid surfaces. These are reconstruction, as was demonstrated for the platinum and silicon crystal surfaces, and there is also relaxation. During relaxation the atoms are contracted in their interlayer distance near the surface region with respect to the interlayer distance in the bulk. However, the atomic locations in... 

All the Co atoms in the second layer become Co" ions with reduced radii and lowered energy states. The interaction between the second layer (Co" ions) and the third layer (metallic Co atoms) is stronger than that between two metallic layers. Therefore, the first interlayer distance expands by an amount that depends on the bond geometry and the second interlayer spacing contracts driven by the enhanced Co -Co interlayer interaction. This mechanism should hold for the Ru(1010)-0 surface though LEED and DFT revealed a different amount of relaxation for the Co(1010)-0 surface. 

The reaction scheme of Bode [11] was derived by comparison of the X-ray diffraction patterns of the active materials with those for the model compounds. How the 8-Ni(OH)2 in battery electrodes differs from the model compound is discussed in Section 5.3.I.3. In recent years, the arsenal of in situ techniques for electrode characterization has greatly increased. Most of the results confirm Bode s reaction scheme and essentially all the features of the proposed a/y cycle. For instance, recent atomic force microscopy (AFM) of o -Ni(OH)2 shows results consistent with a contraction of the interlayer distance fiom 8.05 to 7.2 A on charge [61-63]. These are the respective interlayer dimensions for the model a-Ni(OH)2 and y-NiOOH compounds. Electrochemical quartz crystal microbalance (ECQM) measurements also confirm the ingress of alkali metal cations into the lattice upon the conversion of a-Ni(OH)2 to y-NiOOH [45,64,65]. However, in situ Raman and surface-enhanced Raman spectroscopy (SERS) results on electrostretching modes that are consistent with a weakening of the O-H bond when compared with results for the model a- and 8-Ni(OH)2 compounds [66]. This has been ascribed to the delocalization of protons by intercalated water and Na ions. Similar effects have been seen in passive films on nickel in borate buffer electrolytes [67]. 

F4 0.7 -F4 0.7 0 0 0 0 sites adsorbate induces moderate buckling of two topmost Mo layers contraction of first interlayer distance... 

The XRD patterns of the as-synthesized HNT, HNT-500, Pd-HNT, and Fe/Pd-HNT samples are shown in Fig. 1. The crystal stracture of HNT corresponds to the H2Ti205.H20 (or HxNa2-xTi205.H20) phase. After calcination, HNT-500 presents a typical XRD pattern of the anatase phase except for several weak peaks of H2Ti020sH20. The interrsity of the diffraction peak near 20 1O°C was decreased and shifted towards a higher angle value indicating a decrease in the interlayer distance. This contraction is due to the release of water molectrles adsorbed and present in the space between layers [5]. 

Recently, it was noted that the addition of poly(vinyl methyl ether) to a clay-salt-water system induced the contraction of the interlayer separation between the clay platelets [18], The K-butylammonium vermiculite used in those experiments provided an ideal structure of regularly spaced and parallel charged colloidal platelets in a salt solution [19,20], Swenson et al. [21-23] studied the effect of the addition of poly(ethylene oxide) (PEO) on the distance between the vermiculite layers and observed that, with increasing PEO concentration, the distance between the vermiculite layers decreased. They estimated that the bridging force per bridge was 1.4 pN. 

Given the volume fraction of the polymer inside the gel, we are now able to propose a quantitative model of bridging flocculation. Because Crawford et al. [4] studied the contraction of the interlayer spacing as a function of uniaxial stress for the same system without any added polymer, we are able to convert the observed d-values to effective uniaxial pressures caused by the bridging polymers. If we assume that we have one polymer bridge when the end-to-end polymer distance l (calculated according to Equation 12.1) exactly matches the d-value with the... 

The bulk crystal form of the alkaline earth oxides is the NaCl structure. The bulk termination at the (100) surface consists of a stack of coplanar bilayers each containing one cation and an anion. CaO(lOO) has been the subject of a LEED study which found a contraction of top interlayer spacing by —1.2% (Prutton et al., 1979). Buckling of the top bilayer was not investigated. Buckling of the top bilayer was found in a recent LEED study of MgO(lOO) (Blanchard et al. 1990). In this case, the oxygen atom moves out of the surface by 0.05 0.025 A whilst the Mg atom sinks into the surface the same distance. The center-of-mass plane of the top MgO bilayer is unrelaxed. The (100) surface of transition metal oxide CoO, which also has... 

A [11] and 0.00 0.02 A [23] for 6, 7 and 8-layer slabs respectively), in disagreement with SXRD and with less reliable theoretical results (Az=-0.09 A [55], -0.14 A [56], -0.16 A [57] all from 3-layer slabs). Direct comparison with data from impact-collision ion-scattering spectroscopy [58] is difficult the finding of negligible O relaxation supports the ck slab results, while the 18% interlayer contraction is closer to SXRD (30%). Further experimental data is discussed in this context in Ref. [1]. No explanation is advanced in Ref. [54] for the imusual Ti-0 distances, 13% shorter/longer than usual, that result from the... 

Bond contraction also happens to the hetero-coordinated atoms. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) measmements revealed a 8 % contraction of distance between the As impurity (acceptor dopant) and the Te sub-lattice in CdTe [54]. At A1 twin grain boundaries, homo-junction dislocations have either compact or dissociated interlayers. The minimum stress op) required for moving an edge dislocation is... 

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