Pseudomorphicity

Hahn E, Kampshoff E, Walchli N and Kern K 1995 Strain driven fcc-bct phase transition of pseudomorphic Cu films on Pd (100) Phys. Rev. Lett. 74 1803... 

Pseudomorphism received methodical study from about 1905. A micro-section taken across the interface between a substrate and an electrodeposit shows the grain boundaries of the former continue across the interface into the deposit (Fig. 12.5). As grain boundaries are internal faces of metal crystals, when they continue into the deposit the latter is displaying the form of the substrate. Hothersall s 1935 paper contains numerous excellent illustrations with substrates and deposits chosen from six different metals, crystallising in different lattice systems and with different equilibrium spacing. Grain boundary continuation and hence pseudomorphism is evident despite the differences. 

Fig. 12.5 Pseudomorphism grain boundaries in the substrate (5) are continued in the electrodeposit (D)...

Electron diffraction investigations showed that epitaxy did indeed exist when one metal was electrodeposited on another, but that it persisted for only tens or hundreds of atomic layers beyond the interface. Thereafter the atomic structure (or lattice) of the deposit altered to one characteristic of the plating conditions. Epitaxy ceased before an electrodeposit is thick enough to see with an optic microscope, and at thicknesses well below those at which pseudomorphism is observed. 

Pseudomorphism has less desirable consequences, and usually means are sought to suppress it. If the substrate has been scratched, ground or abrasively polished, or if it has been cold rolled or cold formed, the surface is left in a peculiar state. Cold working reduces the surface grain size, and produces deformed, shattered and partly reoriented metal. It may produce microcrevices between the deformed grains, and, with some processes, non-metallic impurities and oxides are embedded in the surface. The disturbed state of the substrate is copied by a pseudomorphic electrodeposit with several consequences (Fig. 12.7). One is aesthetic it has often been noted that almost invisible abrasion of the substrate develops as more prominent... 

Fig. 12.7 The disturbed structure of a scratch, with fragmented and distorted grains, is perpetuated by a strongly pseudomorphic electrodeposit...

Fig. 12.8 A fairly strongly pseudomorphic bright tin deposit (left) has its brightness impaired by the shattered surface layer produced on steel by cold rolling. When this layer is removed, the deposit is mirror bright (right). Coating S m thick...

Fig. 12.9 Corrosion resistance of tin-nickel electrodeposit impaired by pseudomorphic porosity originating on cold-rolled steel surface (left). Panel on right has had the shattered grain surface removed by chemical polishing (0-125 iim removed). Coating thickness 15 iim-, panels exposed 6 months to marine atmospheric corrosion (Hayling Island)...

The results of a thermodynamic analysis of the interactions in Equations (127) and (128), as presented in [452], show that a coherent shell of tantalum and niobium hydroxides is formed on the surface of the columbite or tantalite during the interaction with sulfuric acid. The formation of the shell drives the process towards a forced thermodynamic equilibrium between the initial components and the products of the interaction, making any further interaction thermodynamically disadvantageous. It was also shown that, from a thermodynamic standpoint, the formation of a pseudomorphic structure on the surface of columbite or tantalite components is preferable to the formation of tantalum and niobium oxysulfates. Hence, the formation of the pseudomorphic phases catalyzes the interaction described by Equation (127) while halting that described by Equation (128). 

Direct kinetic measurements from the changes in diffracted beam intensities with time during heating of the reactant are illustrated in the work of Haber et al. [255]. Gam [126] has reviewed the apparatus used to obtain X-ray diffraction measurements in thermal analysis. Wiedemann [256] has designed equipment capable of giving simultaneous thermo-gravimetric and X-ray data under high vacuum. X-Ray diffraction studies enable the presence, or absence, of topotactic relationships between reactant and product to be detected [92,102,257—260], Results are sometimes considered with reference to the pseudomorphic shape of residual crystallites. 

The effects of adsorbate coverage (film thickness) on the Pd 3d5/2 XPS peak positions of the Pd/W(l 1 0), Pd/ Re(0001), and Pd/Mo(l 10) systems were systematically investigated [63]. The peak positions reported for Pd coverage in excess of 1 ML represent a product of electrons emitted from surface and subsurface atoms. For the case of Pd(lOO), theoretical calculation suggest that the Pd 3d5/2 XPS BE of the surface atoms is 0.4 eV lower than that of bulk Pd. A similar difference has been observed experimentally for Ni and Pt surfaces. These shifts in BE are a consequence of variations in the coordination number of the surface atoms compared to bulk atom. If we reference the combined peak of bulk and surface atoms in 40 ML of Pd on W(1 1 0) to that of Pd(l 00) a difference of —0.8 eV is obtained between the Pd 3ds/2 BE of a pseudomorphic monolayer of Pd on W(110) and that of the surface atoms of Pd(l 00). The corresponding shifts... 

In our recent work, we have used Pd deposited on t hkl) in order to characterize thin metal films [Arenz et al., 2002] and to test their catalytic activity (Fig. 8.16). We have employed both methodologies thermal evaporation in UHV and electrochemical deposition. For the Pd/Pt(l 11) system, in situ SXS measurements have been used to show that after the formation of 1 ML of pseudomorphic Pd film, three-dimensional pseudomorphic crystalline islands of pure Pd begin to aggregate. 

Figure 14.5 STM data (140 nm x 140 nm) (a-e) and corresponding cyclic voltammograms (f-j) of Ru((X)01) and Ru(OOOl) covered by increasing amounts of pseudomorphic Pt submonolayers. (Reprinted with permission from [Hoster et al., 2(X)4]. Copyright 2004 American Chemical Society.)...

Pseudomorphic Pt monolayers on Ru(0001) interact very weakly with H pd, OHad, or Oad, because of electronic ligand (vertical ligand effects) and strain effects (tensile strain), in agreement with results obtained under UHV conditions and in DPT calculations. Therefore, base CVs on these surfaces do not show pronounced voltammettic features. 

LischkaM, Mosch C, Gross A. 2007. Tuning catalytic properties of bimetallic surfaces Oxygen adsorption on pseudomorphic Pt/Ru overlayers, Electrochim Acta 52 2219. 

At high oxygen exposures at 295 K, the surface consists predominantly of hexagonal structures, but also present as a minor component are square lattice structures (Figure 4.10) reminiscent of the cubic structure associated with MgO smoke formed by the oxidation of magnesium at high temperature.20 Therefore, two pseudomorphic oxide overlayers form at Mg(0001) at room temperature, but what factors control their separate growth are not known. 

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