Epitaxis and Pseudomorphism

When a crystal is influenced by another compound, either epitaxis or pseudomorphism may occur. If the new compound shares some of the lattice points with the original crystal, it may grow in the same direction and is referred to as an epitaxial overgrowth. When the 

The goethite pseudomorph is an easy one—the original pyrite is in the isometric system. Incidentally, these particular pseudomorphs are sometimes referred to as limonite. The term limonite is not an accepted mineral name and is used when the composition of the iron oxide is uncertain. The names of minerals are 

The original azurite appears to have a two-fold symmetry axis and belongs to the monoclinic system. Malachite belongs to the same system, but very rarely forms large crystals (it is usually massive or forms thin fibrous needles). 

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Yet another factor that has substantial influence on electrodeposits, including their structure and properties, is the nature of the substrate on which the plating occurs. Two phenomena are of importance in this context epitaxy and pseudomorphism. The former refers to the systematic structural keenship between the atomic lattices of the substrate and the deposit at or near the interface. In other words, epitaxy refers to the induced continuation of the morphology and structure of a substrate... 

Crystals frequently do not grow in perfect single crystals that contain one or more CTystal forms. There are many deviations from such normality, only four of which will be discussed here parallel growth, twinning, epitaxy, and pseudomorphism. The first two involve crystals of the same compound, whereas in the latter two another compound is involved. 

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. 

A number of workers have studied the epitaxial relationships of ZnO on Zn (57-61). The earlier reports of pseudomorphism (58) have not been confirmed by later workers. The most careful study with attention to the surface preparation of the metal was that of Lucas (57) and his results for room temperature oxidation are included in Table IL This work emphasized the importance of surface preparation, and showed conclusively that different orientations could occur on a surface as a result of facet formation. At higher temperatures Raether (59) reported an orientation in which the (0001) planes of the ZnO were normal to the surface rather than parallel as in the case of room temperature oxidation. Yang (61), however, reported the usual parallel orientation at 350°C. 

The catalytic reactivity in the range of 1 to 3 ML Pd on Au(llO) is surprisingly low taking into account the noticeable amount of Pd measured by LEIS on the surface (Fig. 17). This low reactivity could be due either to a low number of active sites (two adjacent Pd atoms) resulting from chemical order or to the tensile stress applied to Pd atoms in pseudomorphic epitaxy with Au substrate. Such a hypothesis would be in agreement with the theoretical predictions of Pallassama and Naurock [21] who propose that Pd stressed in... 

At low coverages, most metallic adsorbates form ordered overlayers with a (1 x 1) surface structure on metal substrates. This implies that the substrate acts as a template and has a significant influence on the growth mode of the deposited material. This effect is usually called epitaxial growth. A more restricted definition of epitaxial growth would include only those examples where the substrate imposes its own crystal structure, orientation, and lattice parameter on the adsorbed overlayer. (This restricted definition is also called pseudomorphic growth.)... 

The supported Pd4 cluster has Pd-Pd distances which are elongated compared with those of the compound in the gas-phase. The optimum geometry of Pd4 adsorbed on the oxide anions of MgO has metal-metal distances of 2.93 A, somewhat shorter than the MgO lattice parameter. " The potential energy for the Pd-Pd stretch is, however, very flat with very little energy it is possible to expand the cluster to a pseudomorphic structure on the MgO surface. Because the clusters experimentally deposited on MgO are usually much larger than the species considered here, it is conceivable that a thin Pd metal film will grow epitaxially on MgO, at variance with Ni and Cu. 

One of the most frequently observed phenomena in epitaxial growth is the formation of strain relief patterns. These are caused by the mismatch of the unit cell size of the substrate and the deposited film. In many cases the strain or stress, which is imposed on the thin film by fhe subsfrafe lattice is relieved by reconstruction of the film. This reconsfrucfion can but must not necessarily lead to a nanopatterned film. An inferesfing example is the growth of Ag on Pt(l 11) (see Fig. 10) [41]. It has been shown for this particular system that the first Ag layer grows pseudomorphically exhibiting an isotropic compressive strain of 4.3% whereas in higher layers this strain is relieved by the formation of a dislocation network [42-47]. In order to improve the long-... 

At room temperature, ZnS bulk material consists predominantly of the cubic phase, often with hexagonal inclusions, leading to polytypic material. Epitaxial ZnS has mostly been grown on GaAs and thus has pseudomorphically assumed the cubic structure of the substrate material. In this section, all data refer to the cubic modification unless expUcitly stated otherwise. 

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