Substitutional pseudomorphs

When the new compound substitutes, even partially, for the original compound, the pseudomorph is called a substitutional pseudomorph. Usually the new compound has an ion in conunon with the original, as is the case of goethite pseudomorph after pyrite (both contain iron as Fe in goethite and as Fe in pyrite) and the malachite pseudomorph after azuiite (both contain Cu, COs ", and OH ) shown in Figure 110. The malachite pseudomorph is particularly interesting, because the two minerals differ only in the stoichiometry (mole ratio) of the ions present. 

The case of Al(100)/Yb, with a very large difference between the atomic radius of the substrate (1.432 A) and adsorbate (1.776 A), and having a stable fee (100) substrate is also of interest. Based on the atomic radii, a simple pseudomorphic substitutional surface alloy would be expected to show a very large buckling of 1.45 A, whereas a much reduced value of 0.37 A is found experimentally [66]. However, while this phase does, in terms of simple stoichiometry, involve only substitution of every fifth surface A1 atom, the (V5xV5)R27° surface phase formed involves substantial lateral displacements of... 

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