Pseudomorphic phase

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). 

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... 

In normal vapor-phase operation the catalyst maintained its activity for 1 to 2 years, although some batches have been used for 5 years. The decrease in activity of the catalyst is caused by the irreversible absorption of high-molecular-weight hydrocarbons. Ash (e.g., iron phenolates) -containing feedstocks cause a more rapid loss of activity. Analyses of used catalysts indicated carbon contents of about 2%. Tungsten disulfide is isomorphous with molybdenum sulfide, and many of the catalytic properties of these two compounds are similar. X-ray analyses of the catalyst showed a hexagonal lattice (13). The primary size of the crystallites is about 3 X 10-7 cm. in height and twice that in width. Microscopic examinations showed that the crystals are pseudomorphous to the... 

The pseudomorphs after fabric were located in an area roughly between H and U on the x axis, and between 3.6 and 9.0 on the y axis. This area corresponds to approximately 52% of the spearpoint s length and 81% of its width. Examination of the spearpoint during the survey phase had already indicated shifts in the geometry of the fabric with respect to the weapon s form (6). 

The conditions under which a high mullite content is created are obvious from the phase diagram. The first necessary condition is a suitable composition of the system. At low temperatures (up to 1300 °C), ihe system cannot be expected to approach the state of equilibrium. When formed from kaolinite, mullite in this case frequently keeps the pseudomorphous shapes of kaolinite. The typical mullitic forms, usually needle-shaped crystals, arise only at higher temperatures. The rate of formation as we]] as the final shape of its crysta]s may be considerab]y aflected by the impurities present or by the non-clay components of the raw materia] mixture. These components also take part in the formation of the melt which is then produced in higher amounts and at lower temperatures than would correspond to the pure system A1203 —Si02. 

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... 

After all, five different surface species have been identified namely A) as the clean (5x1) Ir surface layer, B) as the clean (1x1) Ir surface layer, C) as the pseudomorph Cu(100)-(lxl) overlayer on unreconstructed Ir(lOO), D) as embedded Ir islands in a Cu( 100)-( 1 x 1) matrix and E) as Cu ad-islands on top of such embedded Ir areas. In fact, surface annealing enhanced the effect of phase separation and the weak alloy formation found before in the occurrence of Ir chains in a Cu monolayer at 300 K is completely overruled by the formation of compact separated areas of Cu and Ir content, respectively. In addition, depending on the initial coverage, the bare surfaces of Ir islands... 

The reduction occurs by direct oxygen removal from the solid oxides (solid-state diffusion). The basic underlying mechanism is not known (diffusion of O, OH, H2O) and is likely to vary for different for different phase transitions. On the final reduction the metal remains pseudomorphous to the starting oxide, forming a polycrystalline metal sponge. Solid-state reactions are characteristic for low reduction temperatures (<750 °C) and the early WO3 - WO2 9 transition ( crystallographic shear transition). 

The structural properties of RuSia have been calculated for three crystalline phases high-temperature tetragonal P4lmmm) a-phase, orthorhombic (Cmcd) 3-phase, and metastable pseudomorphic cubic (Fm3m) y-phase with 3, 24 and 3 atoms in the unit cell, respectively [9]. Table 1 summarizes the optimized lattice constants of the mthenium silicides. 

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. 

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