Metastable formation

Fig. 1. If equilibrium is maintained on cooling, CO will be converted largely to CH4 (solid lines) before metastable formation of more complex hydrocarbons by the Fischer-Tropsch reaction becomes possible (dashed lines). However, the reaction is very slow in the absence of catalysts, and may not have begun until about 400 K, when catalysts such as serpentine and magnetite became available through the hydration of olivine. Thus CO may have persisted metastably between 600 and 400 K...

Fig. 14. Variation in the rate constants for electron impact acetylene dissociation, methane dissociation, argon ionization and argon metastables formation along the axis of the tube.

Field Free Zones and the Formation of Metastable Ions... 

By introducing a collision gas into Q2, collision-induced dissociation (CID) can be used to cause more ions to fragment (Figure 33.4). For example, with a pressure of argon in Q2, normal ions (mj ) collide with gas molecules and dissociate to give mj ions. CID increases the yield of fragments compared with natural formation of metastable ions without induced decomposition. 

Under equiUbrium vapor pressure of water, the crystalline tfihydroxides, Al(OH)2 convert to oxide—hydroxides at above 100°C (9,10). Below 280°—300°C, boehmite is the prevailing phase, unless diaspore seed is present. Although spontaneous nucleation of diaspore requires temperatures in excess of 300 °C and 20 MPa (200 bar) pressure, growth on seed crystals occurs at temperatures as low as 180 °C. For this reason it has been suggested that boehmite is the metastable phase although its formation is kinetically favored at lower temperatures and pressures. The ultimate conversion of the hydroxides to comndum [1302-74-5] AI2O2, the final oxide form, occurs above 360°C and 20 MPa. 

Solutions of anhydrous stannous chloride are strongly reducing and thus are widely used as reducing agents. Dilute aqueous solutions tend to hydrolyze and oxidize in air, but addition of dilute hydrochloric acid prevents this hydrolysis concentrated solutions resist both hydrolysis and oxidation. Neutralization of tin(II) chloride solutions with caustic causes the precipitation of stannous oxide or its metastable hydrate. Excess addition of caustic causes the formation of stannites. Numerous complex salts of stannous chloride, known as chlorostannites, have been reported (3). They are generally prepared by the evaporation of a solution containing the complexing salts. 

Zirconia prepared by the thermal decomposition of zirconium salts is often metastable tetragonal, or metastable cubic, and reverts to the stable monoclinic form upon heating to 800°C. These metastable forms apparently occur because of the presence of other ions during the hydrolysis of the zirconium their stabiUty has been ascribed both to crystaUite size and surface energy (152—153) as well as strain energy and the formation of domains (154). 

Although hydration under hydrothermal conditions may be rapid, metastable iatermediate phases tend to form, and final equiUbria may not be reached for months at 100—200°C, or weeks at even higher temperatures. Hence, the temperatures of formation given ia Table 6 iadicate the conditions under saturated steam pressure that may be expected to yield appreciable quantities of the compound, although it may not be the most stable phase at the given temperature. The compounds are Hsted ia order of decreasiag basicity, or lime/siHca ratio. Reaction mixtures having ratios C S = 1 yield xonotHte at 150—400°C. Intermediate phases of C—S—H (I), C—S—H (II), and crystalline tobermorite ate formed ia succession. Tobermorite (1.13 nm) appears to persist indefinitely under hydrothermal conditions at 110—140°C it is a principal part of the biader ia many autoclaved cement—silica and lime—silica products. 

When hydrogen peroxide is added to an acid solution of Cr(VI), a deep blue color, iadicating the formation of chromium (VI) oxide diperoxide [35262-77-2] is observed. This compound is metastable and rapidly decomposes to Cr(III) and oxygen at room temperature. The reaction... 

There is no unanimity in regard to the exact mechanism of ECC formation under high pressure. Wunderlich et al. [11-18] suggested that when a flexible polymer molecule crystallizes from the melt under high pressure, it does not grow in the form of a stable extended chain, rather it deposits as a metastable folded chain. 

The crystallization of glassy Pd-Ni-P and Pd-Cu-P alloys is complicated by the formation of metastable crystalline phaf s [26]. The final (stable) crystallization product consists of a mixture of a (Pd,Ni) or (Pd,Cu) fee solid solution and more than one kind of metal phosphide of low crystallographic symmetry. Donovan et al. [27] used transmission electron microscopy (TEM) and X-ray microanalysis to study the microstructure of slowly cooled crystalline Pd4oNi4oP2o- They identified the compositions of the metal phosphides to be Pd34Ni45P2j and Pdg8Ni[4Pjg. 

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