Crystal formation metastable phases

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. 

Finally, at even lower transformation temperatures, a completely new reaction occurs. Austenite transforms to a new metastable phase called martensite, which is a supersaturated solid solution of carbon in iron and which has a body-centred tetragonal crystal structure. Furthermore, the mechanism of the transformation of austenite to martensite is fundamentally different from that of the formation of pearlite or bainite in particular martensitic transformations do not involve diffusion and are accordingly said to be diffusionless. Martensite is formed from austenite by the slight rearrangement of iron atoms required to transform the f.c.c. crystal structure into the body-centred tetragonal structure the distances involved are considerably less than the interatomic distances. A further characteristic of the martensitic transformation is that it is predominantly athermal, as opposed to the isothermal transformation of austenite to pearlite or bainite. In other words, at a temperature midway between (the temperature at which martensite starts to form) and m, (the temperature at which martensite... 

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

As introduced above, different forms of the same molecule can be observed in the solid state. The phenomenon is known as polymorphism, i.e., the concurrent presence of more crystal forms, only one of which is thermodynamically stable at a given pressure and temperature. However, more polymorphs can be observed simultaneously when kinetic conditions allow formation of metastable phases together with (or even in the absence of) the thermodynamically stable one. It might even occur that metastable phases are not recognized as such, simply because the most stable polymorph is (as yet) unknown. This might produce the extraordinary phenomenon of disappearing polymorphs [97]. 

All crystal growth takes place in low-temperature, low-pressure aqueous solution (at 1 atmospheric pressure and room temperature). This suggests a higher probability of formation of an amorphous state, phases of low crystallinity, and metastable phases as precursors, and therefore subsequent transformation to stable or metastable phases. 

Zeolite formation depends on reaction conditions 2-4). It is generally believed that most zeolites are formed as metastable phases. According to Barrer (3), the course of the synthesis, beginning with the type of starting material, determines the structure of the zeolite formed. The studies of Zhdanov 2, 5) on the composition of liquid and solid phases of hydrogels indicate that the kind and composition of the zeolite formed depend on the hydrogel composition and that the results of crystallization of aluminosilicate gels obtained in the same way are reproducible. 

Reactive sputter deposition is a useful technique not only for coating but also for obtaining metastable phases, especially nitrides. Nitrogen gas is activated in a plasma to enhance nitride formation and heating at high temperature is not required. It is possible to obtain metastable nitrides by sputter deposition in which species in the plasma are quenched on substrates. Compounds with different crystal structures normally do not form solid solutions, but by co-sputter deposition metastable solid solutions may form. 

Although marble migration is only mentioned as an example, this process is not far removed from the crystallization process. In the formation of single crystals, individual ions/atoms/molecules slowly come into contact with one another, and nucleate from thermodynamically favored positions. Sometimes metastable phases may be obtained if one does not allow such preferential migration to occur, through rapid cooling events, for instance. 

Penn RL, Banfield JF (1998b) Oriented attachment and growth, twinning, polytypism, and formation of metastable phases Insights from nanociystalline Ti02. Am Mineral 83 1077-1082 Post JE, Bish DL (1989) Rietveld refinement of crystal stmctnres nsing powder X-ray diffraction data. Rev Mineral 20 277-308... 

---