Metastable phase amorphous solids

Mechanical alloying has been shown to cause significant microstructural changes, including the formation of a nanocrystalline grain structure, extended solid solubility, disordering, formation of metastable phases, amorphization and disproportionation of pre-... 

Palladium acetate, [PdO —02CCH3)2l3, possesses a unique quality that makes it attractive for solid state decomposition studies as well as technological applications. It can be spin-coated from solution to form a homogeneous, apparently amorphous solid film. This provides large uniform areas over which we can study the effects of various irradiation sources on the chemical nature of the film. The bulky structure of palladium acetate, shown in Figure 1 (8), may offer a partial explanation of the molecule s ability to achieve an amorphous metastable phase upon rapid evaporation of solvent. 

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. 

In this chapter, general aspects and structural properties of crystalline solid phases are described, and a short introduction is given to modulated and quasicrystal structures (quasi-periodic crystals). Elements of structure systematics with the description of a number of structure types are presented in the subsequent Chapter 7. Finally, both in this chapter and in Chapter 6, dedicated to preparation techniques, characteristic features of typical metastable phases are considered with attention to amorphous and glassy alloys. 

At a given (low) temperature and pressure a crystalline phase of some substance is thermodynamically stable vis a vis the corresponding amorphous solid. Furthermore, because of its inherent metastability, the properties of the amorphous solid depend, to some extent, on the method by which it is prepared. Just as in the cases of other substances, H20(as) is prepared by deposition of vapor on a cold substrate. In general, the temperature of the substrate must be far below the ordinary freezing point and below any possible amorphous crystal transition point. In addition, conditions for deposition must be such that the heat of condensation is removed rapidly enough that local crystallization of the deposited material is prevented. Under practical conditions this means that, since the thermal conductivity of an amorphous solid is small at low temperature, the rate of deposition must be small. 

Secondary phases predicted by thermochemical models may not form in weathered ash materials due to kinetic constraints or non-equilibrium conditions. It is therefore incorrect to assume that equilibrium concentrations of elements predicted by geochemical models always represent maximum leachate concentrations that will be generated from the wastes, as stated by Rai et al. (1987a, b 1988) and often repeated by other authors. In weathering systems, kinetic constraints commonly prevent the precipitation of the most stable solid phase for many elements, leading to increasing concentrations of these elements in natural solutions and precipitation of metastable amorphous phases. Over time, the metastable phases convert to thermodynamically stable phases by a process explained by the Guy-Lussac-Ostwald (GLO) step rule, also known as Ostwald ripening (Steefel Van Cappellen 1990). The importance of time (i.e., kinetics) is often overlooked due to a lack of kinetic data for mineral dissolution/... 

Metastable amorphous solids can in general be prepared from stable phases by bringing in excess free energy [5]. In the case of water, amorphous solids have been prepared from stable phases in all three aggregate states from the gas, the liquid, and the crystalline solid [131]. 

Figure 19. Schematic phase relations of Si. Solid lines are the boundaries between liquid and crystalline or crystalline and crystalline phases. Dashed lines denote possible boundaries between amorphous and amorphous or liquid and hquid (metastable) phases. The filled circle denotes the hypothesized second critical point. Note that the scale of pressure and temperature is uncertain.

The transformation of the crystalline into the glassy state by solid-state reactions is extensively reviewed in its theoretical and experimental aspects. First, we give some historical background and describe the thermodynamics of metastable phase formations, adding as well the kinetic requirements for the amorphization process. Then we discuss the different experimental routes into the amorphous state hydriding, thin diffusion couples, and other driven systems. In the discussion and the summary, we close the gap between the melting phenomena and the amorphization and provide a tentative outlook. 

The non-equilibrium condition of most groundwater systems with respect to many primary minerals, or similarly the metastability which exists with respect to many semi-crystalline or amorphous phases are common problems, especially for silicates. Some clear identification is needed for system reaction time, or the rate at which equilibrium is approached, and similarly identification is needed for metastable plateaus of pseudo-equilibrium, especially for compounds such as amorphous silica, cristobalite, quartz, clay minerals, etc. The likely magnitude of saturation indices which could apply to a given mineral could be specified for a variety of conditions. In this volume, Glynn, and elsewhere others, have recently shown that some error occurs in the calculated saturation values for trace elements when pure end member minerals are assumed to be present, when actually the phases are solid solutions. The consensus among modelers appears to be that error is present and significant the challenge is to develop procedures that quantify the error, so models become tools that provide realistic and interpretable results. 

Beside these unary, binary and ternary phases, 8-C3N4 has been theoretically predicted by Liu and Cohen [22] as a solid phase but has not been found experimentally yet. Numerous different metastable, amorphous phases of binary, ternary or quaternary compositions have been obtained by gas phase synthesis or polymer thermolysis. Bulk materials, fibers and coatings have been produced by these methods [5-15]. To analyze the crystallization behavior of such materials at high temperatures (T > 1800 K) a knowledge of the phase equilibria with regard to the materials gross composition is required. Theoretical work on metastable phases, e.g. Si3B3N7 [23], is also documented. 

Another area where structural energy calculations can be of great utility concerns the solid-state order-disorder transformation. Such transformations can be induced by pressure. At high pressures, a-quartz subsists as a metastable phase which gradually transforms to an amorphous form and, subsequently to a rutile-like crystalline structure[34]. Evidence for the onset of the amorphization has been reported at about 15 GPa from single crystal analysis[35].In powder measurements[36], the transition is observed to be complete by 35 GPa. Experiments performed on powered samples at pressures above 60 GPa indicate a crystalline structure which is thought to resemble the stishovite structure[37]. 

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