Solid metastable phases

Six crystalline phases of non-solvated AlHg can be produced by the desolvation of aluminium hydride etherate in the presence of small quantities of LiAlH4. The most stable form, a-AlHg, is produced from the solid metastable phases, or by recrystallization from a refluxing mixture of and Et20. ... 

FIG. 16 Negative normal stress T.. as a funetion of substrate separation from grand eanonial ensemble Monte Carlo simulations at T = 1.00, ii = —11.0, and = 0.0 ( )(Pbuik = 0.486) the solid line represents a eubie spline fit to the dis-erete data to guide the eye. Also shown are three isobars —T% = 0.000, 0.598, and 1.196, indieated by horizontal lines. Interseetions between the isobars and the eurve Tzzi z) correspond to stable and metastable phases of the eonfined fluid (see text) (from Ref. 66). 

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... 

Figure 5. Concentrational sections M gdr Rh, x)4B4 phase boundaries (schematic) of different structure types. Open circles indicate the occurrence of superconductivity filled symbols mean T = 1.2 K (no superconductivity observed above 1.2 K). Circles denote the existence of a CeCo4B4-type phase the thiek solid line represents the (metastable) phase boundary of the CeCo4B4-type structure the dashed line encloses the superconductivity region. ...

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. 

Sinha, A. K., Giessen, B. C., and Polk, D. E., Metastable Phases Prodiced by Rapid Quenching from the Vapor and the Liquid. Treatise on Solid State Chemistry, ed. N. B. Hannay. Vol. 3. 1976. 

On heating from a crystalline phase, DOBAMBC melts to form a SmC phase, which exists as the thermodynamic minimum structure between 76 and 95°C. At 95°C a thermotropic transition to the SmA phase occurs. Finally, the system clears to the isotropic liquid phase at 117°C. On cooling, the SmC phase supercools into the temperature range where the crystalline solid is more stable (a common occurrence). In fact, at 63°C a new smectic phase (the SmF) appears. This phase is metastable with respect to the crystalline solid such phases are termed monotropic, while thermodynamically stable phases are termed enantiotropic. The kinetic stability of monotropic LC phases is dependent upon purity of the sample and other conditions such as the cooling rate. However, the appearance of monotropic phases is typically reproducible and is often reported in the phase sequence on cooling. It is assumed that phases appearing on heating a sample are enantiotropic. 

It is worth noting that a monotropic polymorphic system offers the potential of annealing the substance to achieve the preferred form of the thermodynamically stable phase. The use of the most stable form is ordinarily preferred to avoid the inexorable tendency of a metastable system to move toward the thermodynamic form. This is especially important especially if someone elects to use a metastable phase of an excipient as part of a tablet coating, since physical changes in the properties of the coating can take place after it has been made. Use of the most stable form avoids any solid-solid transition that could... 

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. 

In the previous chapter we looked at some questions concerning solid intermetallic phases both terminal (that is solubility fields which include one of the components) and intermediate. Particularly we have seen, in several alloy systems, the formation in the solid state of intermetallic compounds or, more generally, intermetallic phases. A few general and introductory remarks about these phases have been presented by means of Figs. 2.2-2.4, in which structural schemes of ordered and disordered phases have been suggested. On the other hand we have seen that in binary (and multi-component) metal systems, several crystalline phases (terminal and intermediate, stable and also metastable) may occur. 

Quenching (front the solid state). Metastable alloys have been very familiar to metallurgists for a long time now. Several alloys employed in everyday applications contain metastable phases. Typical examples are quenched steels and precipitation hardened aluminium alloys. Until the 1960s, metastable alloys were always obtained by quenching (rapid cooling) from the solid state. 

The traditional techniques for retaining a metastable phase in the solid alloy generally make use of the high-heat transfer from the hot solid to a fluid (a gas or a liquid). 

Metastable crystalline phases frequently crystallise to a more stable phase in accordance with Ostwald s rule of stages, and the more common types of phase transformation that occur in crystallising and precipitating systems include those between polymorphs and solvates. Transformations can occur in the solid state, particularly at temperatures near the melting point of the crystalline solid, and because of the intervention of a solvent. A stable phase has a lower solubility than a metastable phase, as indicated by the solubility curves in Figures 15.7a and 15.7/ for enantiotropic and monotropic systems respectively and,... 

Figure 2.14 Electrochemical phase diagram for chalcopyrite with elemental sulphur as metastable phase. Equilibrium lines (solid lines) correspond to dissolved species at 10 mol/L. Plotted points show the upper and lower limit potential of collectorless flotation of chalcopyrite reported from Sun (1990), Feng (1989) and Trahar (1984)...

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]. 

Trivial examples of metastability are solid solutions. Because these are inherently defect systems, they cannot be thermodynamically stable at low temperatures. Most of our high Tc superconductors need to be regarded as solid solutions which are then necessarily metastable phases. We could dismiss this as an irrelevant observation on the basis that solid solutions are merely required in order to adjust the carrier concentration to appropriate levels. However, we seem unable to generally make stable high Tc superconductors. One could even suggest that there is a correlation between Tc and metastability the higher the Tc, the more unstable. 

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/... 

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