Transformation amorphous-crystalline

Figure 5.3. Effects of soil pH on the Zn amount bound to the Fe oxide fraction (amorphous/crystalline Fe oxide and overall Fe oxide bound fractions) in soils from China with pH 3.73-8.1 and 0-14.7% CaC03 (after Han et al., 1995. Reprinted from Geoderma, 66, Han F.X., Hu A.T., Qi H.Y., Transformation and distribution of forms of zinc in acid, neutral and calcareous soils of China, p 128, Copyright (1995), with permission from Elsevier)...

M. Khayyat, G. Banini, D. Hasko, and M. Chaudhri, Raman microscopy investigations of structural phase transformations in crystalline and amorphous silicon due to indentation with a Vickers diamond at room temperature and at 77 K, J. Phys. D—Appl. Phys. 36, 1300-1307 (2003). 

The most obvious future data needs concern the missing, uncertain, and conflicting data identified above. Additional experimental investigations are needed in the case of Fe(III) and Zr(IV) carbonate complexation, and in the case of the Sn(IV)/Sn(II) and the Se(0)/Se(-II) redox couples. The molecular structure of metal silicate complexes needs clarification in order to remove ambiguities in the speciation scheme of these complexes. A rather challenging topic concerns the supposed transformation of crystalline tetra-valent actinide oxides, AnOz(cr), to solids with an amorphous surface layer as soon as the An4+ ion hydrolyses. The consequences of such... 

Consider the isothermal transformation of a disordered (amorphous) solid to an ordered (crystalline) solid. Obviously, there must be enough thermal energy to allow individual atoms to move around and reorient themselves, but assuming this is possible, it is generally found that at constant temperature, the amount of amorphous material transformed to crystalline material, dx (on a volume basis) per unit time, dt, is given by... 

When calcium phosphate is precipitated from aqueous solutions of high supersaturation and pH values above 7, the solid phase appearing initially is an ACP with the formula Ca9(P04)61U u2 - If this amorphous precipitate is allowed to remain in contact with the solution, it transforms to crystalline HA through a process of dissolution, nucleation and crystal growth77, unless stabilized in some manner. 

Termine, J. D., Peckauskas, R. A., Posner, A. S. Calcium phosphate in vitro. II. Effects of environment of amorphous-crystalline transformation. Arch. Biochem, Biophys. 140, 318 (1970)... 

Eanes, E. D., Termine, J. D., Nylen, M. U. An electron microscope study of the formation of amorphous calcium phosphate and its transformation to crystalline apatite. Calc. Tiss. Res. 12, 144 (1973)... 

Amorphous Tellurium is a brownish-black powder usually obtained by precipitation methods, for example by reducing a solution of tellurium dioxide with sulphurous acid.1 On.heating it is transformed into crystalline tellurium with development of heat ... 

Due to the lack of vinyl monomers giving rise to crystalline segment by cationic polymerization, amorphous/crystalline block copolymers have not been prepared by living cationic sequential block copolymerization. Although site-transformation has been utilized extensively for the synthesis of block copolymers, only a few PIB/crystalline block copolymers such as poly(L-lactide-fc-IB-fc-L-lactide) [92], poly(IB-fr- -caprolactone( -CL)) [93] diblock and poly( -CL-fr-IB-fr- -CL) [94] triblock copolymers with relatively short PIB block segment (Mn< 10,000 g/mol) were reported. This is most likely due to difficulties in quantitative end-functionalization of high molecular weight PIB. 

Utilizing an amorphous form of an API is not, however, universally desirable. Amorphous compounds are often metastable. As a result, there is a real risk that they will transform to crystalline materials in the final dosage form. Novobiocin again provides a case in point. The amorphous form, in aqueous suspension, will transform on standing into the inactive crystalline form.10 Similarly, the highly soluble metastable crystals of riboflavin revert to less soluble forms if they are washed with water above 10°C.9... 

Eanes, E.D., Gillessen, I.H., and Posner, A.S. Mechanism of conversion of non-crystalline calcium phosphates to crystalline hydroxyapatite, p 373-375, "Proc. Int. Cong, on Crystal Growth, Boston, 1966. Pergamon Press, Oxford, 1967. Termine, J.D., Peckauskas, R.A., and Posner, A.S. Calcium phosphate formation in vitro. II. Effect of environment on amorphous crystalline transformation. Arch. Biochem. Bio-phys. 140. 318-325 (1970). 

The rate of diagenetic transformations of iron cherts is determined not only by T and P, but also by the particulars of deposition, form of migration, dispersion of the particles, and pH of the environment. The time it takes to transform amorphous sediments into crystalline minerals is very great. For iron hydroxides it is several million years, and for silica it may be still more. 

Because of its ability to probe the atomic environment in compounds which do not possess long-range order, MAS NMR has become a technique of choice for studying the constitution of amorphous materials, including gels and glasses which contain relatively sensitive NMR-active nuclei, and their thermal transformation into crystalline compounds. 

For a less supersaturated solution, Fig. 2-12 indicates that the amorphous solid can be generated after aging of the oil droplets. An amorphous solid can also be formed by rapid cooling of a saturated solution, as in lyophilization or freeze-drying operations. Like oil droplets, amorphous solids are unstable and will transform into crystalline material upon aging under proper conditions. As shown in Example 12-5, the amorphous imipenem can be transformed into crystalline material even under freezing conditions. 

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