Structural phase transformations, amorphous

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). 

Solid phase growth is used where only structural phase transformations occur and long-range transport of atoms or removal of product species is not required. This is necessary because in solids atoms caimot move very far and the solid material must serve as its own atom source and sink. Solid phase growth is only applied significantly in current semiconductor processing for crystallization of amorphous materials or solid phase reactions, as in formation of silicides by reaction of a metal with silicon. 

Many crystalline solids can undergo chemical transformations induced, for example, by incident radiation or by heat. An important aspect of such solid-state reactions is to understand the structural properties of the product phase obtained directly from the reaction, and in particular to rationalize the relationships between the structural properties of the product and reactant phases. In many cases, however, the product phase is amorphous, but for cases in which the product phase is crystalline, it is usually obtained as a microcrystalline powder that does not contain single crystals of suitable size and quality to allow structure determination by single-crystal XRD. In such cases, there is a clear opportunity to apply structure determination from powder XRD data in order to characterize the structural properties of product phases. 

McNicol et al. (49) used luminescence and Raman spectroscopy to study structural and chemical aspects of gel growth of A and faujasite-type crystals. Their results are consistent with a solid-phase transformation of the solid amorphous network into zeolite crystals. Beard (50) used infrared spectroscopy to determine the size and structure of silicate species in solution in relationship to zeolite crystallization. 

Figure 4 presents the isothermal phase transformation diagram of the template-free syntheses in which the SiCL/Alo ratio and the time t of crytallization are varied. The Siof/Nafcr and l O/SiCL ratios are 10 and 30, respectively. The pentasil phase could only be synthesized for n = SiC /A O, =30-50 and t = 36 - 72 h. Outside of this area amorphous material, mordenite, sheet structures similar to kenyaite, quartz and crystobalite can be found. For values of n less than 25 the crytalline product is mordenite. For 30 < n < 50 a yield of 95% (related to the SiC content) ZSM-5 type, which was proved by X-ray diffraction pattern, could be found. Depending on n and the crystallization time, t, a more or less large amount of amorphous material is produced. This is shown in Figure 5. A long crystallization time causes recrystallization and is harmful to the yield of ZSM-5 products.

The potential of Eq. (1) with parameters determined in Refs. [10, 11] was thoroughly tested in computer simulations of silica polymorphs. In Ref. [10], the structural parameters and bulk modulus of cc-quartz, a-cristobalite, coesite, and stishovite obtained from molecular dynamics computer simulations were found to be in good agreement with the experimental data. The a to / structural phase transition of quartz at 850 K ha.s also been successfully reproduced [12]. The vibrational properties computed with the same potential for these four polymorphs of crystalline silica only approximately reproduce the experimental data [9]. Even better results were reported in Ref. [5] where parameters of the two-body potential Eq. (1) were taken from Ref. [11]. It was found that the calculated static structures of silica polymorphs are in excellent agreement with experiments. In particular, with the pressure - volume equation of state for a -quartz, cristobalite, and stishovite, the pressure-induced amorphization transformation in a -quartz and the thermally induced a — j3 transformation in cristobalite are well reproduced by the model. However, the calculated vibrational spectra were only in fair agreement with experiments. 

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