Phase transition recrystallization

Recrystallization. The recrystallization of a solid may result in the production of a higher temperature lattice modification, which permits increased freedom of motion of one or more lattice constituents, e.g. a non-spherical component may thereby be allowed to rotate. Such reorganizations are properly regarded as premelting phenomena and have been discussed by Ubbelohde [3]. The mechanisms of phase transitions have been reviewed by Nagel and O Keeffe [21] (see also Hannay [22]). 

X-ray diffraction studies are usually carried out at room temperature under ambient conditions. It is possible, however, to perform variable-temperature XPD, wherein powder patterns are obtained while the sample is heated or cooled. Such studies are invaluable for identifying thermally induced or subambient phase transitions. Variable-temperature XPD was used to study the solid state properties of lactose [20], Fawcett et al. have developed an instrument that permits simultaneous XPD and differential scanning calorimetry on the same sample [21], The instrument was used to characterize a compound that was capable of existing in two polymorphic forms, whose melting points were 146°C (form II) and 150°C (form I). Form II was heated, and x-ray powder patterns were obtained at room temperature, at 145°C (form II had just started to melt), and at 148°C (Fig. 2 one characteristic peak each of form I and form II are identified). The x-ray pattern obtained at 148°C revealed melting of form II but partial recrystallization of form I. When the sample was cooled to 110°C and reheated to 146°C, only crystalline form I was observed. Through these experiments, the authors established that melting of form II was accompanied by recrystallization of form I. 

F. Three-stage (partial melting, recrystallization, and total melting) phase transition theory... 

In DSC the sample is subjected to a controlled temperature program, usually a temperature scan, and the heat flow to or from the sample is monitored in comparison to an inert reference [75,76], The resulting curves — which show the phase transitions in the monitored temperature range, such as crystallization, melting, or polymorphic transitions — can be evaluated with regard to phase transition temperatures and transition enthalpy. DSC is thus a convenient method to confirm the presence of solid lipid particles via the detection of a melting transition. DSC recrystaUization studies give indications of whether the dispersed material of interest is likely to pose recrystallization problems and what kind of thermal procedure may be used to ensure solidification [62-65,68,77]. 

Crystal phase transitions are a possible target with present day computational means, when the transition is a smooth one and does not involve melting of the mother phase and subsequent recrystallization into the daughter phase. For crystalline OL-norleucine, an MD simulation has provided a detailed picture of the mechanism of a solid-solid second-order transition between two polymorphic crystal forms, showing concerted molecular displacements involving entire bilayers [61]. 

Reconstructive phase transitions occur when major changes are made in the topology, i.e. when the bond graph is reorganized. The transitions usually observed in structures with lattice-induced strain are displacive and often second order (no latent heat). Reconstructive transitions arise when two quite different structures with the same composition have similar free energies. Unlike the displacive transitions they involve the dissolution of one structure and the recrystallization of a quite different structure. These phase transitions possess a latent heat and often display hysteresis. 

Crystallization and phase transition, minerals deposit and evaporate. Deposition of minerals as they crystallize is a start of the (ESR) clock.8 Their partial recrystallization gives sometimes misleadingly young ages. Phase transitions also remove the signals in some minerals.22... 

Fig. 4.12 A DSC measured at four different heating rates. At the slowest rate (0.5 °C min ) both a solid-solid phase transition (III —I) (A) and the melting of the more stable phase I (B) can be seen. At the fastest heating rate of 64 °C min modification III melts directly (E), but the heating rate is sufficiently fast to prevent the crystallization of Form I. At 4 °C min Form II melts (C) and recrystallizes to Form I (D) which subsequently melts. At the intermediate rate of 16 °C min the system does not reach equilibrium, so the recrystaUization of Form I is masked by the direct melting of Form III. (From Burger 1975, with permission)...

In conclusion, the Raman features observed and calculated for iron oxide crystals have been used as reference to identify Raman modes in their counterpart nanomaterials. Furthermore, it is known that the high power density from a laser excitation source can excessively heat a sample during a Raman experiment, as discussed previously. This effect becomes even more important for micro-Raman experiments of nanomaterials, where laser beams are focused to a spot size with a diameter of only a few micrometers, and nanoparticulates do not dissipate heat well. Moreover, an increase in the local sample temperature may cause a frequency shift in the Raman bands, or it may cause material degradation as the result of oxidation, recrystallization, order-disorder transitions, phase transition, or decomposition. 

In the case of solvates, binary phase diagrams of temperature versus concentration of the solvent (or water) at a given pressure are useful for the understanding of the phase transitions. The characterization of solvates and hydrates need the use of both DSC and TG. Desolvatation can be complex melting of the solvate followed by exothermic recrystallization into the anhydrous form or solid-state transformation with... 

The DSC, TG curves of solvates and hydrates are related to the phase diagrams between substance and solvent (or water). Eutectic are observed. Fusion or decomposition of the solvate may occur during heating. Therefore, one may observe the melting of the solvate followed by recrystallization into the anhydrous form or the endothermic desolvatation in the solid state. In certain cases both phenomena may over-lapp. Details about experimental factors and examples can be found in Ref. If the anhydrous form is metastable, further phase transitions follow the desolvatation. If several solvates or hydrates exist, the transitions observed depend on the pressure, as demonstrated by Soustelle in the case of copper sulfate pentahydrate. Depending on the pressure, the direct dehydration into the anhydrous or the dehydration via the monohydrate, or the three dehydration steps trihydrate, monohydrate and anhydrous forms may be obtained. Hydrates have been the subject of... 

Vessal (1991) has studied the simulation of amorphization of crystalline A1P04 berlinite using the constant pressure MD method. When the system is depressurized gradually, amorphous A1P04 is observed to recrystallize back to the crystalline phase. These observations are in agreement with recent experiments (Kruger and Jeanloz, 1990). The simulation was undertaken with a pressure interval of 1 GPa between two consecutive cycles. During pressurization, the behaviour of the system was studied to detect a phase transition. After amorphization, the amorphous material was pressurized further to see if there... 

The internal energy is plotted against the pressure in Fig. 12.20. The crystal is pressurized from a to b and amorphizes between b and c the amorphous material is pressurized from c to d. The system is then depressurized from c to g. The amorphous material recrystallizes between e and f. A clear first order transition occurs between points b and c from which the transition pressure and the heat of amorphization La = U(c) — (7(b) are obtained. The observed heat of amorphization is 58 kJ/mol. During the depressurization of the system from c to g another first order phase transition occurs between points e and f from which the transition pressure and the heat of recrystallization La = t/(f) — t/(e) are obtained. The observed heat of recrystallization is — 58 kJ/mol. 

It can be seen from Fig. 12.20 that berlinite amorphizes at a pressure of 33 GPa and amorphous aluminium phosphate recrystallizes at a pressure of 27 GPa. The observed phase transitions are in accord with the experimental results (Kruger and Jeanloz, 1990), but the transition pressures are different from experiment (15 + 3 GPa and 5 GPa, respectively, with a hysteresis of about 13-15 GPa). In this study the hysteresis is about 6 GPa. This discrepancy could be attributed to several factors. First, the fact that a perfect crystal is... 

The four main peaks in the Al-Al RDF are located at 4.52, 5.07, 5.56, and 5.39 A at zero pressure (see Fig. 12.23). As the pressure on the system is increased these peaks shift to shorter distances. The peak at 5.56 A will disappear completely at a pressure of 24 GPa. At a pressure of 33 GPa the shape of the RDF changes drastically, which indicates the amorphization of berlinite, where three broad peaks at 3.36, 4.65, and 5.63 A are observed in the Al-Al RDF at 33 GPa. During the decompression process the reverse trend is observed. At 28 GPa there is clearly a phase transition and the system becomes more ordered. The four Al-Al peaks that are observed for the recrystallized material are at exactly the same positions as the peaks for the starting crystalline material. 

A. For samples W5, WIO and W15, recrystallized in the presence of water, a broadening of the pore radius distribution and a shift of the mean pore radius to lower values is observed. For these samples, an increase of the dV/dr values with decreasing pore radius for pore radii below 15 A indicates that the irregular micropores of the parent SBA-15 are still preserved. With the increase of the water content, such irregular micropores are not observed for samples W30 and W40, which contains mesopores with a relatively narrow radius distribution centered at 19 A. The sharp peaks at 8 A observed in the pore radius distribution of samples W30 and W40 are probably artifacts caused by a phase transition ( solidification ) of the nitrogen adsorbed in the ZSM-5 micropores [10] and therefore confirming the formation of crystalline ZSM-5 in these samples, already proved by their WAXRD patterns (Figure 2). 

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