Polymorphic transition temperature

The pressure-temperature phase diagrams also serve to highlight the fact that the polymorphic transition temperature varies with pressure, which is an important consideration in the supercritical fluid processing of materials in which crystallization occurs invariably at elevated pressures. Qualitative prediction of various phase changes (liquid/vapor, solid/vapor, solid/liquid, solid/liquid/vapor) at equilibrium under supercritical fluid conditions can be made by reference to the well-known Le Chatelier s principle. Accordingly, an increase in pressure will result in a decrease in the volume of the system. For most materials (with water being the most notable exception), the specific volume of the liquid and gas phase is less than that of the solid phase, so that... 

MAR/TSA] Martynyuk, M. M., Tsapov, V. I., Overheating of the hexagonal modification of zirconium above the polymorphic transition temperature under pulse heating conditions, Sov. Phys., 14, (1972), 1558-1559. Cited on page 93. 

Since oxidation of lead is an exothermic reaction, considerable amounts of heat are produced in the reaction pot. The reaction temperature should be controlled carefully as it determines the type of the resulting leady oxide. The processing temperature should be kept below 487 °C for the production of tet-PbO, which is the oxide modification preferred by the battery industry. Since the oxidation must be carried out at temperatures below the polymorphic transition temperature of 487 °C, the reactor pot is kept within the temperature range of 460—470 °C. Under these conditions, small amounts (less than 15%) of orthorhomb-PbO are formed and the subsequent parameters of the battery are virtually unaffected. 

The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

Focusing attention on PTEB, it has been found that, similar to the case of PDTMB, the mesophase experiences a very slow transformation into the crystal. Thus, only the isotropization is observed in a sample freshly cooled from the melt [27]. However, after a long time at room temperature, the transformation mesophase-crystal is produced, owing to a glass transition temperature of about 14°C. Moreover, several endotherms were obtained before the final isotropization for a sample of PTEB annealed at 85°C for 12 days, i.e., PTEB shows enantiotropic behavior. The different endotherms may arise from polymorphism or melting-recrystallization phenomena [30]. 

Polymorphic transition may occur very rapidly (e.contact with the second form, accelerate the process. 

In the fourth section the influence of comonomeric units on the polymorphic behavior of some polymers is described. In particular it is shown that comonomeric units can produce changes in the solid-solid transition temperatures as well as variations of the crystalline forms, which can be obtained for a given crystallization procedure. In the same section, some recent studies, showing that the polymorphic behavior of some polymers can be altered in particular miscible blends, are also reviewed. 

Second, x-ray diffraction will, directly, give spacings in the crystal and reveal differences between samples. Finally, solubility curves can be carried out, and if a nick in the solubility curve is found (Fig. 9), this is a transition temperature. If no nick is found, there is no transition temperature, but if the dissolution curves are as shown in Fig. 5, there is polymorphism, and it may... 

An enantiotropically related pair of polymorphs was obtained for p-aminoben-zoic acid, with the system being characterized by a transition temperature of 25 °C [31]. The a-form was obtained as fibrous needles, while the /M orm was obtained in the form of prisms. The solubilities of the two forms are almost the same, indicating the existence of comparable values for AG, which in turn explained the slow transformation of the a-form into the [>-form. Nucleation of the a-form was found to be favored, which is reasonable considering that the structural motif of the a-form consists of carboxylic acid dimmers that would be expected to be stable association species in solution. 

The structures of two polymorphs of pleconaril, enantiotropically related with a transition temperature of 35.7°C, have been reported [36], Form I was described as consisting of a network of dimers, while Form III was described as a three-dimensional network of monomers. The two forms contradicted the density rule, and the solid solid transition could occur only through a destructive-reconstructive mechanism. A quantitative differential scanning calorimetry method was also described that enabled the quantitative determination of Form I in bulk Form III to be made at levels as low as 0.1%. 

In the enantiotropic case the relative solubility and stability changes at a specific transition temperature where the two solubility curves cross. A given polymorph may be more soluble (less stable) above the transition point, but less soluble below and vice versa, Figure 4. An example of this relationship is the Form C and A or B relationships for Cimetidine in the case study. 

A plot of the ideal solubility curves can be used to identify the transition temperatures for the enantiotropic relationships. Form C is the most stable polymorph at high temperature, with a transition to Form B at 20 °C. The Form C to A transition occurs at 11 °C. 

Y. Hu, H. Wikstrom, S.R. Bym and L.S. Taylor, Estimation of the transition temperature for an enantiotropic polymorphic system from the transformation kinetics monitored using Raman spectroscopy, J. Pharm. Biomed. Anal, 45, 546-551 (2007). 

Incidentally, it must be noted that, although most olivine compounds are stable (at P = 1 bar) up to the melting temperature, in some compounds polymorphic transitions are observed at temperatures below the melting point (for instance, Ca2Si04 has a transition to a monoclinic polymorph at 1120 K). Forsterite at 1 bar melts at 7) = 2163 K, whereas fayalite melts at a much lower temperature Tf = 1490 K). The significance of this difference, as shown by Hazen (1977), must be ascribed to different polyhedral thermal expansions for polyhedra containing Mg and Fe cations. Hazen (1977) pointed out that, if the curve of volumes of the members of the (Mg,Fe)2Si04 mixture is extrapolated at various... 

We have already stated that the a-[3 transition of quartz may be described as a A transition overlapping a first-order transition. The heat capacity function for the two polymorphs is thus different in the two stability fields, and discontinuities are observed in the H and S values of the phase at transition temperature T rans cf section 2.8). For instance, to calculate the thermodynamic properties of ]8-quartz at T = 1000 K and P = bar, we... 

Differential scanning calorimetry (DSC) and x-ray diffraction (XRD) are the techniques most widely used for the characterization of crystallinity and polymorphism of solid lipid particles. Although DSC is usually more sensitive in detecting crystalline material, XRD is much more reliable in determining the type of polymorph present in the dispersions because it provides structural data. In contrast, DSC can detect the type of polymorph only indirectly via the transition temperatures and enthalpies. Because these parameters may be different from those observed in the bulk material, particularly for small colloidal particles [1,62], assigmnent of polymorphic forms in DSC curves should be supported by x-ray data. 

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]. 

Since a polymorphic transition (a/p) of the mixed iron and cobalt molybdate occurs in the temperature range of the catalytic reaction (10,13,14), and since the high temperature form (p) can metastably be maintained at low temperature,... 

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