Slow-crystallized sample temperature dependences

The spontaneous but slow crystallization of monoclonic selenium (Se8) from red amorphous Se, prepared by quenching of selenium vapor (1000°C) in liquid nitrogen, depends on the storage temperature of the samples. Below 303 K, red amorphous Se(vap) is completely transformed into the monoclinic phase, whereas above 303 K transformation into the metallic phase takes place. (The relative Se8 content of the material can be determined by DSC.) In contrast, chemically prepared red amorphous Se(redn) is transformed only into the metallic phase, even below 303 K (21). 

When organic (drug) molecules crystallize from a solvent, the crystal structure is dependent upon the speed of crystallization, temperature, polarity of the solvent, concentration of the material, etc. Since the energy of the crystal affects the (physiological) rate of dissolution and thus the potency and activity of the drug, polymorphism is an important pharmaceutical concern [39]. The most common tool to determine crystal form is differential scanning calorimetry (DSC). Unfortunately, DSC uses small samples and may not represent the bulk of the material. X-ray diffraction is another excellent technique, but quite slow and sometimes difficult to interpret. 

Arrhenius plots showing the temperature dependence of diffusivity for n-butane in various 5A samples are shown in figure 4 while figure 5 shows the trend of activation energy with carbon number. For linear paraffins higher than n-butane, diffusion, even in the large 5A crystals, is too slow to measure by the NMR PFG method so it is only for butane that a direct comparison between ZLC and PFG NMR data is possible. 

The temperature dependences of the IR bands of PE were quite different for the slow-crystallized and quenched samples. This effect was most noticeable for the crystalline rocking modes. The peak heights of the 720 and 731 cm" components of this doublet increased with increasing temperature. This was not as anticipated. The intermolecular forces lessen with thermal expansion. Therefore, the induced dipole moment of the interaction decreases with increasing temperature (45,46). The intensity of a band which is enhanced by the dipole-dipole interaction is expected to decrease. 

Figure 5.13 (a) Temperature jump cell for the X-ray scattering measurement, (b) time dependence of sample temperature (solid line 600°C/min, broken line slow cooling), and (c) simultaneous experimental system of WAXD and SAXS for the isothermal crystallization... 

The origin of the small Sy content of all commercial sulfur samples is the following. Elemental sulfur is produced either by the Frasch process (mining of sulfur deposits) or by the Claus process (partial oxidation of HyS) [62]. In each case liquid sulfur is produced (at ca. 140 °C) which at this temperature consists of 95% Ss and ca. 5% other sulfur homocycles of which Sy is the main component. On slow cooling and crystalhzation most of the non-Ss species convert to the more stable Ss and to polymeric sulfur but traces of Sy are built into the crystal lattice of Ss as sohd state defects. In some commercial samples traces of Ss or Sg were detected in addition. The Sy defects survive for years if not forever at 20 °C. The composition of the commercial samples depends mainly on the coohng rate and on other experimental conditions. Only recrystalhzation from organic solvents removes Sy and, of course, the insoluble polymeric sulfur and produces pure a-Ss [59]. 

After evaporation of the excess liquid, the reaction flask is disconnected and the product removed. The product will vary from a light yellow cake to dark red crystals depending on the rate of evaporation of the pyridine. Slow evaporation is most favorable to the formation of large crystals. The yield is approximately 10.3 g. (100%). The sample may be stored indefinitely at room temperature in a tightly stoppered bottle in the dark. Anal. Calcd. for 2C5H6N-Cr03 Cr, 20.14. Pound total Cr, 20.48, 20.58 Cr6+, 20.15, 20.35. 

A consequence of the afore-mentioned slow transitions would be a rather strong dependence of the actual structure observed at room temperature upon the previous thermal history of the sample. For example, quenched samples around Na0i35WO3 may be cubic, while slowly annealed samples would be tetragonal (II). Conversely, samples of low sodium content may be cubic if they have been prepared by extracting sodium vapor at temperatures of 600° to 700°. The existence of two-phase regions may account for some of the difficulties we (and others) have had in growing single crystals of low-sodium bronzes by the electrolytic method. 

The hysteresis that may appear depends on the direction or the scan rate of temperature change. This tendency is based on the phase transition or slow diffusion process of sample. Remarkably such hysteresis can even appear where a sample shows crystallization within the measurement temperature range. Therefore the ionic conductivity measurement is usually performed at each temperature after reaching the constant value. On the other hand, ionic conductivity is measured at scan rate of 1° to 2° C min. Therefore, when hysteresis appears during the heating or cooling process, the relationship between the phase transition and the ionic conductivity can be used in the analysis at this scan rate with the DSC measurement. It is better to use a small cell design to avoid the temperature distribution in samples. 

---