Crystal habit characterization

In order to bridge the gap between an SHG-active material and one optimized for use in an optoelectronic device, many compounds have been synthesized, characterized, modified and then ultimately rejected during the past decade (i-3). This is not surprising, since the ideal material must fulfil a plethora of stringent requirements (4-7). The most critical condition for an SHG-active material is that it must form noncentrosymmetric structures however, thermal stability, in volatility, transparency, lack of colour, mechanical strength and crystal habit are also crucial properties for materials to be incorporated into practical devices. 

These minerals have different stacking of the silica and alumina layers, as well as, incorporating metal hydrates of Na, K, Mg, Al, or Fe between the silica and alumina layers. Clay minerals can also be characterized according to their morphological features including crystal habit (i.e., plates, rods, or rolled-up platelets) stacked in either a house of cards or blocklike aggregates giving a partide-size distribution. 

The most general use of the microscope for the study of polymorphism is simply for observing the homogeneity or diversity of a crystalline sample. Variations in size, shape or colour may indicate the presence of polymorphism and the need for further examination (see, for instance Fig. 3.2). As noted in Section 2.4.1, differences in crystal habit are not necessarily indicative of polymorphism. Further physical characterization of the individual crystals may involve measurements such as optical constants (Hartshorne and Smart 1960, 1964 Wahlstrom 1969 McCrone et al. 1978) or interfacial angles (Winchell 1943 Porter and Spiller 1951 Terpstra and Codd 1961). 

It is a pharmacopoeial requirement that suspensions should be redispersible if they settle on storage. However, the pharmacopoeias do not offer a suitable test that can be used to characterize this aspect of the formulation. In an attempt to remedy this situation, Deicke and Stiverkrtip (1999) have devised a mechanical redispersibility tester, which closely simulates the action of human shaking. The crystal habit may also affect the physical stability of the formulation Tiwary and Panpalia (1999) showed that trimethoprim crystals with the largest aspect ratio showed the best sedimentation volume and redispersibility. 

Crystal Morphology Predictive Techniques To Characterize Crystal Habit Application to Aspirin (C9Hg04)... 

Van Rosmalen, G.M. and Bennema, P. (1990) Characterization of additive performance on crystallization habit modification. Journal of Crystal Growth, 99, 1053-1060. 

When crystalline, all the 1 1 minerals show a bladed to acicular crystal habit. Often, the crystals form radiating sprays sometimes up to 5 cm in diameter. Individual crystals are rare usually needles are many crystals in near-parallel orientation. Except for cuprosklodowskite, which is green, and kasolite, which is yellow-orange, the minerals are yellow to pale yellow. Uranophane and beta-uranophane show a weak yellow-green fluorescence in shortwave ultraviolet. Colour and crystal habit are the best field guides for identification, but X-ray diffraction is the best method for positive characterization. 

Fig. 26 (a) Schematic illustration of two kinds of nanoplates and the microstructure of their surfaces, (b) SEM characterization of the crystal habit-tuned Li[Lii/3 2x/3NixMn2/3-x/3]02 (LNMO). (c, d) XRD patterns and discharge capacity at a 6C rate of (1) LNMO particles, (2) conversional nanoplate-LNMO and (3) crystal habit-tuned nanoplate-LNMO. Reproduced with permission from ref. 150. Copyright 2010 Wiley-VCH. 

The existence of two polymorphs was reported for a NO-releasing derivative of acetyl-salicylic acid [28]. Selection crystallization of one form or the other was achieved from a number of solvent systems (14 solvents and 3 preparative methods), but several systems were identified that yielded mixtures of the two forms. The single-crystal structure of Form I was reported, but the habit of the Form II crystals precluded their characterization. The transition point of the two forms was calculated from intrinsic dissolution data to be higher than the melting points of both polymorphs and thus the two forms bear a monotropic relationship. 

It is recommended that concentration measurements for this type of modeling work are based on analytical standards of mole or mass fraction, to avoid the conversion error caused by density effects. The excess solid phase should always be characterized by a suitable analytical technique, before and after the equilibrium solubility measurements, to confirm that the polymorphic form is unchanged. It should be noted that the crystal shape (habit) does not always change significantly between different polymorphic forms, and visual assessments can be misleading. 

---