Crystal impurities, effects

AIChESymp. Ser. (a) 65 (1969) no. 95, Crystallization from solutions and melts (b) 67 (1971) no. 110, Factors affecting size distribution (c) 68 (1972) no. 121, Crystallization from solutions Nucleation phenomena in growing crystal systems (d) 72 (1976) no. 153, Analysis and design of crystallisation processes (e) 76 (1980) no. 193, Design, control and analysis of crystallisation processes (f) 78 (1982) no. 215, Nucleation, growth and impurity effects in crystallisation process engineering (g) 80 (1984) no. 240, Advances in crystallisation from solutions. 

KlMURAH, H. J. Cryst. Growth 73 (1985) 53-62. Impurity effect on growth rates of CaCl2-6H20 crystals. 

Reif 97) has observed the effects of point defects on nuclear resonance lines of Br , Br, Na , and Li in cubic crystals. The effect of temperature on the line widths and spin-lattice relaxation times was investigated for various impurity levels in AgBr and found to be quite pronounced due to vacancy association and diffusion. 

The work discussed in the previous paragraphs provides the framework for the prediction of crystal habit from internal structure. The challenge is to add realistic methods for the calculation of solvent and impurities effects on the attachment energies (hence the crystal habits) to allow this method to provide prediction of crystal habit. Initial attempts of including solvent effects have been recently described (71. 721. The combination of prediction of crystal habit from attachment energies (including solvent and impurity effects) and the development of tailor made additives (based on structural properties) hold promise that practical routine control and prediction of crystal habit in realistic industrial situations could eventually become a reality. 

Since the edge free energies, y, are different for the vapor and solution phases, and particularly for solute-solvent interaction energies, the same crystal species will exhibit different Tracht and Habitus in different ambient phases and different solvents. If impurities are present in the system, this affects y and the advancing rates of steps. There are two opposite cases in impurity effects, and, depending on the interface state, some will promote growth, whereas others will suppress growth. 

An example of the size of the impurity effects that may arise is shown in Fig. 1, which gives the electrode kinetics for the ferro-ferricyanide reaction on three different zinc oxide single crystals of varying conductivity. Each of the crystals was in excess of 99.999% pure. As can be seen, each crystal gives a linear Tafel plot under cathodic bias. However, the exchange currents, i.e, the extrapolations back to the reversible potential (+. 19 volts), differ by a factor of about 1000 and... 

Whatever the details of the kinetic mechanism, impurities cause crystal habit modification. Buckley [65] has classified many impurity effects on different crystal habit modifications. In most cases, impurities decrease the growth rate of specific crystal faces, which lead to a change in the crystal habit because the slowest growing faces will dictate the crystal morphology. In some exceptional cases, impurities can increase the growth rate of a particular crystal face. For example, 1% Fe added... 

In the semiconductors of greater polarity, the dielectric constants are smaller and the effective masses larger, and the same evaluation leads to 0.07 eV in zinc selcnidc, for example many of the impurity states can be occupied at room temperature. As the energy of the impurity states becomes deeper, the effective Bohr radius becomes smaller and the use of the effective mass approximation becomes suspect the error leads to an underestimation of the binding energy. Thus, in semiconductors of greatest polarity- and certainly in ionic crystals— impurity states can become very important and arc then best understood in atomic terms. We will return to this topic in Chapter 14, in the discussion of ionic crystals. 

Since its formulation, solid state theory has been concerned also with non-strictly-periodic systems, due principally to the theoretical and technological importance of defects (point impurities, color centers, dislocations, surfaces, etc.). However, most of these theoretical studies and approaches exploit the results of the ideal periodic crystal as the basic ingredient on which to include impurity effects. 

With its determination carried out at constant ionic strength the pH, value, so obtained, provides an assessment of the acidity of the amphoteric ion exchanger. This pH, relatable to the average value of pKj and pK2 with Eq. (16), is very close to the value of the isoelectric point 0 )- Paries [105] has pointed out that the relationship between the lEP of a solid surface and the valency-effective ionic radii, when corrected for crystal field effects, coordination, hydration, and other factors, is quite good. He also has indicated that the broad probable lEP range characteristic of a cation oxidation state may be selected from the data in Table 2 as shown below. It is known that the lEP for amphoteric oxides is affected by the presence of impurities, crystallinity and the chemical species under investigation. 

Williams-Seton, L. Davey, R.J. Liebermann, H.F. Pritchard, R.G. Disorder and twinning in molecular crystals impurity-induced effects in adipic acid. J. Pharm. Sci. 2000, 89 (3), 346-354. 

Direct measurements of A H of Sr(cr) by Mah (8) gave -144.44 0.4 kcal mol". The negative bias of about 3 kcal mol" presumably resulted from inadequate allowance for side reactions, e.g., with combustion products of Mylar used to contain the Sr. Parker (2 ) noted that the combustion value is incompatible with data for SrCl2(cr) and related compounds. We find that the combustion value is also less consistent with equilibrium data for SrCl2(t and g). There is a similar, but much larger, discrepancy for Ba compunds (cf, BaO, crystal). Although impurity effects are of concern in all studies, the evidence predominantly favors the solution calorimetry. 

Jang, S.M., Solvent and Impurity Effects in Crystallization Process, Polytechnic University, New York, New York, 1996. 

Kaneko, N., T. Horie, S. Ueno, J. Yano, T. Katsuragi, and K. Sato, Impurity Effects on Crystallization Rates of n-Hexadecane in Oil-in-Water Emulsions, J. Crystal Growth 197 263-270 (1999). 

It is also worthwhile to note that it is also possible to establish a close relation between the crystal field effects, covalent effects (overlap between the wave functions of an impurity ion and ligands) and electron-phonon interaction and JT effects [52-54]. It was shown in these works that it is possible to distinguish and analyze separately different contributions (arising from the point charge and exchange interactions) to the vibronic effects. 

Solvent and impurity effects must also be considered. Solvent effects are important and may play a key role in inclusions and in affecting the width of the metastable zone, as discussed in Example 11-1. However, variations in impurity composition can have more influence and can dominate the course of crystallization in many ways. 

Increasing solubility because of increased concentration of impurities will result in a similar equilibrium change, although in some cases, the effect could be much greater. In extreme cases, when the residual solvent concentration is reduced to less than a critical value, the substrate could melt or solidify, depending on the melting point and the impurity effect. This condition is often used in laboratory preparations for convenience in changing solvents and is referred to as concentration to dryness. It is obviously not a scalable operation in a stirred vessel. Specialized tubular evaporators with close-clearance or scraped-surface rotors are available for these applications and have been successfully used by the authors for concentration but not for simultaneous crystallization. 

We have emphasized that minor chemical modifications stabilize different structural minima in Fig. 21. The MBP-TCNQ complexes in Section 3.4 may illustrate, if supported by structure determinations, different structures for the same complex, presumably due to different preparative conditions. That one modification is metastable hardly matters in the solid state where there is no interconversion between different minima in Fig. 21. The precise control of crystallization conditions is obviously crucial. The impurity effects leading to MP-TCNQ or HMP-TCNQ, or to different MjP-TCNE adducts in Section 4.4, represent different structures for different complexes. We are not aware of definitive evidence for different structures of the scone complex, but suspect such behavior to be possible among phenazine complexes. 

Harano, Y., and Yamamoto, H. (1982). Impurity Effect of Amino Acids on Formation and Growth of L-Glutamic Acid , Industrial Crystallization, vol. 81 (Jancic, S.J., and de Jong, E.J., eds.), pp. 137-145, North Holland Publ. Co. 

The expectation of obvious positive effects from the development of all-silicon microphotonics increases the activity in the search for the ways of manufacturing of silicon lasers and light amplifiers. Among these approaches it is the most commonly encountered a use of specific properties of low-size silicon crystals, impurity rare-earth ions, and the Raman effect [1-3]. In the present paper, we consider (and propose for development) the promising, but insufficiently studied approach to the creation of silicon lasers. It is based on emission features of specific structural nanoscale imperfections of silicon crystals which can be called emissive structural defects (ESDs). No mention has been made of this approach in reviews [1,2] devoted to the problem. 

The combined influence of supersaturation and impurity concentration on crystal growth can be quite complex, but two basic cases may be considered (Kubota, Yokota and Mullin, 2000) (i) growth is only suppressed in the low range of supersaturation while at higher supersaturations the impurity effect disappears completely and (ii) growth rate suppression occurs throughout a very wide range of supersaturation. The first case may be explained by... 

Boistelle, R. (1982) Impurity effects in crystal growth from solution. In Interfacial Aspects of Phase Transformations, B. Mutaftschiev (ed.), 621 38, Reidel, Dordrecht. 

Eidelmann, N., Azoury, R. and Sarig, S. (1986) Reversal of trends in impurity effects on crystallization parameters. Journal of Crystal Growth, 74, 1-9. 

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