Organic Crystals, Germanates, Semiconductors by L. N. Demianets, G. A. Emelchenko, J. Hesse, N. Karl, A. N.

By L. N. Demianets, G. A. Emelchenko, J. Hesse, N. Karl, A. N. Lobachev, H. Maier (auth.)

1.1 Preface natural chemistry had its starting place in chemical compounds that are synthesized by way of residing cells. those chemical substances encompass molecules whose skeletons are equipped up of carbon atoms. the rest valences are attached with ligands corresponding to hydrogen, halo­ gens, -OH,==O, -NH . the various skeletal carbon atoms will be changed via non­ 2 metals corresponding to oxygen, nitrogen, or sulfur {"heteroatoms"}. it truly is attribute for the residing international, to not be in a crystalline country. but it is feasible to acquire unmarried crystals from many natural compounds either one of normal and artificial foundation. For a few years the physics and chem­ istry of those crystals have prompted basic learn on a quickly starting to be scale. the nice number of attainable natural buildings {as in comparison to inorganics} opens up a wide box of latest fabrics and of novel fabric houses; for earlier literature experiences and information compilations see 1-40) and Chap. 6. The artwork of manufacturing reliable and natural natural unmarried crystals has built hand­ in-hand with the ever transforming into standards of simple learn, bobbing up from its curiosity in basic interactions within the sturdy country. Interactions show up themselves in a truly distinctive method by means of strength transfer.

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The product was found to be essentially free of those impurities which are present in "natural" fluorene. The over-all reaction yield is high: triplet exciton lifetime (cf. 6) reached a value of 120 ms after zone refining 134). 3 Practical Results of Combined Purification Methods The highest over-all purity of any organic material seems to have been obtained for naphthalene. Persistent impurities such as /3-methylnaphthalene and thionaphthene can be removed w€(ll below 10- 7 mol/mol. 01 ppm). Therefore in what follows, ultrapurification will be exemplified by a prescription which has been succe,ssfully used for naphthalene 120).

The main condition, however, is that S is not incorporated into the crystal lattice of the compound, A, to be purified. A suitable mixture A(B)S is transferred to the liquid state by heating. Subsequent cooling leads to the formation of crystals, usually of the component A. The first fractions have usually a higher purity than the initially added material. However, the starting point in the phase diagram could also be selected in such a way 42 High Purity Organic Molecular Crystals that the added component (S) crystallizes first, incorporating and thus withdrawing the impurity from the melt.

1 General Aspects One of the most successful methods for the growth of high purity crystals is that devised by Bridgman (l925)155). Although first used for metals, the method has turned out to be suitable for the growth of organic crystals such as naphthalene and anthracene as well, see for instance 156 ). Bridgman growth is essentially normal freezing i. e. slow unidirectional cooling of the molten material, as discussed in Sect. 1 (cf. Fig. 7a), except for the provision of seed selection in order to obtain a single crystal.

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