Variants, crystal transformation

Whenever a crystal transforms (usually on cooling) to a lower symmetry structure in which a number of orientational variants are possible, transformation twins are an almost invariable consequence if the new phase is nucleated at several different places in the original structure. The formation of Dauphine twins at the jS-to-a transformation in quartz is a classic example, which has already been mentioned briefly in Section 8.6. We now consider the twins and modulated structures associated with the monoclinic-to-triclinic transformation in the K feldspars, KAlSisOg. 

Transmission electron microscopes (TEM) with their variants (scanning transmission microscopes, analytical microscopes, high-resolution microscopes, high-voltage microscopes) are now crucial tools in the study of materials crystal defects of all kinds, radiation damage, ofif-stoichiometric compounds, features of atomic order, polyphase microstructures, stages in phase transformations, orientation relationships between phases, recrystallisation, local textures, compositions of phases... there is no end to the features that are today studied by TEM. Newbury and Williams (2000) have surveyed the place of the electron microscope as the materials characterisation tool of the millennium . 

To a certain extent, the formation of the tetrabasic variant is desired, because 4PbO PbS04 forms fairly large crystals when transformed into lead dioxide (PbO,). This results in a mechanically stable active material, but there are disadvantages, because it is more difficult to transform this material into lead dioxide, i.e., the formation process (see below) is more expensive (and takes longer) and the initial capacity is slightly reduced (cf., e.g., Ref. [19]. For "long-life batteries" (Bell systems cell), a special process has been developed to produce pure tetrabasic material [20]. 

A very selective variant of these methods has been proposed [52]. Mercury is extracted into benzene as a molecular species, Hgt or HgBr2, which is transformed into a coloured ion-associate by equilibration of the extract with an aqueous basic dye solution also containing iodide or bromide ions. The highest sensitivities have been obtained with Brilliant Green, Crystal Violet and Butylrhodamine B. 

To a certain extent, the formation of the tetrabasic variant is desired, because 4Pb0 PbS04 forms fairly large crystals when transformed into lead dioxide (Pb02). 

Renewed examination by TLC and electrophoresis of calebassinine, another curare alkaloid, but also known to be present in S. solimoesana (Table 1.4), has now revealed that it consists of two components, the major one calebassinine 1 together with a small proportion of calebassinine 2. X-ray crystal analysis of calebassinine 1 (436) has shown it to possess the quinoline structure 46a. The suggestion has been made that it could arise through a number of oxidative transformations from 2,16-dihydroakuammicine (47) a variant on the route originally proposed has been outlined (437). Isolation of the alkaloid from S. solimoesana (325) would thus imply the occurrence of an akuammicine derivative in the plant, which has not yet been demonstrated. 

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