Pseudomorphic product crystals

Quite recently, Nakanishi et al. have reported an example of crystalline-state dimerization for which the product matrix is essentially of single-crystal character63. On the other hand, it may be assumed that any solid-state polymerization of diolefinic crystals, which results in an amorphous product, gives a pseudomorph. 

Single crystals of MoOj.HjO ("a-molybdic acid") were transformed [130] on heating at 433 K into perfect pseudomorphs composed of MoOj crystalhtes. This reactant to product transition is explained by a topotactic reaction in which the M0O3 structure was formed by comer linking of isolated double chains after water... 

From a photomicrographic study, Figlarz and Vincent [41] concluded that the dehydration of Co(OH)2 is initiated at randomly-distributed sites on the surfaces of the approximately hexagonal crystals of reactant. Following the topotactic advance of the reaction interface, alignment of the (111) fece of CoO with (0001) of Co(OH)2 preserves the external form of the reactant particles to yield pseudomorphic particles of product. Appreciable quantitites of water are retained in the residual material. 

Though salt dehydration was not accompanied [27] by particle disintegration, the anhydrous pseudomorph was shown by X-ray diffiaction measurements to be very poorly crystallized (a characteristic feature of many nickel carboxylates). Decomposition in air (554 to 631 K) proceeded at a constant rate (0.1 < nr < 0.8 and = 96 kJ mol" ), ascribed to the operation of an autocatalytic mechanism. The reaction in vacuum (562 to 610 K) gave a sigmoid ar-time curve which was fitted by the Prout-Tompkins equation. Because the activation energy was the same as that for reaction in air, it was concluded that the same mechanism operated. The reaction in air yielded residual nickel oxide, while reaction in vacuum gave the carbide with excess carbon and some oxide. In addition to carbon dioxide, the volatile products of decomposition included water and acetic acid. 

Ultramicroscopic and x-ray studies show that oxides obtained from carbonates and oxalates at low decomposition temperatures are pseudomorphous with the crystals of the starting material [10, 12]. Since the molar volume of the new substance is as a rule considerably smaller than that of the starting material, the particles of the product are usually very porous, that is, provided the reduced decomposition temperature T/T , < 1/3 (compare p. 1611). If the decomposition temperature is higher, aggregate crystallization can be expected to an increasing degree [19], except when the starting substance (which must in this case be a uniform fine powder) is heated for such a short time that only the desired... 

The structure of a Hofmann type inclusion compounds is shown in scheme (C). The reaction of (HI) and (IV) with oxidants such as Fe + in water results in a color change from pale violet to black. When single crystals are used as starting materials, the resulting products, retain the original crystal shape (pseudomorphs), but they lose their single crystal nature and under SEM microscopic examination show small cracks and other defects. The proper formulation of the oxidation products would be [Ni(CN)2NH3] (polymer)(A)x y where polymer=polypyrrole, polyaniline ... 

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