Polycrystalline state

Obviously, the major difference in the single-crystal and polycrystalline (crystallite) state is a matter of size. For the single-crysted, the size is leu ge (> 10 cm), wherecis in the polycrystalline state, the size of the cr3rstals is small (10 (im = 0.001 cm.) The methods for obfriining one or the other differ considerably. They include formation from ... 

The problems of obtaining a seed-crystal 2ire not simple. We can freeze the melt to a polycrystalline state. When cool, we examine the boule (after first removing the crucible) to try to find a single crystal Icurge enough for a seed ( 3-6 mm.). We could also cast the melt into a mold and then look for seeds. We could also freeze a polycrystalline rod by pulling it... 

Traps and recombination centers which depend on purity, crystal defects and preparation, can exert an influence, and electrode contacts, carrier injections, and other factors can interfere with measurements. Yet there is no doubt that the photoconductive gain (quantum yield) G can be reproduced by different methods. As in the case of dark conductivity, the photoconductivity properties are related to the electronic and structural behavior of pure and doped organic compounds, also those in the polycrystalline state. 

Mossoba MM, Rosenthal I, Riesz P (1981) ESR and spin-trapping studies of dihydropyrimidines, y-Radiolysis in the polycrystalline state and UV photolysis in aqueous solution. Int J Radiat Biol 40 541-552... 

The extensively studied (especially during the recent years) transitions of solids from the metastable amorphous state to the polycrystalline state (see ref. 58 and the references therein) are of autowave character and resemble very much the above regimes of solid-state cryochemical reactions. The action of autodispersion, which facilitates phase transition by allowing it to proceed on the surface of a fracture instead of in the glass volume, cannot be excluded in the case of those processes either. Actually, the two classes of processes are similar in their physical nature both are connected with rearrangement of the solid matrix and are of exothermic character, differing only in the extent of the thermal effect. It should be added that fracturing and autodispersion of the sample are very typical of the autowave destruction of amorphous states and can be seen even by the unaided eye. 

The ground-state vibrational normal modes of thymine have been extensively studied, both experimentally and computationally. Vibrational spectra of thymine in the polycrystalline state [96-104], in Ar and N2 matrices [105-109], and in the gas phase [110] have been measured. In the least interactive environments, only the 1 -d, 3-d, and 1,3-d2 derivatives have been measured, while a number of 2H and 15N isotopomers in the polycrystalline state have been measured for thymine [104], Semi-empirical [111,112] and ab initio [98,113-115] calculations have been used to assign the vibrational bands for natural abundance thymine. However, the most robust reconciliation of experiment and computation is a recent attempt to computationally reproduce the experimentally observed isotopic shifts in 10 different isotopomers [116] of thymine. The success of that attempt is an indication of the reliability of the resulting force field and normal modes. The resonance Raman vibrations of thymine, and their vibrational assignments, are given in Table 9-1. 

As mentioned above, some arylalkenes, such as stilbene, form complexes with Ag+. Such complexes are also formed between Ag+, as added silver perchlorate, and simpler alkenes. Typical of this is the interaction with 1-methylenecyclohexane when a crystalline complex is formed. Irradiation of this complex in the polycrystalline state or in solution in methanol affords isomerization to 1-methylcyclohexene by a 1,3-hydrogen migration path. Further irradiation brings about the stereospecific formation of the exo,trans,exo-dimer (24) of 1-methylcyclohexene. Less specific photodimerization is also reported for the irradiation of the /S-pinene (25)/silver perchlorate complex69 The mechanism was thought to involve a silver/cyclohexenyl radical similar in type to that observed in the y-radiolysis of silver/cycloalkene complexes70,71. 

All the above dimers are readily formed from the respective monomer complexes, e.g., TcO(EG) (TCTA) (where EG is ethylglicolate). When NaBH4 in aqueous solution is added the dimeric TCTA-complex of Tc(IV) is formed. The Tc(III/IV) compounds are blue (Amax 600 nm), while the Tc(IV/IV) compounds are pink (2max 500 nm) [54-57]. All the complexes are rather stable in aqueous and alcohol solutions. The Tc(III/IV) may be oxidized electrochemically, to the respective dimers Tc(IV/IV) or by the action of K2S208. The reverse reduction easily occurs in the presence of hydrazine. The Tc(III/IV) compounds are paramagnetic and in the polycrystalline state at — 100 °C give a broad ESR signal with a weak hyperfine structure, composed of 18 bands, which are due to two equivalent technetium atoms with a nuclear spin of 9/2. 

Table 2.3 contains an overview of the elastic constants for some metals and ceramics. As can be seen, the anisotropy factor of tungsten is 1.0, so it is (almost) isotropic even as a single crystal. For most other materials, almost isotropic properties can only be found in a polycrystalline state. The direction dependence of Young s modulus for selected materials is plotted in figure 2.10. 

Imperfect crystals are close to the asymmetric stage of matter but being variant in the characteristics of symmetrical state, they demonstrate some important properties that an ideally perfect symmetrical state fail to give. Therefore, attention is then shifted from single crystal state to polycrystalline state and some of their characteristic properties. 

Figures 31 and 32 indicate the SEM and TEM images of CdS and ZnS NPs, respectively [149], The shape is spherical, and the diameter is about 450nm and almost mono-dispersed in both cases. From SEM images, the surface of CdS NPs is a little rough, while ZnS NPs have a smooth surface. In addition, the ED patterns suggest polycrystalline state in any case of CdS and ZnS NPs.

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