Phase clusters, crystallization

Most of the chalcogenide studies have involved binary, ternary, and pseudo ternary metal clusters. The ternary Chevrel-phases can be represented by MxMo6X8 (M = transition metal, X = chalcogen atom), where the transition metal fills the cavities in the Mo6X8 crystal structure. The pseudo-ternary (or pseudo-binary) Chevrel-phase clusters can be represented by MxMo6 xX8, where the Mo atoms are substituted by transition metal atoms.185 The crystal structure of ternary and pseudo ternary Chevrel phases are shown in Fig. 4. The carbonyl-chalcogenide clusters189 192 will be discussed in the next section. 

In Ref. 8, crystals ca. 5 mn in size were deposited from a nitrilotriacetate (NTA)-complexed bath (no ammonia) at 40°C (a lower temperature than most CdS depositions). The composition of the bath was such that Cd(OH)2 was present as a colloidal phase (cluster mechanism-see Chap. 3). Under conditions where no hydroxide phase was present and the reaction proceeded via an ion-byion mechanism, much larger crystals (>70 mn) and a red-shifted spectrum were found. See Section 10.2.2 for more detail on the dependence of crystal size on the deposition mechanism. 

In another DFT study devoted to the reaction mechanism of FeIV=0-catalyzed hydroxylation of L-Phe, the gas-phase cluster model was based on the X-ray crystal structure of the catalytic domain of hPAH containing ferric iron (hPAH-Fe111) (PDBid 1PAH (97)) (119). The water molecule distal to His290 was replaced by the oxo-ligand and only ligands of the first coordination sphere were included in the model. The amino acid residues were truncated in the model to include the side chains only. This model was also used as the QM part in a QM/MM study of the hydroxylation of L-Phe. In the latter study, the MM part included the complete catalytic domain (119). 

Our recent DFT investigations of the AAH catalytic cycle (121-123) will be the main focus of the discussion to come. The gas-phase cluster model that constitutes the starting point for these studies is based on the X-ray crystal structure of the catalytic domain of hPAH in binary complex with the cofactor (hPAH-Fe11-BH4) (PDBid 1J8U) (98). The active site iron and the six ligands,... 

A bivariant linear eq uation, contain ing factors such as phase percentages, crystal sizes, morphology and cluster indexes, for predicting the 28-day mortar strength was presented by Sinha, Rao, and Akhouri (1991). The standard deviation was said to be 17.9 kg/ cm agreeing closely with the actual strength. 

Surface science Surface structure, adhesion of materials, phase structures, crystal structures, catalytic processes, clusters... 

Allegra (Allegra 1977,1980 Allegra and Meille 1999,2005) proposed statistical thermodynamic theory for the mesophase of small crystallites or crystal cluster in the metastable disordered phase before crystallization. The cluster will first grow into a stable size and then joins into the crystal, as illustrated in Fig. 4.27. The thickness of lamella is decided by the cluster size. Zhang and Muthukumar (2007) performed simulations of clusters to form single crystals grown in dilute solutions, consistent with the experimental observations. 

More generally, one may wish to ensure that the choice of the soft, intersegment potential, v, results in a liquid state. The phase diagram of fluids of soft particles has been explored and exhibits quite a rich behavior [106-108]. In addition to the liquid and crystal phases that occure in fluids with harsh repulsions exhibit at low temperatures, these fluids exhibit either re-entrant melting or formation of cluster crystals at high densities. The latter phases are thermodynamically stable crystal phases that feature a lattice constant that is independent of density and, in turn, the occupancy of a lattice site increases linearly with In the present context, a coarse-... 

Mladek et al., 2007] B. M. Mladek, P. Charbonneau, and D. FVenkel. Phase coexistence of cluster crystals Beyond the Gibbs phase rule. Physical Review Letters 99 235702, 2007. [Needham, 2002] P. Needham. The discovery that water is H2O. International Studies in the... 

One way to model a solid is to use software designed for gas-phase molecular computations. A large enough piece of the solid can be modeled so that the region in the center for practical purposes describes the region at the center of an inhnite crystal. This is called a cluster calculation. 

The growth of crystals—or more generally the solidification of a sohd from a fluid phase—is definitely not an equilibrium problem. Why, therefore, should we discuss here equihbrium thermodynamics, instead of treating directly, for example the coagulation of two atoms and then simply following the growth of the cluster by adding more particles with time ... 

At this point a third intermediate approach deserves mentioning. It is due to Allegra [43] who proposed that polymer crystallization is controlled by a metastable equilibrium distribution of intramolecular clusters, the so-called bundles , forming in the liquid phase. These subsequently aggregate to the side surfaces of the crystals, driven by van der Waals interactions. The lamellar thickness is determined by the average contour length of the loops within the bundles. Although the model can... 

Sodium carboxymethyl chitin and phosphoryl chitin had most evident influences on the crystallization of calcium phosphate from supersaturated solutions. They potently inhibited the growth of hydroxyapatite and retarded the rate of spontaneous calcium phosphate precipitation. These chitin derivatives were incorporated into the precipitate and influenced both the phase and morphology of the calcium phosphate formed (flaky precipitate resembling octacalcium phosphate instead of spherical clusters in the absence of polysaccharide) [175]. 

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