Crystal form types

Ultimately the various crystal form types give rise to solids that can exhibit markedly different performance properties , as evident by changes in solubility and dissolution or differences in chemical and physical stability. The solubility, dissolution rate and overall bioavailability of a drag will be greatly influenced by the crystal packing adopted and the surface chemistry this exposes. The crystal form can also affect the hygroscopicity... 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Other factors also impact the type of crystals formed upon cooling of hot soap. Water activity or moisture content contribute to the final crystal state as a result of the different phases containing different levels of hydration. Any additive that changes the water activity changes the crystallization pathway. For example, the addition of salt reduces the water activity of the mixture and pushes the equiUbrium state toward the lower moisture crystal stmcture. Additionally, the replacement of sodium with other counter cations influences the crystallization. For example, the replacement of sodium with potassium drives toward the formation of 5-phase. 

As with chemical etches, developing optimum conversion coatings requires assessment of the microstructure of the steel. Correlations have been found between the microstructure of the substrate material and the nature of the phosphate films formed. Aloru et al. demonstrated that the type of phosphate crystal formed varies with the orientation of the underlying steel crystal lattice [154]. Fig. 32 illustrates the different phosphate crystal morphologies that formed on two heat-treated surfaces. The fine flake structure formed on the tempered martensite surface promotes adhesion more effectively than the knobby protrusions formed on the cold-rolled steel. 

The other type of porous glass that has cylindrical pores is mesoporous silicate (MPS) (14,15). The advantage of MPS is in its feasibility to make a small pore diameter, typically below 10 nm. A columnar-phase liquid crystal, formed from surfactant molecules with a long alkyl chain tail and silicate molecules, is calcined to remove hydrocarbons. At the end, a hexagonal array of straight and uniform cylindrical holes is created in a crystalline order. MPS is not available commercially either. 

S Tantalum and niobium are present in the crystal structure in the form of complex ions. The lowest coordination number, 6, corresponds to the formation of slightly distorted octahedrons. The linking and packaging of the octahedrons depends on the X Me ratio, where X is the total number of oxygen and fluorine atoms, and Me is the total number of tantalum or niobium ions as well as other metals that can replace tantalum or niobium in the octahedral polyhedron. The crystal structure type can be defined based on the X Me ratio, as follows ... 

Thermal properties and decomposition mechanisms depend on the crystal structure type. Compounds with a crystal structure that includes shared octahedrons decompose forming tantalum- or niobium-containing gaseous components, while island-type compounds release light atoms and molecules into the gaseous phase. 

For the diolefin crystals, including unsymmetrical diolefin crystals, each packing of the a- and j8-types is further classified into translation- and centrosymmetry-type packings. Of the photoproducts derived from unsymmetrical diolefins, the cyclobutane ring which has the same substituent on a ring is called a homo-adduct, and that which has different substituents is called a hetero-adduct. Corresponding to the molecular arrangements of these diolefin crystals, four types of photoproducts (a- and jS-types, and homo- and hetero-adducts) are expected to be formed based on the topochemical principle, as shown in Scheme 2. 

An alloy is said to be of Type II if neither the AC nor the BC component has the structure a as its stable crystal form at the temperature range T]. Instead, another phase (P) is stable at T, whereas the a-phase does exist in the phase diagram of the constituents at some different temperature range. It then appears that the alloy environment stabilizes the high-temperature phase of the constituent binary systems. Type II alloys exhibit a a P phase transition at some critical composition Xc, which generally depends on the preparation conditions and temperature. Correspondingly, the alloy properties (e.g., lattice constant, band gaps) often show a derivative discontinuity at Xc. 

Knowledge of polymorphic forms is of importance in preformulation because suspension systems should never be made with a metastable form (i.e., a form other than the stable crystal form). Conversely, a metastable form is more soluble than a stable modification, and this can be of advantage in dissolution [Eq. (9)]. There are two types of polymorphism, a fact illustrated in the following discussion. 

The structure of the parent compound 43 of the 1,1 -binaphthyl host family has been determined many times in independent laboratories and in different (racemic and resolved) crystal forms 60,61,72,73). A common feature of both types of structure is that molecules of 43 form helices. Lateral contacts between such helices play an important role in the respective crystals. These spatial arrangements also emphasize the importance of the question concerning the presence of alpha or beta substituents positioning as mentioned earlier 28 ... 

Scheme 5 shows a group of alkynylgold(i) complexes for which the studies focused on the UV-VIS electronic absorption and emission properties. Most of these compounds are of the type [(L)AuC=CR], for which the methods of synthesis have been summarized above. The products were found to show phosphorescence in various polymorphs and crystal forms of solvates. Although there are no metallophilic interactions discernible in the crystal between most of the monomers due to the steric effect of the large tertiary phosphines, there is nevertheless strong excitonic coupling based on other weak interactions, which depend on the organization of the molecules in the crystal.105,106... 

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

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