Hexagonal phases

Zr fAl3, hP7, structural type (a tetrahedrally close-packed phase) Hexagonal, space group P6, N. 174. 

Zr-based AB2 Laves phase alloys consist of cast polycrystalline and nanocrystalline structures. The nanocrystalline microstructure could be obtained for quenching the melt-spun alloys after annealing151. Figure 1 shows SEM Micrographs of the cast Polycrystalline ABi Alloys. Microstructures of AB2-1 alloys consist of cubic C15 Laves phase, hexagonal C14 Laves phase and of AB2-4 is only Cl5. The white one of non Laves in AB2-I and AB2-4 is the phase Zr9Nil 1 and Zr7NilO respectively. 

AB2 ZrV2, ZrMn2, TiMn2 ZrV2H5.5 Laves phase, hexagonal or cubic... 

ZIRCONIUM. [CAS 7440-67-7]. Chemical element symbol Zr. at. no. 40, at. wt. 91.22, periodic table group 4. mp l,853°C, bp 4,376°C, density 6.44 g/cm3, 6.47 g/cm3 (single crystal). Metallic zirconium is allotropic. Up to about 863°C, the alpha phase (hexagonal close-packed) is stable above this temperature, the metal assumes the beta phase (body-centered cubic). The most common impurity, oxygen, tends to stabilize the alpha phase. 

Refractive index data are very useful for the quantitation of isotropic (liquid and cubic liquid crystal) phases, and for the calibration of cell thickness and nonflatness. Hovever, the analysis of birefringent phases using refractive index data has been found to be unreliable (9). A problem arises from the fact that the orientation of such phases relative to the direction of the light path, as veil as the system variables, influence refractive indices. In order to use refractive index data for quantitation, a phase must spontaneously orient in a reproducible fashion. Such orientation does occur in the case of fluid lamellar phases (as in short chain polyoxyethylene nonionic systems (7)), but viscous lamellar phases, hexagonal phases, and crystal phases do not orient to a sufficient degree. 

For mesoporous silica (pore 0>2O A), successful results were obtained by the synthesis of the MCM family of silicates and aluminosilicates (of Section II.B). Their preparation is achieved by liquid crystal templating and micellar phases (hexagonal MCM-41, cubic MCM-48, laminar MCM-50)36,37 and further developments are currently reviewed38. 

When temperature is raised, the membrane bilayer not only becomes increasingly fluid due to enhanced motions of acyl chain, but it also tends to shift increasingly towards forming lipid aggregates in the inverted hexagonal phase (.hexagonal II, Hn, phase) (Hazel, 1995). The temperature at which this type of phase change... 

FIGURE 21.6 Ternary phase diagram of the sodium octanoate-decanol-water system at 25°C. There are two isotropic solution phases, micellar and reversed micellar (rev mic), and three liquid crystalline phases, hexagonal (hex), lamellar (lam), and reversed hexagonal (rev hex) (from Ref. 17). 

Figure 6. Pressure-temperature diagrams of n-alkanes and polyethylene. Shaded areas Indicate regions of stability of the meso-phase ("hexagonal" or "rotator"). The figures Indicate the number of carbon atoms In the chain (from ref. 6).

Self-assembly-based networks Ordered superlattices composed of nanosized semiconducting sulfides have been synthesized within lyotropic phases. Hexagonal-packed arrays of nanocrystalline CdS (or similar structures such as ZnS, Cdi cZn tS, and CdSe) have been produced, a mineral copy of an (ethylene oxide)lo-oleyl/water mesophase presenting periodicities ranging between 7 and 10 nm. 

With nonionic surfactants of the efhoxylate type, an increase in the temperature of a solution at a given concentration causes dehydration of the PEO chains and, at a critical temperature, the solution will become cloudy. This is illustrated in Figure 3.6, which shows the phase diagram of Cyj E. Below the CP curve it is possible to identify the different liquid crystalline phases hexagonal-cubic-lamellar, which are shown schematically in Figure 3.7. 

Silver iodide undergoes a first order structural phase transition at 420 K from the / -phase (hexagonal Wurtzite structure), which is metastable with respect to the / -phase (cubic sphalerite structure), to the a-phase where the I - ions occupy a bcc lattice within which the Ag+ ions jump rapidly between a number of possible sites. The ionic conductivity is very high upon melting it actually decreases. Agl is probably the most widely studied fast ion conductor, with much of the work concentrating on determination of the exact distribution of Ag+ sites and conduction pathways. 

Figure 2. SBS (S) TEM micrograph of spherical styrene domains (white phase) hexagonally packed in grains of about 1.5 x 0.5 im (48,000 x).

T lamellar-i- cubic phase hexagonal + cubic phase... 

It is clear ffran the data presented in this chapter that the trivalent lanthanide metals show the following structural sequence hep Sm-type dhep - fcc —> effee (hR24 phase—hexagonal with 24 atoms celt) with increasing pressure. The long standing debate oti the structure of the dfcc phase now appears to be settled with... 

There are a number of liquid crystalline phases formed by amphiphilic molecules, notably surfactants, polar lipids and block copolymers, including discrete and bicontinuous cubic phases, hexagonal phases (and their reversed counterparts), lamellar phases, intermediate phases, etc. A number of these phases are interesting from a drug delivery point of view. This is due to the frequently large solubilization capacity of both hydrophilic and hydrophobic substances, possibilities to control the drug release rate, favourable rheological properties, suitable water transport rates, excellent stability, etc. 

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