Cubic phases

A progressive etching technique (39,40), combined with x-ray diffraction analysis, revealed the presence of a number of a polytypes within a single crystal of sihcon carbide. Work using lattice imaging techniques via transmission electron microscopy has shown that a-siUcon carbide formed by transformation from the P-phase (cubic) can consist of a number of the a polytypes in a syntactic array (41). 

RfLl phases The existence of these phases (cubic AuCu3 type) had been reported for R = La, Ce, Pr, Nd and Sm. Subsequently, however, Buschow and van Vucht (1967) found that many of the R3A1 phases do not form, unless some carbon is present. C atoms occupy the body-centred site, which in the AuCu3 type structure is normally vacant, while the Au atoms occupy the corners, and the Cu atoms the face-centred positions. When the face-centred position is completely filled, the structure is known as the anti-perovskite structure. This occurs for R = Nd, Sm, Gd, Tb, Dy Ho and Er. They also noted that neither N nor O would stabilize these compounds. Notice that Ce3Al and Pr3Al are truly binary compounds and C is not required for these two phases to form. Independently, Nowotny (1968) found that the anti-perovskite structure could be formed by C and N additions to R3M alloys, where M = Al, Ga, In, Tl, Sn and Pb. 

Zirconia exists in three solid phases-cubic, tetragonal and monoclinic. It undergoes the following transformations... 

Light microscopy has been used in a number of contexts to characterize block copolymer morphology. For crystalline block copolymers, spherulitic structures that result from organization of crystalline lamellae can be examined using microscopy. In solutions, polarized light microscopy can reveal the presence of lamellar and hexagonal-packed cylindrical micellar phases. Cubic micellar phases are optically isotropic, and consequently cannot be distinguished from sols only on the basis of microscopy. 

In summary, information about the interactions between layers allowed one to identify the domains where the lamellar phase is unstable. A positive derivative off with respect to di or d2 implies that the region is inaccessible to a lamellar phase. In this case, a water or oil phase will separate until the allowed values for dj and 82 will be reached. In addition, a negative surface tension indicates that another phase (cubic,hexagonal, microemulsion, etc.) is stable. It should be, however, emphasized that when y > 0, a phase other than the lamellar one may be the thermodynamically stable one. 

Litvinov D., Clarke R., Reduced bias growth of pure-phase cubic boron nitride, Appl. Phys. Lett., 71 (1997) pp. 1969-1971. 

In addition, anode crystallographic information after PEVD was obtained with XRD. The XRD spectrum of the PEVD composite anode is presented in Eigure 33. Three phases exist in the XRD spectrum a yttria stabilized zirconia phase (cubic), a pure zirconia phase (monoclinic) and a metallic Pt phase. The Pt phase shows up in the spectrum because the PEVD product on top of the Pt is thin enough to allow x-rays to penetrate the product phase to reach the underlying Pt phase. Based on the relative peak intensity, yttria stabilized zirconia is the major phase in the PEVD product. However, a certain amount of pure zirconia is evident. Thus, the zirconia in the PEVD product... 

In addition to the cubic and/or inverse cubic forms described previously, further transitional forms exist between the lamellar phase and the hexagonal meso-phase (cubic, type II) or inverse hexagonal mesophase (cubic, type III). In contrast to the discontinuous phases of types I and IV, cubic mesophases of type II and type III belong to the bicontinuous phases (Fig. 4F). A range of lyotropic mesophases are possible, depending on the mesogen concentration, the lipophilic or hydrophilic characteristics of the solvent... 

A central issue in the field of surfactant self-assembly is the structure of the liquid crystalline mesophases denoted bicontinuous cubic, and "intermediate" phases (i.e. rhombohedral, monoclinic and tetragonal phases). Cubic phases were detected by Luzzati et al. and Fontell in the 1960 s, although they were believed to be rare in comparison with the classical lamellar, hexagonal and micellar mesophases. It is now clear that these phases are ubiquitous in surfactant and Upid systems. Further, a number of cubic phases can occur within the same system, as the temperature or concentration is varied. Luzzati s group also discovered a number of crystalline mesophases in soaps and lipids, of tetragonal and rhombohedral symmetries (the so-called "T" and "R" phases). More recently, Tiddy et al. have detected systematic replacement of cubic mesophases by "intermediate" T and R phases as the surfactant architecture is varied [22-24]. The most detailed mesophase study to date has revealed the presence of monoclinic. 

Use of liquid crystalline phases Surfactants produce liquid crystalline phases at high concentrations. Three main types of Hquid crystals can be identified hexagonal phase (sometimes referred to as middle phase) cubic phase and lamellar (neat phase). All of these structures are highly viscous and also show elastic responses. If produced in the continuous phase of suspensions, they can eliminate sedimentation of the particles. These Hquid crystalline phase are particularly useful for application in liquid detergents which contain high surfactant concentrations. Their presence reduces sedimentation of the coarse builder particles (phosphates and silicates). 

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. 

Since transition through an optically isotropic phase, cubic, is excluded by x-ray data (12), the 57°C. extinction of rotation indicates the formation of a phase with disorder normally associated with a melt. This indicates that the chain separation and dislocation during the transition from the perpendicularly oriented q l hexagonal structure to the obliquely oriented 13l triclinic structure, Figure 2, are larger than has previously been described or implied. 

Farneth et al. have investigated the mechanism of the solid-state conversion of a series of II-VI precursors of general formula (R4N )4[S4Mio(SPh)i6]" (R = Me, Et M = Gd, Zn) to the bulk metal sulphide structure. The transformation, as followed by combined TGA and mass spectroscopy, proceeds in two discrete reaction steps. In the case of cadmium derivative, the loss of countercations around 200 °G produces a new molecular solid, which was characterized (X-ray) to be GdioSi6Phi2. This intermediate composition gave a broad X-ray diffraction pattern that indicated very small (<25 A) sphalerite-phase (cubic) crystals of GdS. The second decomposition reaction eliminates S6Phi2 around 350 °G and produces phase-pure GdS (wurtzite) (Equation (5)). 

The UC2 phase is stable above about 1500°C and undergoes phase transition from a (tetragonal CaCj type) to p phase (cubic KCN type) at 1765°C (92). The C/U ratio of the UCj phase never reaches 2.00, the ratio being usually near 1.90 in coexistence with graphite (93). There exists a miscibility gap between UC. and UC2 below 2050°C, while above this temperature a complete solid solution is formed, as described above. 

Figure 29. Phase diagram of SDS/water (reproduced from [62]). (H, hexagonal phase two-dimensional monoclinic phase rhombo-hedral phase cubic phase tetragonal phase C, C and C" refer to different polymorphic varieties for the same SDS hydrate otherwise as for Fig. 14.)...

If we turn to the isotropic phases, several possibilities exist with respect to both liquid solution phases and cubic liquid crystalline phases. Cubic phases, which have long-range order, can be distinguished from microemulsions in giving rise to low-angle X-ray diffraction patterns. A number of different cubic phase structures are possible, but the X-ray low-angle... 

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