Cubic from hexagonal

Fig. 1 (a) Phase diagram for the 51% CjeEv-DjO system, Hj, V, and L designate hexagonal, cubic and lamellar phases, respectively. The corresponding SANS patterns are shown in (b) hexagonal, (c) cubic from hexagonal, (d) lamellar and (e) cubic from lamellar phases... 

Cubic boron nitride is obtained from hexagonal boron nitride at high pressure and temperature in the presence of lithium nitride as a catalyst. It is almost as hard as diamond and has superior chemical resistance and a much higher oxidation threshold.Efforts to... 

Doping is important for semiconductors in order to tune their optical and electrical properties for the potential applications in biotechnology and solar cells [65]. Ag-doped hexagonal CdS nanoparticles were successfully obtained by an ultrasound-assisted microwave synthesis method. Here, the doping of Ag in to CdS nanoparticles induced the evolution of crystal structure from cubic to hexagonal. Further support from photocatalytic experiment also clearly indicates the doping of Ag clusters into the CdS matrix. 

A few examples of the moduli of systems with simple symmetry will be discussed. Figure 1A illustrates one type of anisotropic system, known as uniaxial orthotropic. The lines in the figure could represent oriented segments of polymer chains, or they could be fibers in a composite material. This uniaxially oriented system has five independent elastic moduli if the lines (or fibers) ara randomly spaced when viewed from the end. Uniaxial systems have six moduli if the ends of the fibers arc packed in a pattern such as cubic or hexagonal packing. The five engineering moduli are il-... 

The same atom-centered polyhedra can be used to describe interstitial diffusion in all the many metal structures derived from both face-centered cubic and hexagonal closest packing of atoms. In these cases the polyhedra are centered upon a metal atom and all the tetrahedral and octahedral interstitial sites are empty. The hardening of metals by incorporation of nitrogen or carbon into the surface layers of the material via interstitial diffusion will use these pathways. 

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

Figure 16.2. Unit cells of the three most important lattice types (a) face-centered cubic (b) hexagonal close-packed (c) body-centered cubic. (From Ref 1, with permission from Noyes.)...

Figure 1.6 The three most common polytypes in SiC viewed in the [1120] plane. From left to right, 4H-SiC, 6H-SiC, and 3C-SiC k and h denote crystal symmetry points that are cubic and hexagonal, respectively.

GaP has a much larger mismatch with CdS (-7%) compared with InP. Yet a fair degree of epitaxy was obtained for CD CdS on the (111) GaP surface [58]. In this case, a mixture of cubic and hexagonal CdS with a large density of stacking faults, presumed due to strain relaxation arising from the large mismatch, was obtained. 

Fig. 1.1 The three commonest elemental structure types (a) face-centred cubic, (b) hexagonal close-packed, and (c) body-centred cubic. From Wells (1986).

This minimum is responsible for the diamond and graphite lattices with = 109° and 120° respectively having the smallest and second smallest values of the normalized fourth moment, and hence the shape parameter, s, in Fig. 8.7. This is reflected in the bimodal behaviour of their densities of states in Fig. 8.4 with a gap opening up for the case of the diamond cubic or hexagonal lattices. Hence, the diamond structure will be the most stable structure for half-full bands because it displays the most bimodal behaviour, whereas the dimer will be the most stable structure for nearly-full bands because it has the largest s value and hence the most unimodal behaviour of all the sp-valent lattices in Fig, 8.7, We expected to stabilize the graphitic structure as we move outwards from the half-full occupancy because this... 

The information obtainable from the Laue symmetry is meagre it consists simply in the distinction, between crystal classes, and then only in the more symmetrical systems—cubic, tetragonal, hexagonal, and trigonal (see Table VI). But it is useful in cases in which morphological features do not give clear evidence on this point. 

Crystallographic data for CdSexTe x, from X-ray powder diffraction. Stoichiometries from relative amounts of starting materials, and from Vegard s law. Percentages of cubic and hexagonal structures estimated from powder diffraction intensities. Lattice dimensions in parentheses are literature data. 

Fig. 4.20 Phase diagram for aqueous solutions of PBO7PEO40PBQ7 (Booth et al. 1997 Yang 1996). The dashed line separates clear solutions and gels from cloudy ones. The boundary enclosing the cubic (C), hexagonal (H), and lamellar (L) phases is indicated. An approximate region of gel extending to low concentrations is denoted S.

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