Wurtzite hexagonal-phases

Boron nitride can also form a superhard hexagonal phase in wurtzite-type (w-BN). This modification is a high pressure phase and was described first by Bundy and Wentorf [19]. 

The structure of Agl varies at different temperatures and pressures. The stable form of Agl below 409 K, y-Agl, has the zinc blende (cubic ZnS) structure. On the other hand, /3-AgI, with the wurtzite (hexagonal ZnS) structure, is the stable form between 409 and 419 K. Above 419 K, ft-Agl undergoes a phase change to cubic a-Agl. Under high pressure, Agl adopts the NaCl structure. Below room temperature, y-Agl obtained from precipitation from an aqueous solution exhibits prominent covalent bond character, with a low electrical conductivity of about 3.4 x 10-4 ohm 1cm 1. When the temperature is raised, it undergoes a phase change to a-Agl, and the electrical conductivity increases ten-thousandfold to 1.3 ohm-1 cm-1. Compound a-Agl is the prototype of an important class of ionic conductors with Ag+ functioning as the carrier. 

The most common way of fabricating GalnN/GaN heterostructures is by means of heteroepitaxy of GaN and GalnN on sapphire or SiC substrates. In fact, this leads to wurtzite GaN, which is now the commonly used modification. Even though there is considerable effort being devoted to producing cubic GaN structures, we will restrict our discussion to the hexagonal phase, since practically all the work on heterostructures concentrates on the wurtzite structure. 

Binary III-VI alloys crystallize in a variety of forms such as defect wurtzite (Ga2E3), layered structures GaE, and a novel cubic form, whereas a, p, and y - Ga2Ss are reported to have a wurtzite, hexagonal, and monoclinic phase respectively. The two predominant binary stoichiometries are AB and A2B3,... 

Figure 14.1 shows each crystal stmcture. It has been reported that electroluminescent material must contain both sphalerite and wurtzite phases (Arterton et al.,1992). The wurtzite type stmcture predominates when the bonding is primarily ionic whereas the more covalent systems favor the sphalerite form. The cubic phase of ZnS is not grown as easily as the hexagonal phase, thus making the hexagonal phase more appealing for EL device applications (Bellotti et al., 1988). 

GaN has excellent properties such as a direct and wide band-gap energy of 3.4 eV at room temperature (RT). It also has high electron mobility, and has thus attracted much attention for its potential use in a wide range of electronic devices. Usually GaN crystal (Fig. 1) has a hexagonal phase (wurtzite, a = 0.319 nm and c = 0.519 nm) (Edgar, 1994 Strite Morkoc, 1992). 

Thinner films a=6.48, zincblende-type. Thicker films additional wurtzite-type phase with a=4.57, c=7.47, c/a= 1.63 and a hexagonal phase with a=4.57, c= 11.27, c/a=2.47. 

Fig. 3.8 XRD patterns (CuKc( source) showing a rich in selenium (x > 0.6) CdSej Tei-jt electrodeposited film, which adopts the hexagonal CdSe wurtzite structure by annealing at 520 °C (b). The as-deposited (from a typical acidic solution) film is rather amorphous (a). Segregation of a Te phase is observed in the solid. (Reprinted from Bouroushian et al. [137], Copyright 2009, with permission from Elsevier)...

A similar one-step process was employed successfully [66] to prepare well-crystallized CdS thin films of optical quality on Au(lll) from an aqueous solution of CdSOa, EDTA, and Na2S at room temperature. A phase transition from cubic (zinc blende) to hexagonal (wurtzite) CdS structure was observed with decreasing pH below 5, while highly preferential orientation along [11.0] directions for the... 

When the atomic size ratio is near 1.2 some dense (i.e., close-packed) structures become possible in which tetrahedral sub-groups of one kind of atom share their vertices, sides or faces to from a network. This network contains holes into which the other kind of atoms are put. These are known as Laves phases. They have three kinds of symmetry cubic (related to diamond), hexagonal (related to wurtzite), and orthorhombic (a mixture of the other two). The prototype compounds are MgCu2, MgZn2, and MgNi2, respectively. Only the simplest cubic one will be discussed further here. See Laves (1956) or Raynor (1949) for more details. 

STRUCTURE. CdS Can exist in three different crystal structures hexagonal (wurtzite), cubic (zincblende)— both tetrahedrally coordinated and cubic (rock-salt), which is sixfold coordinated. Except in a few cases, the rocksalt modification of CdS has been observed only at very high pressures CD films of this phase have never been reported. The other two phases have been reported to occur in CD films under various conditions. The wurtzite phase is thermodynamically slightly more stable, and invariably forms if the zincblende phase is heated above 300-400°C. The low-temperature CD method therefore can allow the formation of the zincblende phase, and this phase is commonly obtained in CD CdS films. Very often, a mixture of wurtzite and zincblende phases has been reported in the literature. There are many variables that affect the crystal structure, including the nature of the complex, the substrate, and sometimes even stirring. 

Alpha silver iodide (a-Agl), a fast ion conductor, is one of the different polymorphic structures of Agl showing a cubic structure [51], where I occupies anionic positions, that is, the Cl- sites in the CsCl-type structure (see Figure 2.19). On the other hand, the low temperature phase, that is, p-Agl, exhibits a hexagonal wurtzite-type structure. 

The majority of unipolar ionic conductors identified to date are polymorphic compounds with several phase transitions, where the phases have different ionic conductivities owing to modifications in the substructure of the mobile ions [28], One of the first studied cationic conductors was a-Agl [21]. Silver iodide exhibits different polymorphic structures. Agl has a low-temperature phase, that is, [3-Agl, which crystallizes in the hexagonal wurtzite structure type, and a high-temperature cubic phase, a-Agl, which shows a cubic CsCl structure type [20,22] (see Section 2.4.5). 

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