Hexagonal wurtzite

In this process, diamond forms from graphite without a catalyst. The refractory nature of carbon demands a fairly high temperature (2500—3000 K) for sufficient atomic mobiUty for the transformation, and the high temperature in turn demands a high pressure (above 12 GPa 120 kbar) for diamond stabihty. The combination of high temperature and pressure may be achieved statically or dynamically. During the course of experimentation on this process a new form of diamond with a hexagonal (wurtzitic) stmcture was discovered (25). 

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... 

Fig. 4.12 (a) CdSe wurtzite unit cell (b) schematic illustration of a hexagonal (wurtzite) CdSe basal plane on a (111) section of the gold lattice, emphasizing the 2 3 lattice match. Note the [111] Au//(0001)CdSe orientation, with the CdSe a-directions aligned along the (llO)Au. The outlined rhombus indicates the projection of a CdSe unit cell. (Adapted from [112])... 

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. 

This study also reported that films deposited on carbon membranes at temperatures >80°C were of hexagonal (wurtzite) structure, with a high density of planar defects, in contrast to the zincblende obtained from both hydroxide and ion-by-ion mechanisms at lower temperatures and to the epitaxial films on InP at all temperatures. 

One example is the tertiary bond found in the wurtzite structure of ZnO (67454). All members of the Zn chalcogenide series crystallize with structures based on the close packing of the chalcogenide ions, with Zn occupying half the tetrahedral cavities. The higher members, ZnSe and ZnTe (31840), crystallize with the cubic sphalerite structure while ZnO crystallizes with the hexagonal wurtzite structure. ZnS (60378, 67453) is known in both forms. 

In the sphalerite structure the anions form a cubic close packed array. The structure has a single adjustable parameter, the cubic cell edge. The 0 ions are too small for them to be in contact in this structure (see Fig. 6.4) so ZnO adopts the lower symmetry hexagonal wurtzite structure which has three adjustable parameters, the a and c unit cell lengths and the z coordinate of the 0 ion, allowing the environment around the Zn " ion to deviate from perfect tetrahedral symmetry. In the sphalerite structure the ZnX4 tetrahedron shares each of its faces with a vacant octahedral cavity (one is shown in Fig. 2.6(a)), while in the wurtzite structure one of these faces is shared with an empty tetrahedral cavity which places an anion directly over the shared face as seen in Fig. 2.6(b). The primary coordination number of Zn " in sphalerite is 4 and there are no tertiary bonds, but in wurtzite, which has the same primary coordination number, there is an additional tertiary bond with a flux of 0.02 vu through the face shared with the vacant tetrahedron. 

The luminescent properties can be influenced by the nature of the activators and coactivators, their concentrations, the composition of the flux, and the firing conditions. In addition, specific substitution of zinc or sulfur in the host lattice by cadmium or selenium is possible, which also influences the luminescent properties. Zinc sulfide is dimorphic and crystallizes below 1020 °C in the cubic zinc-blende structure and above that temperature in the hexagonal wurtzite lattice. When the zinc is replaced by cadmium, the transition temperature is lowered so that the hexagonal modification predominates. Substitution of sulfur by selenium, on the other hand, stabilizes the zinc-blende lattice. 

The zinc blende and wurtzite structures. Zinc sulfide crystallizes in two distinct lattices hexagonal wurtzite (Fig. 4.2a) and cubic zinc blende (Fig. 4.2b). We shall not elaborate upon them now (see page 121), but simply note that in both the coordination number is 4 fbr both cations and anions. The space groups are Ptync and F43m. Can you tell which is which ... 

Different from ZnS, the nanocrystalline of ZnO synthesized under mild conditions generally belongs to the hexagonal wurtzite structure. Lanthanides ions have been successfully incorporated into its lattice. 

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). 

The unique properties of the stable d5 Mn2+ ion is reflected by the fact that MnS and MnSe crystallize in three modifications, in the rocksalt, the cubic sphalerite and the hexagonal wurtzite structure. While in the NiAs structure of MnTe the cations occupy the octahedral holes of a hexagonal close-packing of anions they occupy half of the tetrahedral holes of this packing in the ZnO type modification of MnS and MnSe. The non-metallic character is evident already from the fact that the structure is undistorted (c/a = 1.61 for MnS and 1.63 for MnSe) and that the cations really are at the centres of one set of tetrahedral holes and not at the centre of the bipyramidal holes composed of two tetrahedra of the two different sets. 

Zinc oxide (ZnO) is an oxidic compound naturally occurring as the rare mineral zincite, which crystallizes in the hexagonal wurtzite structure P63inc [16]. The mineral zincite was discovered in 1810 by Bruce in Franklin (New Jersey,... 

TABLE 1 Lattice parameters of III-N compounds (hexagonal-wurtzite and cubic-zincblende structures). For GaN bulk crystals, the errors indicate variations between various samples, as the measurement accuracy was of about 5 parts per million. For cubic AIN and InN, the given lattice parameters are estimated from bond-lengths of the wurtzite phase. For all epitaxial layers, the given values are relaxed lattice parameters calculated from the measured ones using EQN (1),... 

AIN exists in two types the hexagonal (wurtzite structure) and the cubic (zincblende structure). The former is more stable, and has been investigated in more detail. The wurtzitic AIN has two formula units per unit cell (4 atoms per cell) and 9 optical branches to the phonon dispersion curves [1] ... 

In addition to the oxides, the other six chalcogenides are also known. Table 15-3 shows the structures of the eight compounds. Clearly, with the sole exception of CdO, the chalcogenides of zinc and cadmium prefer tetrahedral coordination, though preference for the cubic zinc blende structure or the hexagonal wurtzite structure varies irregularly. 

One of the most widely studied types of semiconductor NCs is CdSe. The hexagonal wurtzite structure that pertains for large CdSe crystals is shown in Fig. la, and a model of a nanocrystal is pictured in Fig. b. The size of such NCs can be controlled by reaction... 

Schematic parabolic band structure for CdSe, which has a band gap of 1.75 eV. The conduction band is labeled C, and several valence bands (V,) are shown. The filled and open circle symbols indicate the position of quantized k values mr/ai allowed for the / = 1 and n = 2 states of an NC with radius a. The solid arrow shows the / = 1 transition in which an electron is excited and a hole is created (open circle). The dashed arrow shows how the position of this n = i transition would change for a nanocrystal of smaller radius 32- (Adapted from Ref. 7.) This simple diagram is for the cubic zinc blend structure the hexagonal wurtzite structure has a small gap k= 0 between the and V2 bands.

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