Hexagonal wurtzite structure

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. 

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 mobility for the transformation, and the high temperature in turn demands a high pressure (above 12 GPa 120 kbar) for diamond stability. 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) structure was discovered (25). 

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. 

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,... 

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.

The way in which these materials change their structures as the anion to cation ratio is varied is of interest. The structures are all variants of the hexagonal wurtzite structure of AIN in which we have alternate sheets of A1 and N atoms packed normal to the hexagonal c-axis. The stacking sequence can be represented by drawing the atom positions on the (110) planes, as all possible atom sites lie on such planes. In... 

Tables 1 and 2 present a summary of available data, obtained both by experiment and by calculation methods "-" for nitrides and carbides with the cubic-NaCi and hexagonal wurtzite structures.

Normally, these compounds in bulk polycrystalline form have the rock salt structure however, evaporation has been used to deposit thin films that have the tetrahedrally bonded hexagonal (wurtzite) structure . ... 

Figure 7.22 Three-dimensional nets (a) the cubic diamond structure (b) the net equivalent to (a) (c) the cubic zinc blende (sphalerite) structure (d) the net equivalent to (c), which is identical to that in (b) (e) the hexagonal wurtzite structure (f) the net equivalent to (e)...

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. 

The chemical deposition of PbS [56, 60] and ZnS [60, 69, 70] produces films which exhibit only a cubic structure. On the contrary Cdj-xZnjS ternary compounds present the hexagonal Wurtzite structure [100]. Chemically deposited PbSe [86], ZnSe [92, 93] and CdSe [41, 78] are generally found with a cubic structure According to A. H. Bid et al. [77] the presence of Cd(OH)2 in the solution could promote the formation of CdSe with an hexagonal structure. Recently H. Cachet et al. [147] found that CdSe films prepared from a solution complexed by sodium nitrilotriacetate could present an hexagonal structure when the temperature is increased up to 80 °C. On the other hand CdSei xSx films where x > 0.6 contain a small amount of the hexagonal modification [112]. 

Presumably, all the compounds, except ZnS, have a cubic zinc blende structure (I) at the vaporization temperatures. The ZnS crystals, as noted in [28], had a hexagonal wurtzite structure (III). To bring the calculated value in agreement with the experimental molar enthalpy (263.8kJ moP ), the following vaporization scheme needs to be assumed ... 

Bulk CdS possesses wurtzite structure ( 4 type) or zincblende structure ( 3 type). Both phases are formed by stacking of close-packed planes of cadmium and sulfur atoms. Hexagonal wurtzite structure can be represented as the periodic sequence AB AB AB... of closed-packed planes. Cubic zincblende is a sequence ABCABC... of closed-packed planes. For both structures the tetrahedral surrounding of both elements, cadmium and sulfur, takes place. According to the available literature data X-ray diffraction patterns of nanostructured CdS films and powders synthesized by different methods have similar features. Analysis of these features shows that the atomic structure of CdS nanoparticles differs from that of the bulk material. 

Aluminum nitride (AIN) differs from other A B compounds in its hexagonal wurtzite structure. The lattice parameters of this compound are a = 3.111 k, c = 4.978 A, and c/a = 1.600 [1]. Thus, the structure of aluminum nitride differs from the perfect wurtzite structure, which consists of regular tetrahedra and is characterized by the axial ratios c/a = 1.633. Moreover, aluminum nitride has distinctive physical and physicochemical properties. 

Figure 3.3 shows a typical XRD pattern of the as-synthesized product. All the diffraction peaks can be indexed to the hexagonal wurtzite structure of AIN crystal (JCPDS No.25-1133). No characteristic peaks of impurities were detected in the pattern. The sharp diffraction peaks indicated the good crystallinity of the product. 

XRD result showed that the as-synthesized product is pure hexagonal wurtzite structure of AIN crystal. EDS pattern illustrated that the product mainly consisted of Al and N elements with a trace of O. The presence of O element probably came from the relatively low vacuum degree of the combustion chamber. The XRD and EDS patterns can be seen in our previous study [7c]. 

Use of the alternative methods whose selection rules admit of spectral activity of the LO modes. One such method is polarized Raman spectroscopy, which is applicable to substances with cubic zinc blend and hexagonal wurtzite structures such as ZnS, ZnSe, CdS, ZnO, ZnTe, and the III-V compounds (see Refs. [47-49] and literature cited therein). The most direct method to measure vlo is inelastic neutron scattering (INS) since there are no selection rules for INS spectroscopy and as a result all modes are allowed [50, 51]. 

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