Crystals/crystallization wurtzite structure

A similar distortion may occur in some crystals with the wurtzite structure. Wurtzite and greenockite show easy prismatic cleavage and difficult basal cleavage, whereas iodyrite, Agl, cleaves perfectly on the... 

The monosulfides of the alkaline earth metals crystallize in the rock salt (MgS, CaS, SrS, BaS) and zinc blende (BeS) structures. BaS is insoluble in water, while the other monosulfides are sparingly soluble but hydrolyzed on warming (except MgS that is completely hydrolyzed). The monoselenides are isomorphous to the sulfides. The monotellurides CaTe, SrTe, BaTe adopt the rock salt stmcture, while BeTe has the zinc blende and MgTe the wurtzite structure. Alkaline earth polysulfides may be prepared by boiling a solution or suspension of the metal hydroxide with sulfur, e.g.,... 

Crystals whose structures are not centrosymmetric are polar because their centers of positive charge are displaced slightly from their centers of negative charge. Examples are crystals with the wurtzite structure which have polar axes along their (0001) directions. Also, crystals with the zincblende structure are polar in their (111) directions. 

The familiar diamond structure, with each atom covalently bonded in a perfect tetrahedral fashion to its four neighbors, is adopted not only by C but also by Si and Ge. Silicon can also adopt a wurtzite structure (see below), an example of a polytype (one of several crystal structures possible for a substance having an identical chemical composition but differing in the stacking of layers, and which may exist in a metastable state after its formation at some different temperature or pressure). 

Explain why MgO crystallizes in the rock salt (NaCl) structure whereas BeO crystallizes in the wurtzite structure. 

Yellow to orange crystal occurs as two polymorphs, hexagonal alpha form and cubic beta form exhibits stable wurtzite structure at lower temperature, and zinc blende type structure at higher temperatures the beta form converts to alpha form when heated at 750°C in sulfur atmosphere sublimes at 980°C practically insoluble in water (1.3 mg/L at 20°C) Ksp 3.6x10-29 dissolves in dilute mineral acids on heating or concentrated acids at ordinary temperatures (decomposes with liberation of H2S). 

II-VI semiconductors, such as CdSe and CdS, normally have the wurtzite structure (see Chapter 1) where each element is tetrahedrally coordinated. Under high pressures (2 GPa), these transform to the six-coordinate NaCl (rock salt) structure. However, if pressure is applied to a CdSe nanocrystal of about 4 nm in diameter, it now takes much more pressure, about 6 GPa, to transform it to the rock salt structure. It is thought that this may be a resistance to the exposure of high-index crystal planes... 

The parameter is obtained by relating the static dielectric constant to Eg and taking in such crystals to be proportional to a - where a is the lattice constant. Phillips parameters for a few crystals are listed in Table 1.4. Phillips has shown that all crystals with a/ below the critical value of0.785 possess the tetrahedral diamond (or wurtzite) structure when f > 0.785, six-fold coordination (rocksalt structure) is favoured. Pauling s ionicity scale also makes such structural predictions, but Phillips scale is more universal. Accordingly, MgS (f = 0.786) shows a borderline behaviour. Cohesive energies of tetrahedrally coordinated semiconductors have been calculated making use... 

The wurtzite structure is closely related to the zinc blende structure, having the same 4 4 tetrahedral coordination arising from a hexagonal close packing of anions in which half the tetrahedral sites are occupied by cations. Examples of AB compounds crystallizing in this structure are CuCl, CuBr, Cul, Agl, BeO, ZnO, ZnS, MnS, MnSe,... 

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. 

By the use of mainly LEED and lately ion scattering techniques the location of many atomic adsorbates, their bond distances and bond angles from their nearest neighbor atoms have been determined. The substrates utilized in these investigations were low Miller Index surfaces of fee, hep and bcc metals in most cases, and low Miller Index surfaces of semiconductors that crystallize in the diamond, zincblende and wurtzite structures in some cases that could be cleaned and ordered with good reproducibility. 

Wurtzite structure. Zinc sulfide can also crystallize in a hexagonal form called wurtzite that is formed slightly less exothermically than the cubic zinc blende (sphalerite) modification (Afff = —192.6 and —206.0 kJ mol-1, respectively) and hence is a high temperature polymorph of ZnS. The relationship between the two structures is best described in terms of close packing (Section 4.3) in zinc blende, the anions (or cations) form a cubic close-packed array, whereas in wurtzite they form hexagonal close-packed arrays. This relationship is illustrated in Fig. 4.13 note, however, that this does not represent the actual unit cell of either form. 

In hexagonal diamond (wurtzite structure) the wrinkled sheets are stacked in an ABABAB sequence, as shown in the Figure 7. Looking down on the stack from above, hexagonal holes can be seen formed by the six-membered carbon rings. The crystal has hexagonal symmetry about this axis, hence the name hexagonal diamond, or wurtzitic carbon. 

Figure 27. Crystal lattice of cadmium pigments (wurtzite structure) a) Sulfur (selenium) b) Cadmium (zinc, mercury)...

Cation holes can also be created by coactivation with trivalent metal ions or by incorporation of oxygen [5.313]. The luminescence band of self-activated zinc sulfide with the zinc-blende structure exhibits a maximum at 470 nm. On transition to the wurtzite structure, the maximum shifts to shorter wavelengths. In the mixed crystals zinc sulfide-cadmium sulfide and zinc sulfide-zinc selenide, the maximum shifts to longer wavelengths with increasing cadmium or selenium concentration. 

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 ... 

Agl, in its low temperature form, crystallizes in the (hep) wurtzite structure. The silver ions occupy tetrahedrally coordinated sites. The fast ion conducting Agl (7 >420K) is bcc. One can stabilize structural disorder at low temperature not only by incorporating cations such as Rb+, NH, etc., but also by adding S2 to reconstruct the anion sublattice and obtain Ag3SI. This compound exists in three different structures. At high temperature (>519K), it is bcc and both the cations and... 

Compounds with the wurtzite structure are shown in Table 6.2. 6.2.2. The 2-2PT Crystal Structure of I/, Ice... 

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,... 

Wurtzite-structure ZnO thin films grown by a variety of deposition techniques, as well as commercially available single crystal bulk samples are discussed. Furthermore, data for ZnO thin films intermixed with numerous elements are reviewed. Most of the results are obtained by SE, which is a precise and reliable tool for measurements of the DFs. The SE results are supplemented by Raman scattering and electrical Hall-effect measurement data, as well as data reported in the literature by similar or alternative techniques (reflection, transmission, and luminescence excitation spectroscopy). 

Undoped and doped ZnO, and most of the ZnO-based alloys crystallize under normal conditions in the wurtzite structure, but ZnO-based alloys can reveal a rocksalt structure for a high content of alloying atoms. One example is Mg-rich Mg, Zni., () Thus, a phase transition with change of coordination... 

Fig. 3.1. (a) Primitive cell (heavy lines) of the wurtzite-structure lattice placed within a hexagonal prism, a and c are the lattice constants, (b) Schematic drawing of surfaces cut from a hexagonal single crystal with different crystallographic orientations (surface planes)... 

Both A - and Ei-modes are Raman and IR active. The two nonpolar E2-modes E and E are Raman active only. The Bi-modes are IR and Raman inactive (silent modes). Phonon dispersion curves of wurtzite-structure and rocksalt-structure ZnO throughout the Brillouin Zone were reported in [106-108]. For crystals with wurtzite crystal structure, pure longitudinal or... 

Table 3.2 summarizes the optical phonons of crystals with wurtzite structure and the scattering configurations, in which the optical phonons are predicted to produce backward signal in first-order Raman scattering. 

The subscript i refers to the two polarization states parallel (i = ) and perpendicular [i = L) to the c-axis, which have to be distinguished for optically uniaxial samples, for instance wurtzite-structure ZnO or sapphire. Cubic crystals, for instance rocksalt-structure Mg Zni- O, are optically isotropic and have only one DF, because the dielectric tensor is reduced to a scalar. 

In the / -spectrum of the ZnO thin film, a similar plateau as in the 3 -spectrum of the ZnO bulk sample is present. However, the phonon modes of the sapphire substrate introduce additional features, for example atw 510, 630, and "-900 cm 1 [38,123]. The spectral feature at w 610 cm-1 is called the Berreman resonance, which is related to the excitation of surface polari-tons of transverse magnetic character at the boundary of two media [73]. In the spectral region of the Berreman resonance, IRSE provides high sensitivity to the A (LO)-mode parameters. For (OOOl)-oriented surfaces of crystals with wurtzite structure, linear-polarization-dependent spectroscopic... 

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