Wurtzite , structure

The relaxation of the outermost surface layer on the (10—10) surface has been determined in an early LEED experiment to be larger for the Zn ions than that for the O ions, the values being Ad(Zn) = —0.45 A and Ad 0) = —0.05 A, respectively, which leads to a tilting of the Zn-O surface dimer of 12° [79]. This result has been confirmed by angle-resolved ultraviolet photoelectron spectroscopic (UPS) 

Polar ZnO Surfaces The polar surfaces of ZnO are the so-called Tasker type 3 (Section 15.2.2) surfaces constituting alternating layers of oppositely charged ions. 

In the purely ionic model, the stabilization of these polar surfaces is achieved via a charge redistribution that increases the formal charge of Zn ions on the Zn-terminated surface from +2 to +3/2 and reduces the formal charge of O ions on the O-terminated surface from —2 to —3/2. The charge compensation can be achieved by electron transfer from the O- to the Zn-terminated surface, by removing surface ions, or by adsorption of charged species. 

An important fact is that the surface hydroxylation as well as the adsorption of O atoms almost completely lifts the relaxation inside the first double layer, which, for the bulk-truncated surface, amounts to 33% of the bulk interlayer distance. It should be noted that in most experimental studies, a slight outward relaxation of a small percentage of the interlayer distance has been determined [95-97], and only recently, an inward relaxation of the Zn-terminated surface, as predicted by calculations, was observed experimentally [98]. 

as in the case of the Zn-terminated surface, also on the O-terminated surface, the formation of OH groups completely Hfts the surface relaxation, which for the bulk-trancated surface was calculated to be almost half of the interlayer distance [99,100]. 

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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.,... 

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)...

By substituting alternately the carbon atoms in cubic diamond by zinc and sulfur atoms, one obtains the structure of zinc blende (sphalerite). By the corresponding substitution in hexagonal diamond, the wurtzite structure results. As long as atoms of one element are allowed to be bonded only to atoms of the other element, binary compounds can only have a 1 1 composition. For the four bonds per atom an average of four electrons per atom are needed this condition is fulfilled if the total number of valence electrons is four times the number of atoms. Possible element combinations and examples are given in Table 12.1. 

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. 

Since these structures are formed by filling the open spaces in the diamond and wurtzite structures, they have high atomic densities. This implies high valence electron densities and therefore considerable stability which is manifested by high melting points and elastic stiffnesses. They behave more like metal-metalloid compounds than like pure metals. That is, like covalent compounds embedded in metals. 

The ethylzinc ethylthiolate cluster consists of an ethyl-studded (ZnS)9 core, having a wurtzite structure, which is capped with an ethylthiolate group on one end and an ethylzinc moiety on the other. All zinc and sulfur atoms are four... 

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). 

The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

The wurtzite structure with hexagonal symmetry depicted in Figure 2.2b has a 4 4 tetrahedral coordination arising from HCP arrangement of anions with half the tetrahedral sites occupied by the cations. Examples include ZnO and BeO. 

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

Sphalerite and wurtzite structures general remarks. Compounds isostructural with the cubic cF8-ZnS sphalerite include AgSe, A1P, AlAs, AlSb, BAs, GaAs, InAs, BeS, BeSe, BeTe, BePo, CdS, CdSe, CdTe, CdPo, HgS, HgSe, HgTe, etc. The sphalerite structure can be described as a derivative structure of the diamond-type structure. Alternatively, we may describe the same structure as a derivative of the cubic close-packed structure (cF4-Cu type) in which a set of tetrahedral holes has been filled-in. This alternative description would be especially convenient when the atomic diameter ratio of the two species is close to 0.225 see the comments reported in 3.7.3.1. In a similar way the closely related hP4-ZnO... 

In the first case, along the direction of the diagonal of the cubic cell, there is a sequence ABC of identical unit slabs ( minimal sandwiches ), each composed of two superimposed triangular nets of Zn and S atoms, respectively. The thickness of the slabs, which include the Zn and S atom nets, is 0.25 of the lattice period along the superimposition direction (that is along the cubic cell diagonal aj3). It is (0.25,3 X 541) pm = 234 pm. In the wurtzite structure there is a sequence BC of similar slabs formed by sandwiches of the same triangular nets of Zn and S atoms. Their thickness is —0.37 X c = 0.37 X 626.1pm = 232 pm). 

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). 

The wurtzite structure is composed of an hep array of sulfide ions with alternate tetrahedral holes occupied by zinc ions. Each zinc ion is tetrahedrally coordinated by four sulfide ions and vice versa. Compounds adopting the structure include BeO, ZnO, and NH4F. 

Below 146°C, two phases of Agl exist y-Agl, which has the zinc blende structure, and (3-Agl with the wurtzite structure. Both are based on a close-packed array of iodide ions with half of the tetrahedral holes filled. However, above 146°C a new phase, a-AgI, is observed where the iodide ions now have a body-centred cubic lattice. If you look back to Figure 5.7, you can see that a dramatic increase in conductivity is observed for this phase the conductivity of a-Agl is very high, 131 S m , a factor of 10 higher than that of (3- or y-AgI, comparable with the conductivity of the best conducting liquid electrolytes. How can we explain this startling phenomenon ... 

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... 

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. 

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