Wurtzite structure


At room temperature the stable form of silver iodide is y-A.gl which has the cubic zinc blende structure (p, 1210). /3-Agl, which has the hexagonal ZnO (or wurtzite) structure (p. 1210), is the stable form between 136° and 146°. This structure is closely related to that of hexagonal ice (p. 624) and Agl has been found to be particularly effective in nucleating ice crystals in super cooled clouds, thereby inducing the precipitation of rain. fi-Agl has another remarkable property at 146° it undergoes a phase change to cubic a-AgI in which the iodide sublattice is rigid but the silver sublattice melts . This has a dramatic effect on the (ionic) electrical conductivity of the solid which leaps from 3.4 X 10 to 1.31 ohm cm, a factor of nearly 4000. The iodide sublattice in a-Agl is bcc and this provides 42 possible sites for each 2Ag+, distributed as follows  [c.1185]

Crystal Structure. Diamonds prepared by the direct conversion of well-crystallized graphite, at pressures of about 13 GPa (130 kbar), show certain unusual reflections in the x-ray diffraction patterns (25). They could be explained by assuming a hexagonal diamond stmcture (related to wurtzite) with a = 0.252 and c = 0.412 nm, space group P63 /mmc — Dgj with four atoms per unit cell. The calculated density would be 3.51 g/cm, the same as for ordinary cubic diamond, and the distances between nearest neighbor carbon atoms would be the same in both hexagonal and cubic diamond, 0.154 nm.  [c.564]

Diamond is an important commodity as a gemstone and as an industrial material and there are several excellent monographs on the science and technology of this material [3-5]. Diamond is most frequently found in a cubic form in which each carbon atom is linked to fom other carbon atoms by sp ct bonds in a strain-free tetrahedral array. Fig. 2A. The crystal stmcture is zinc blende type and the C-C bond length is 154 pm. Diamond also exists in an hexagonal form (Lonsdaleite) with a wurtzite crystal structure and a C-C bond length of 152 pm. The crystal density of both types of diamond is 3.52 g-cm.  [c.4]

W = wurtzite, hexagonal ZnS structure (p. 1210).  [c.255]

Stoichiometry but also on the relative sizes of the atoms involved and the propensity to form p,r double bonds to oxygen. In structures which are conventionally described as ionic , the 6-coordinate radius of (140 pm) is larger than all 6-coordinate cation radii except for Rb Cs Fr, Ra and Tl though it is approached by (138 pm) and Ba" (135 pm). Accordingly, many oxides are found to adopt structures in which there is a close-packed oxygen lattice with cations in the interstices (frequently octahedral). For cations , which have very small effective ionic radii (say <50 pm), particularly if they carry a high formal charge, the structure type and bonding are usually better described in covalent terms, particularly when n interactions enhance the stability of terminal M=0 bonds (M = C, N, P, etc.). Thus, for oxides of formula MO, a coordination number of 1 (molecular) is found for CO and NO, though the latter tends towards a coordination number of 2 (dimers, p. 446). With the somewhat larger Be and Zn" the wurtzite (4 4) structure is adopted, whereas monoxides of still larger divalent cations tend to adopt the sodium chloride (6 6) structure (e.g. = Mg, Ca, Sr, Ba, Co,  [c.641]

Figure 29.1 Crystal structures of ZnS. (a) Zinc blende, consisting of two, interpenetrating, cep lattices of Zn and S atoms displaced with respect to each other so that the atoms of each achieve 4-coordination (Zn-S = 235 pm) by occupying tetrahedral sites of the other lattice. The face-centred cube, characteristic of the cep lattice, can be seen — in this case composed of S atoms, but an extended diagram would reveal the same arrangement of Zn atoms. Note that if all the atoms of this structure were C, the structure would be that of diamond (p. 275). (b) Wurtzite. As with zinc blende, tetrahedral coordination of both Zn and S is achieved (Zn-S = 236 pm) but this time the interpenetrating lattices are hexagonal, rather than cubic, close-packed. Figure 29.1 Crystal structures of ZnS. (a) Zinc blende, consisting of two, interpenetrating, cep lattices of Zn and S atoms displaced with respect to each other so that the atoms of each achieve 4-coordination (Zn-S = 235 pm) by occupying tetrahedral sites of the other lattice. The face-centred cube, characteristic of the cep lattice, can be seen — in this case composed of S atoms, but an extended diagram would reveal the same arrangement of Zn atoms. Note that if all the atoms of this structure were C, the structure would be that of diamond (p. 275). (b) Wurtzite. As with zinc blende, tetrahedral coordination of both Zn and S is achieved (Zn-S = 236 pm) but this time the interpenetrating lattices are hexagonal, rather than cubic, close-packed.
ZnO is by far the most important manufactured compound of zinc and, being an inevitable byproduct of primitive production of brass, has been known longer than the metal itself. It is manufactured by burning in air the zinc vapour obtained on smelting the ore or, for a purer and whiter product, the vapour obtained from previously refined zinc. It is normally a white, finely divided material with the wurtzite structure. On heating, the colour changes to yellow due to the evaporation of oxygen from the lattice to give a nonstoichiometric phase Zni+ cO (jc < 70 ppm) the supernumerary Zn atoms produce lattice defects which trap electrons which can subsequently be excited by absorption of visible light.Indeed, by doping ZnO with an excess of 0.02-0.03% Zn metal, a whole range of colours — yellow, green, brown, red — can be obtained. The reddish hues of the naturally  [c.1208]

Tolbert S H and Aiivisatos A P 1995 The wurtzite to rock salt structural transformation in CdSe nanocrystais under high pressure J. Chem. Rhys. 102 4642  [c.2924]

In the (Cd, Zn) (S, Se, Te) family of compounds the principal crystal structure is sphalerite, a face-centred cubic structure of the metal atoms with the Group VI elements along the cube diagonals in four coordination with the metal atoms. The exceptions are CdS and CdSe which have the related wurtzite (h.c.p) structure. The lattice parameters of the sphalerite phases range between 0.54 nm for ZnS to 0.64 nm for InSb. There are therefore a number of solid solutions which can be formed between drese compounds, where the lattice parameters differ by less than about 15%, such as GaAs-InAs. A practical example is tire solid solution between CdTe (band gap = 1.44 eV, a — 0.6488 nm) and HgTe (band gap = 0.15eV, a — 0.6459 nm). This solution has a variable band gap as a function of the Cd/Hg ratio, and a wide range of miscibility. Another example is the solid solution between InP (a — 0.5969 nm) and GaAs (a — 0.5654 nm) in which the band gap varies direcdy as the mole fraction of each component. These solutions are best prepared by vapour phase deposition under ulU a-high vacuum conditions, using separate Knudsen cells for each element because of the widely differing vapour pressures of the constituent elements.  [c.158]

The predominantly ionic alkali metal sulfides M2S (Li, Na, K, Rb, Cs) adopt the antifluorite structure (p. 118) in which each S atom is surrounded by a cube of 8 M and each M by a tetrahedron of S. The alkaline earth sulfides MS (Mg, Ca, Sr, Ba) adopt the NaCl-type 6 6 structure (p. 242) as do many other monosulfides of rather less basic metals (M = Pb, Mn, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Th, U, Pu). However, many metals in the later transition element groups show substantial trends to increasing covalency leading either to lower coordination numbers or to layer-lattice structures. Thus MS (Be, Zn, Cd, Hg) adopt the 4 4 zinc blende structure (p. 1210) and ZnS, CdS and MnS also crystallize in the 4 4 wurtzite modification (p. 1210). In both of these structures both M and S are tetrahedrally coordinated, whereas PtS, which also has 4 4  [c.679]

The principal compounds in this category are the monochalacogenides, which are formed by all three metals. It is a notable indication of the stability of tetrahedral coordination for the elements of Group 12 that, of the 12 compounds of this type, only CdO, HgO and HgS adopt a structure other than wurtzite or zinc blende (both of which involve tetrahedral coordination of the cation — see below). CdO adopts the 6-coordinate rock-salt structure HgO features zigzag chains of almost linear O-Hg-0 units and HgS exists in both a zinc-blende form and in a rock-salt form.  [c.1208]

Zinc blende, ZnS, is the most widespread ore of zinc and the main source of the metal, but ZnS is also known in a second naturally occurring though much rarer form, wurtzite, which is the more stable at high temperatures. The names of these minerals are now also used as the names of their crystal structures which are important structure types found in many other AB compounds. In both structures each Zn is tetrahedrally coordinated by 4 S and each S is tetrahedrally coordinated by 4 Zn the structures differ significantly only in the type of close-packing involved, being cubic in zinc-blende and hexagonal in wurtzite (Fig. 29.1). Pure ZnS is white and, like ZnO, finds use as a pigment for which purpose it is often obtained (as lithopone ) along with BaS04 from aqueous solution of ZnS04 and BaS  [c.1209]

McLT82 McLarnen, T. J., Baur, W. H. Enumeration of Wurtzite derivatives and related dipolar tetrahedral structures. J. Solid State Chem. 42 (1982) 283-299.  [c.143]


See pages that mention the term Wurtzite structure : [c.119]    [c.613]    [c.766]    [c.433]    [c.2238]    [c.386]    [c.59]    [c.114]    [c.208]    [c.413]   
Chemistry of the elements (1998) -- [ c.679 , c.1209 ]