Crystals rocksalt

The rocksalt stmcture is illustrated in figure Al.3.5. This stmcture represents one of the simplest compound stmctures. Numerous ionic crystals fonn in the rocksalt stmcture, such as sodium chloride (NaCl). The conventional unit cell of the rocksalt stmcture is cubic. There are eight atoms in the conventional cell. For the primitive unit cell, the lattice vectors are the same as FCC. The basis consists of two atoms one at the origin and one displaced by one-half the body diagonal of the conventional cell. 

The archetype of the ionic ceramic is sodium chloride ("rocksalt"), NaCl, shown in Fig. 16.1(a). Each sodium atom loses an electron to a chlorine atom it is the electrostatic attraction between the Na ions and the CF ions that holds the crystal together. To achieve the maximum electrostatic interaction, each Na has 6 CF neighbours and no Na neighbours (and vice versa) there is no way of arranging single-charged ions that does better than this. So most of the simple ionic ceramics with the formula AB have the rocksalt structure. 

Figure 10.1 Schematic layers of Ti and C atoms in the rocksalt crystal structure of TiC.

Perhaps the most simple crystals in this class are the alkaline earth oxides. They are II-VI compounds and have rocksalt crystal structures. Data for their hardnesses versus their bond moduli (optical band gaps per molecular volumes) are displayed in Figure 11.4. 

In the KNCS (FT) complex (58) the anions are somewhat disordered in the crystal but the general arrangement is that of a distorted rocksalt structure. In the complex cation, Figs. 8 and 9, the molecule has S4... 

Rocket propulsion oxidizers, 18 384-385 Rocks, weathering of, radiation and, 3 299 Rocksalt, crystal structure of, 2 6, 29 Rock-salt-type alkali halide crystals, dissolution process, 39 411 19 alkali chlorides, 39 413, 416 alkali fluorides, 39 413-415... 

Few oxide superconductors were known prior to 1985 and we shall now return to these so that we can discuss these materials in reference to their crystal structure classes. There are only three broad structural categories in which most of the oxide superconductors occur. The important structural types include sodium chloride (rocksalt, or Bl-type), perovskite (E2X), and spinel (Hlx). 

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. 

PbS and PbSe are almost always found in the rocksalt (RS) crystal form. All structural investigations on CD films have shown this form, with one exception PbSe deposited from hydroxide complex at high hydroxide concentrations and at rela-... 

Solid solntions of Cd and Pb snlphides have been popnlar in ternary CD. The crystal structnres of the individnal snlphides are different PbS crystallizes in the rock-salt structnre, while CdS forms tetrahedrally bonded sphalerite or wnrizite structure (a rocksalt form of CdS exists but normally only at high pressures). This suggest that the solubihty range of the alloys will be limited. 

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 NaCl structure is also found in compounds like TiO, VO and NbO, possessing a high percentage of cation and anion vacancies. Ternary oxides of the type MggMn 08 crystallize in this structure with of the cation sites vacant. Solid solutions such as Li,j )Mg Cl (0 x 1) crystallize in the rocksalt structure stoichiometric MgCl may indeed be considered as having a defect rocksalt structure with 50% of ordered cation vacancies. 

The above simple picture of solids is not universally true because we have a class of crystalline solids, known as Mott insulators, whose electronic properties radically contradict the elementary band theory. Typical examples of Mott insulators are MnO, CoO and NiO, possessing the rocksalt structure. Here the only states in the vicinity of the Fermi level would be the 3d states. The cation d orbitals in the rocksalt structure would be split into t g and eg sets by the octahedral crystal field of the anions. In the transition-metal monoxides, TiO-NiO (3d -3d% the d levels would be partly filled and hence the simple band theory predicts them to be metallic. The prediction is true in TiO... 

However, in a cubic structure the value of G will be equal to C44 only when slip is on the 110 <001> slip system (Kelly et al 2000). In rocksalt-structured nitrides and carbides, slip in indentation at room temperature occurs on the 110 <110> slip system (Williams and Schaal, 1962 Molina-Aldareguia etal., 2002). The appropriate value of 6 is related to the different single crystal elastic constants, cy, by... 

Figure 5.12 Reduced Madelung Energy A/d as a function of dimensionless interionic distance, for CsCl, rocksalt and zincblende crystals.

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. 

It is noteworthy that the corresponding silver compounds do not exist in this structure. AgLuS2 (468), which we expect to be non-metallic, crystallizes in a disordered NaCl structure (high-temperature modification ). The structure of AgYS2, on the other hand, is a monoclinic, strongly distorted but ordered version of the rocksalt type (468). 

MnS and MnSe are the only transition-element compounds which have a zinc blende modification. The ZnS structure is the cubic version of the ZnO structure, i. e. the cations occupy half the tetrahedral holes in a cubic close-packed anion sublattice. As in the rocksalt structure the anion and the cation sublattices are identical to one another, i.e. the NaCl and ZnS structures are their own antitype. Like in the case of ZnS itself one should expect several polytypes to occur for MnS and MnSe. MnTe can be stabilized in the zinc blende structure by adding B3-type tellurides. Cubic mixed crystals Zni Mn Te were synthesized up to x = 0.86 171), Cdi-zMnzTe up to x — 0.75 172) and Hgi- Mn Te up to x = 0.8 172). 

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... 

The rocksalt crystal structure belongs to the cubic system with space group 0 (Pm3m). It consists of two face-centered cubic (fee) sublattices, which are occupied by one atom species each. The two sublattices are shifted along one half of the diagonal of the primitive unit cell against each other. The rocksalt lattice is sixfold coordinated. 

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... 

The T-point optical phonons of a crystal with rocksalt structure belong to the irreducible representation... 

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. 

Alg, Zn i, 0 crystallizes in the wurtzite or in the rocksalt structure, depending on the Mg mole fraction x. The alloys remain direct-gap materials over the whole composition range. The wurtzite-structure part reflects a valence-band structure, which is similar to ZnO. For the rocksalt-structure part the... 

Knapp et al. (144) show that for oxides containing 3d elements in spinel, perovskite, rocksalt, or zircon-type structures, the K-edge XANES spectra are quite independent of 3d electron occupation but instead nicely correlate with the crystal structure type. Various studies of Ti K edges of titanium oxides and other titanium compounds have been reported (40,158,172,177, 297). 

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