Rocksalt structures

Fig. 16.1. Ionic ceramics, (a) The rocksalt, or NoCl, structure, (b) Magnesia, MgO, has the rocksalt structure. It can be thought of as an f.c.c. packing with Mg ions in the octahedral holes. ( ) Cubic zirconia ZrOj an f.c.c. packing of Zr with O in the tetrahedral holes, (d) Alumina, AljOj a c.p.h. packing of oxygen with Al in two-thirds of the octahedral holes.

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. 

Thallous halides offer a unique possibility of studying the stereochemistry of the (chemically) inert electron pair, since their structures and their pressure and temperature-dependent phase transitions have been well established. Thallium (1) fluoride under ambient conditions, adopts an orthorhombic structure in the space group Pbcm which can be regarded as a distorted rocksalt structure (Fig. 2.4). In contrast to TIF, the thallium halides with heavier halogens, TlCl, TlBr and Til, adopt the highly symmetric cubic CsCl structure type under ambient conditions [46]. Both TlCl and TlBr, at lower temperatures, undergo phase transitions to the NaCl type of structure [47]. 

It should be noted that these types of spectra are expected only for quadrupolar nuclei of semiconductors in non-cubic axially-symmetric forms such as the WZ structure cubic forms such as ZB or rocksalt structures ideally lack any anisotropy, and the ST peaks overlap the CT peak. However, defects in such cubic structures can produce EFGs that have random orientations, and the resulting ST are spread out over a wide range. 

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... 

The stoichiometry of Mn-S precipitated from solution is normally MnS. The stable form of MnS is green a-MnS, which has the rocksalt structure. However, the pink form, which is the form that usually precipitates from solution, is a mixture of p-MnS (zincblende) and 7-MnS (wurtzite), both of which are metastable. MnSe behaves analogously. 

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. 

Figure 5.12 (a) Ordered defects in monoclinic TiOfTij gOj g) (b) ordered defects in (orthorhombic) nonstoichiometric TiOj l (c) coherent intergrowth of (a) and (b) along the (120) planes of rocksalt structure. Lines indicate unit cell faces of the superstructures. (After Anderson, 1984.)... 

Figure 5.13 The 4 1 cluster of one tetrahedral cation and four vacant octahedral sites in one octant of the rocksalt structure.

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... 

Superexchange describes interaction between localized moments of ions in insulators that are too far apart to interact by direct exchange. It operates through the intermediary of a nonmagnetic ion. Superexchange arises from the fact that localized-electron states as described by the formal valences are stabilized by an admixture of excited states involving electron transfer between the cation and the anion. A typical example is the 180° cation-anion-cation interaction in oxides of rocksalt structure, where antiparallel orientation of spins on neighbouring cations is favoured by covalent... 

Monoxides of 3d transition metals, TiO to NiO, possess the rocksalt structure and exhibit properties shown in Table 6.3. While TiO and VO exhibit properties characteristic of itinerant (or nearly itinerant) d electrons, MnO, FeO, CoO and NiO show localized electron properties. The properties can be understood in terms of the possible cation-cation and cation-anion-cation interactions in the rocksalt structure (Fig. 6.12(a)). Direct cation-cation interaction can occur through the overlap of cationic t2g orbitals across the face diagonal of the cubic structure. When this interaction is strong R < and b > b, cationic t2g orbitals are transformed into a cation sublattice t%g band if this band is partly occupied, the material would be... 

Table 6.3. Properties of 3d metal monoxides with rocksalt structure ...

Figure 6.12 (a) Orbitals in the (100) plane of rocksalt structure showing cation-cation and cation-anion-cation interaction (b) schematic energy band diagram of TiO. 

We now turn to the formation of some of these hydride structures. The majority of them are based on a fee array of metal atoms, as shown by the open circles in Figure 2. The dihydride structure comes from filling the tetrahedral interstice (large solid circles) in the lattice with hydrogens and gives the well known CaF2 or calcite structure. Similarly, if one fills the octahedral interstice (small solid circles), one gets the NaCl or rocksalt structure found in nickel hydride and palladium hydride, which we will discuss near the end of this chapter. 

Figure 8.11 (a) Rocksalt structure of KC1 and AgBr with (100) planes delineated. 

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... 

This increase in hardness has been associated with the formation of a rocksalt cubic (c) form of AIN, stabilized by the reduction of the interfacial energy with the rocksalt-structured TiN (Madan etal., 1997). As the thickness increases the effect is offset by the increase in the volume free energy, so that the cubic form is stable only at layer thicknesses of less than 2 nm, although some increase is possible by using rocksalt-structured compounds such as VN with a lower mismatch (Li et al., 2002, 2004). Stabilization with other materials such as W (Kim et al., 2001) or ZrN (Wong et al., 2000) is also possible, and other compounds such as CrN (Yashar et al., 1998) can also show stabilized cubic forms. 

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