Rock salt-type structure

Cadmium Sulfide. CdS [1306-23-6] is dimorphic and exists in the sphalerite (cubic) and wurtzite (hexagonal) crystal structures (40). At very high pressures it may exist also as a rock-salt structure type. It is oxidized to the sulfate, basic sulfate, and eventually the oxide on heating in air to 700°C, especially in the presence of moisture (9). 

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. 

Hydrides of the types AnHi (An = Th, Np, Pu, Am, Cm) and AnHs (Pa —> Am), as well as ThaHis (i.e. ThHs.yj) have been so obtained but are not very stable thermally and are decidedly unstable with respect to air and moisture. Borides, carbides, silicides and nitrides (q.v.) are mostly less sensitive chemically and, being refractory materials, those of Th, U and Pu in particular have been studied extensively as possible nuclear fuels.Their stoichiometries are very varied but the more important ones are the semi-metallic monocarbides, AnC, and mononitrides, AnN, all of which have the rock-salt structure they are predominantly ionic... 

Madelung constant (A) A number that appears in the expression for the lattice energy and depends on the type of crystal lattice. Example A = 1.748 for the rock-salt structure. 

As was discussed in Chapter 7, there are numerous solids that can exist in more than one form. It is frequently the case that high pressure is sufficient inducement for the structure to change. An example of this type of behavior is seen in KC1, which has the sodium chloride (rock salt) structure at ambient pressure, but is converted to the cesium chloride structure at high pressure. Other examples illustrating the effect of pressure will be seen throughout this book (see especially Chapter 20). It should be kept... 

The fourth and final crystal structure type common in binary semiconductors is the rock salt structure, named after NaCl but occurring in many divalent metal oxides, sulfides, selenides, and tellurides. It consists of two atom types forming separate face-centered cubic lattices. The trend from WZ or ZB structures to the rock salt structure takes place as covalent bonds become increasingly ionic [24]. 

Figure 11.7 shows schematically the resulting calculated variation of H with p for the NaCl-type and the CsCl-type phases of CaO. The NaCl-type structure, which is stable at low pressures, is the rock salt structure in which the Ca and O atoms are 6-coordinate. In the CsCl structure, stable at high pressures, both cation and anion are 8-coordinate. In the static limit where the entropy is set to zero, the thermodynamically most stable phase at any pressure is that with the lowest value of H at the thermodynamic transition pressure, ptrs, the enthalpies of the two phases are equal. For CaO the particular set of potentials used in Figure 11.7 indicates a transition pressure of 75 GPa between the NaCl-type and CsCl-type structures, which compares with experimental values in the range 60-70 GPa. 

The simplest of structures is the rock salt structure, depicted in Figure 2.2a. Magnesium oxide is considered to be the simplest oxide for a number of reasons. It is an ionic oxide with a 6 6 octahedral coordination and it has a very simple structure — the cubic NaCl structure. The structure is generally described as a cubic close packing (ABC-type packing) of oxygen atoms in the (111) direction forming octahedral cavities. This structure is exhibited by other alkaline earth metal oxides such as BaO, CaO, and monoxides of 3d transition metals as well as lanthanides and actinides such as TiO, NiO, EuO, and NpO. 

For o-Li ,Mn02, 50% of the Mn ions need to change position in order to form spinel. The characteristics of the orthorhombic, a-NaFe02-type layered, and spinel structures as well as many other ordered rock-salt structures have been covered in detail by Thackeray. ... 

For later use in this section it is sufficient to recall that both these structures are composed of cations in cubic eutaxy. In the fluorite type the anions occupy all the tetrahedral interstices in the cation array while, in the rock salt structure they occupy all the octahedral interstices. All the structures considered in this section are lamellar intergrowths of these two types (or of c.c.p. cations and B1). 

On the other hand, before and/or after a layer (AX) there may be a layer of type (BX2) or one of type (AX). This last case is possible because (AX) is characteristic of both the perovskite and the rock salt structures and, therefore, is structurally coherent with both (BX2) and (AX). A sequence. .(AX)c o(AX)o c... can consequently substitute the single layer (AX)co, thus increasing the thickness of the rock salt monolayers (AX) present in the structure of perovskite. In this way, we may derive the structure of A2BX4 (K2NiF4 - type) from that of perovskite by substituting each (AX)C 0 in expression (6) with bilayers... 

Following the above discussion, it is now possible to define a general structural type built with alternating blocks having the perovskite and the rock salt structures, according to the scheme ... 

In this large class of materials the blocks R = m (AX) with the rock-salt structure are made by two or more layers of type (AX) which may be identical to each other or have different chemical compositon. The blocks P = (BX2)oc (n-1) [(AX)c o(BX2)o c] with the perovskite structure may have different values of n, and the layers (AX), sandwiched between layers (BX2), may or may not be defective. The important homologous series with the rock salt-perovskite structure are listed in the scheme of Figure 9where they are compared with each other and with the basic structure of perovskite. 

Fig. 5.44). The coordination number of each type of ion is 8, and overall the structure has (8,8)-coordination. The cesium-chloride structure is much less common than the rock-salt structure, but it is found for Csl as well as CsCl. 

Magnesium oxide (MgO) is a good example of a catalyst with rock salt structure, that is, NaCl-type structure (Figure 2.8). 

Structures for some common crystal types in which the ratio of cation to anion is 1 1. (a) The NaCI or rock salt structure, (b) The CsCI structure, (c) The zinc blende structure for ZnS. (d) The wurtzite structure for ZnS. 

In Figure 3.1, the reflectance spectrum of a MgO monocrystal is reported and is compared with the transmission/ absorption spectrum of a MgO powder. MgO (periclase) has the rock-salt structure as the other rock-salt type solids, MgO is Raman-silent (as far as the fundamentals are concerned, see below) and only shows one IR-active fundamental mode. The reflection by the MgO monocrystal, is strong between two limits. The upper limit is the so-called LO while the lower limit is the TO. 

Vibrational spectroscopies give rise to interesting information on the microscopic structure of soUd-solution mixed oxides. For example, the state of vanadium in soUd solution in Ti02 anatase catalysts [59], the partial ordering of cations in comndum-type Fe-Cr oxides [60], the real presence of Ti" in the silicalite framework of TSl catalysts [58] and the solubility of AT ions in the NiO rock-salt structure [61] have been objects of IR spectroscopic studies. 

Coarse-grained or single-crystalline (bulk) PbS possesses a rock salt structure (51 type). This phase is formed by the stacking of close-packed planes of lead and sulfur atoms and can be represented as the periodic stacking sequence ABC ABC... of either lead or sulfur planes. These structure features correspond to the octahedral coordination for lead and sulfur. 

These are sometimes considered under two heads, those with complex ions, such asCOg and NOa", and those without complex ions. Only the latter will be discussed here. They are of two main types. In the first, the lattice structures are those of simple compounds in which random replacement of metal ions has occurred. LigTiOg has a random rock-salt structure with two thirds of the metal ion positions occupied by Li and one third by Ti. FeSb04 has a random rutile structure. 

Table 1 Summary of experimental information for Li , Cu" " and Ag" " impurities in atkab halide lattices. All lattices present rock salt structure except CsQ and CsBr that have CsCl-type structure.

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