NaCI-type structure

Fig. 8.2 S implified structures of CaC2 l-lll (from (a) to (c)), displayed as derivatives of the NaCI type structure, neglecting the relation of their true crystallographic unit cells.

Looking at the trends in atomic properties, we can see that K atom is bigger than Ca and consequently that Ca is smaller than K but bigger than Ca. Thus, the ionic radius for Ca can be approximated as in between that of K and Ca. Since KCl has an NaCI-type structure, we can safely assume that CaCl would have the same, so we are using cationic radii for coordination six (138 + 100)pm/2 = 119 pm. We have ... 

Checking the frequency of occurrence of the B1 (NaCI)-type structure among the 1 1 compounds of the metals (Manes and Benedict 1985) shows that at ambient pressure and temperature practically all LnX and AnX compounds have this structure, while only f of the other MX compounds (M metal) crystallize in this structure type. This may indicate that either the incomplete f shells or the low d-electron counts favor the formation of the Bl-type structure. 

Fig. 2.9 S implified interaction scheme of ir with X in the solid, (a) Hypothetical TIP in the NaCI type of structure,...

Figure 10.3 Carbide hardnesses vs. characteristic vibrational energy densities derived from average force constants (entropic specific heat). After Grimvall and Theissen (1986). The crystal structures are of the NaCi type. The hardness data are fromTeter (1998).

In the case of an f.c.c. AI structure, with u = w = 1, the state of complete occupation of interstitial carbon corresponds to a MC carbide with the NaCI lattice. For the c.p.h. A3 structure, with = 1 and v = 0.5, complete occupation of the interstitial sites by carbon gives the MjC carbide with a hexagonal Fe2N-type structure. 

It is convenient to draw micro-twin structures derived from the micro-twinning of a CCP lattice projected on the normal plane to the pillars of the trigonal prisms. Let us consider a twin structure derived from a NaCl-type structure, in which both anions and cations have a CCP arrangement. Figure 2.67 shows the structure of a NaCl-type (a) is projected on (OOl)Naci and (b) on (110). It can be easily shown that the twin plane (203) ,j in the ortho-hexagonal expression corresponds to the (113) plane in the cubic one. Figure 2.68 shows a twin structure of NaCl-type,... 

Tab. 3.1 Experimental and theoretically predicted lattice parameters (A) for CaO in the [ZnS]/[NaCI]/[CsCI] structure types derived from GGA calculations, ionic radii tables, bond-valence calculations, and volume increments.

The space group of GeTe is a subgroup common to the two branches of the family tree. The structure can be obtained from the NaCI type by introducing a small asymmetry in the distance between alternating layers of Na and Cl atoms. Alternatively, the structure can be derived from a-As by an ordered substitution of As by Ge and Te in the layers parallel to the trigonal axis. 

For results where comparisons could be made, the interfacial tension behavior was practically independent of the type of heavy oil used. Interfacial tensions strongly depended on the surfactant type, temperature, and NaCI and CaCI2 concentrations. Changes in the structure of the amphiphile at the oil/water interface is affected by these variables and accounted for some of the experimental observations. 

Fig. 3.4 The Phiilips-Van Vechten structure map ( c, ) for the sp-valent octet AB compounds. The four-fold coordinated zinc blende and wurtzite structure types are separated from the six-fold coordinated NaCI structure type by the straight line corresponding to the degree of ionicity a = 0.785. (After Phillips and Van Vechten (1969).)...

Fig. 2.68 Twin structure of NaCl-type (CCP of anions and cations) projected on (HO)Naci- Twin plane is (113)Naci> corresponding to (203) . ...

Thus increased covalent bonding resulting from Fajans-type phenomena can lower the transition temperatures. For example, the alkali halides (except CsCI, CsBr. and Csl) and the silver halides (except Agl) crystallize in the NaCI structure. The sizes of the cations are comparable Na = M6 pm. Ag = 129 pm, K = 152 pm, yet the melting points of the halides are considerably different (Table 8.6). The greater covalent character of the silver halide bond (resulting from the electron confi J ra-... 

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. 

Structure Linear, type D A. NaCy 40.7 H20 48.0 Dec. 11.3 texture Type E Structure Hexagonal, type E A. 

Fig. 3.3 Band structure (a), density-of-states (b) with Ca contributions in biack, and Ca-0 crystai orbitai Hamiiton popuiation (c) of CaO in the [NaCi] structure type, according to a TB-LMTO-ASA caicuiation and the LDA.

The first nomenclature for inorganic structure types has been proposed by Ewald Hermann in 1931. They used letters to designate the kind of chemical compound and numerals to distinguish among compounds with the same general formula. The chemical elements are designated by the letter A, the binary compounds by the letter B, the AB2 compounds by C and so on. Examples are A1 for Cu, A10 for Hg, B1 for NaCI, B3 for ZnS (sphalerite), HI2 for Mg2Si04 (olivine), G1 for CaCOa (calcite). This notation has not received much acceptance, possibly due to its lack of self explanatory structural information. 

Illustration of the closepacking of two types of motifs, such as the Na" " and cr ions in NaCI as proposed in 1898 just before it became possible to analyse crystal structures by X-ray diffraction. 

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