Rock salt lattice

Note that in part of the literature and in Eq. (9.12), the Madelung potential is defined such that iioiji.unii ccii = — where r is the nearest distance between the cation and the anion. For the rock-salt lattice, ft = p/2. 

The physical properties of the thallium halides have been tabulated.1 Crystalline T1F has a distorted rock-salt lattice, with two independent thallium sites.333-335 The thermodynamic properties of the solid have been reassessed.336 At high temperature, the vapor phase contains TLF and T12F2 molecules,337 and matrix isolation studies have demonstrated the presence of both T1F and T12F2 in the solid the dimers have linear symmetry.338... 

There are some apparent exceptions. In the ammonium halides, which have the rock salt lattice, the ammonium ions, which do not themselves have centers of symmetry, lie at lattice points which would, were the ammonium ions not present, be centers of inversion. Depending on the particular halide and the temperature, either the ammonium ions are freely rotating, so that the time-averaged symmetry is centric, or else ammonium ions at different sites have different orientations so that, although any particular site is not centrosymmetric, the crystal as a whole appears to be centrosymmetric. In this brief discussion we shall consider only cases in which the crystals are completely ordered and contain no rotating molecules. 

In a simple lattice such as the rock-salt lattice, etc., Pa is equal to Pc [absolute value] since the positions of both kinds of ions are equivalent. In the calculation of the potential the summation must be made over the contributions of all the surrounding ions ... 

Only the alkalis and alkaline earths form sulfides that appear to be mainly ionic. They are the only sulfides that dissolve in water and they crystallize in simple ionic lattices, for example, an anti-fluorite lattice for the alkali sulfides and a rock salt lattice for the alkaline earth sulfides. Essentially only SH" ions are present in aqueous solution, owing to the low second dissociation constant of H2S. Although S2" is present in concentrated alkali solutions, it cannot be detected below 8 M NaOH owing to the reaction... 

The 20 A diameter monodisperse crystallites isolated Ifom S. pombe and C. glabrata have been calculated to contain 85 CdS pairs in the lattice coated with approximately 30y-EC peptides (Figure 10). The particle size was determined using transmission electron microscopy and powder X-ray analysis. The analysis of the X-ray diffraction patterns was inconclusive in discriminating between a four-coordinate zinc blende and six-coordinate rock salt lattice structure. The presence of a short coherence length (8 A) can be attributed either to internal disorder or to deviation from pure crystallinity. The absorption red edge of the crystallites is located at about 320 inn. The other properties characteristic of semiconductor nanoparticulates such as luminescence with emission near 460 mn, and electron transfer to methyl viologen... 

As an example of a pseudopotential simulation of boundary conditions for clusters representing some ionic compounds, let us consider the MgO case [79], which has been considered previously [80], [81] within a bare cluster model. The MgO crystal (the rock-salt lattice) was simulated by a cluster MggOg (see Fig. 3) placed in several embeddings comprised of a different number of pseudoatoms possessing the effective potentials and placed at the lattice sites. 

James, R. W., Waller, I., and Hartree, D. R. Existence of zero-point energy in the rock-salt lattice by an X-ray diffraction method. Proc. Roy. Soc. London) A118, 334-350 (1928). 

All the alkali metal halides except the cliloride, bromide and iodide of caesium form cubic crystals with the rock salt lattice and show a co-ordination number of 6. The exceptions are also cubic, but have the caesium chloride structure (Fig. 133) characterised by a co-ordination number of 8. The radius ratio for CsCl, Cs /Cl" = 0.93, allows 8 co-ordination, but is so near the ratio for 6 co-ordination that caesium chloride is dimorphous, changing, at 445°, from the caesium chloride to the rock salt structure. The crystalline halides are generally markedly ionic, though, as expected, lithium iodide is somewhat covalent, for iodide is the largest and most easily polarised simple anion and lithium, the smallest alkali metal cation, possesses the strongest polarising power. 

ZnO has the wurtzite, CdO the rock-salt lattice. Ikjth the yellow and red forms of HgO are orthorhombic and indistinguisliable in structure. A rhombohedral form of HgO, orange in colour, has been obtained as well-formed crystals by slow precipitation from aqueous solution (Laruelle, 1955). It is converted irreversibly to the orthorhombic form above 200°. When heated ZnO becomes yellow but without change in the structure of the phase. The hot oxide is bleached by oxygen and its colour is restored by zinc vapour. The coloured oxide has about 0.03% of zinc over the stoichiometric ratio the zinc is present as atoms occupying interstitial sites (p. 147). 

In the gas molecule, in consequence of the one-sided action of the electric forces, the electron clouds of the two ions are of course very much deformed, so that the analysis of the ionic binding becomes extremely difficult. The circumstances are simpler in crystals, especially in the highly symmetrical ones of the rock salt (cubic face-centred, fig. 9) and similar types. In these the deformation disappears on account of the symmetry in a rock salt lattice the same force is exerted on four sides on a chlorine ion by the neighbouring sodium ions, and a similar result holds for the more distant ions. Thus... 

NajNbO have been recorded and interpreted on the basis of a rock salt lattice containing a disordered distribution of Na and Nb atoms among the cation sites. A corrected atomic position parameter for Ba3Si4Nbg026 has been given. ... 

Fig. 4,1-133 The Brillouin zone of the zinc blende and rock salt lattices...

For alkali halide clusters it was first proposed to classify species corresponding to (i) cuboid or deformed sections of the rock salt lattice with the localized excess electrons in the vacancy (surface F-centers), for which relatively high IPs were measured and a strong absorption in the visible-infrared region, in analogy with the bulk color centers, is expected (ii) filled cuboids with excess electrons highly delocalized... 

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