Alkali halide crystals hardnesses

As mentioned above alkali halide crystals are strongly hardened by small additions of divalent impurities. Data are available for 12 cases, the host crystals NaCl, NaBr, KC1, and KBr with additions of Ca2+, Sr2+, and Ba2+ (Chin, et al., 1973). It was found that the hardness increases depend only on the concentrations of the additions and not on the divalent specie (Ca, Sr, or Ba). However, a dependence on the valence (1, 2, or 3) is observed. It was also found that hardness increment is proportional to the square root of the concentration, (C1/2). 

In alkali halide crystals containing color-centers (F-centers) illumination with light of appropriate energy causes transient changes of hardness (Nadeau, 1964). This effect apparently results from changes in the effective sizes of the F-centers when they become excited. 

The most remarkable aspect of Eq. (14-10) is that the dependence upon bond length and therefore, the dependence upon which alkali halide is being considered—has cancelled out. Another consequence is that the ion. softening does not depend upon pre.ssurc. This explains why theories of the crystal-field splitting and its pressure dependence that arc based upon hard ions have been successful the 0.51 factor can be absorbed in an undetermined scale parameter, depending upon the shape of the orbitals being split, since the factor 0.51 does not change with distortion. 

Ionic crystals may be viewed quite simply in terms of an electrostatic model of lattices of hard-sphere ions of opposing charges. Although conceptually simple, this model is not completely adequate, and we have seen that modifications must be made in it. First, the bonding is not completely ionic with compounds ranging from the alkali halides, for which complete ionicity is a very good approximation, to compounds for which the assumption of the presence of ions is rather poor. Secondly, the assumption of a perfect, infinite mathematical lattice with no defects is an oversimplification. As with all models, the use of the ionic model does not necessarily imply that it is true , merely that it is convenient and useful, and if proper caution is taken and adjustments are made, it proves to be a fruitful approach. 

Three metal oxides with fee lattices and rocksalt stmetures have been examined by HAS NiO [78, 79], CoO [51, 80], and MgO [81-83], Because the ions in these materials are divalent, the atomic interaction forces are very much stronger than those for the alkali halides. This results in substantial differences in a number of physical properties, including the very high melting and boiling points of the oxides and the hardness of their crystals. Nonetheless, they still cleave along (100) planes like the alkali halides to form high-quality surfaces. 

Ionic Cations and anions Electrostatic, non-directional Hard, brittle, crystals of high m.t. moderate insulators melts are conducting Alkali metal halides... 

We will now consider an important group of compounds, namely the halides of the alkali metals. Assuming that the atoms behave as completely hard spheres, one would expect that the shortest distance between two atoms in a crystal of NaCl will be equal to the... 

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