Rock-salt compounds

A typical powder diffraction apparatus is shown in Fig. 14. The desirable requirements of high beam intensity and high resolution are rather incompatible and conventional instruments often have fairly poor resolution. This is advantageous in experiments where peaks are well separated as count rates can be high, and was the situation for example, in the measurements on Mn2+ and Ni2+ rock-salt compounds (56, 61) and Cr2+, Fe + and Mn + oxide perovskites (62), where only a few low-angle peaks were measured. High resolution is essential in profile analysis refinement, however, or when many intensities must be measured to... 

Such anion-anion overlap causes a broadening of the Compton profile, and it has been shown that quite good agreement between experiment and a free ion model may be obtained for other rock-salt compounds if the effect is included, although it is always smaller than in LiH. Thus for LiF and LiCl< > the anion-anion overlap is calculated to have a 10% effect on the Compton profile at low momentum values but only a 1 % effect on x-ray scattering factors. As expected, MgO shows larger overlap effects than the halides. In other, more covalent compounds, for example BeO< > and A1N< >, the expected large departures from ionic model behavior have been observed but have yet to lead to improved wavefunctions for the solids. 

Studies of solid solutions have been particularly rich in detail when considering the actinide rock salt compounds. Some seven pages in the work by Rossat-Mignod et al. (1984) are devoted to these studies and references go back to the 1960s. The principal reasons for the popularity of these studies are as follows ... 

The metallic rock salt compound GdSe and the semiconducting rock salt compound EuSe form a continuous series of solid solutions GdxEu xSe. The change of the lattice constant over the whole composition range from EuSe with divalent Eu to GdSe with trivalent Gd is shown in Fig. 135, Holtzberg et al. [4, 5]. 

Sodium is not found ia the free state ia nature because of its high chemical reactivity. It occurs naturally as a component of many complex minerals and of such simple ones as sodium chloride, sodium carbonate, sodium sulfate, sodium borate, and sodium nitrate. Soluble sodium salts are found ia seawater, mineral spriags, and salt lakes. Principal U.S. commercial deposits of sodium salts are the Great Salt Lake Seades Lake and the rock salt beds of the Gulf Coast, Virginia, New York, and Michigan (see Chemicals frombrine). Sodium-23 is the only naturally occurring isotope. The six artificial radioisotopes (qv) are Hsted ia Table 1 (see Sodium compounds). 

X-ray structural studies have played a major role in transforming chemistry from a descriptive science at the beginning of the twentieth century to one in which the properties of novel compounds can be predicted on theoretical grounds. When W.L. Bragg solved the very first crystal structure, that of rock salt, NaCl, the results completely changed prevalent concepts of bonding forces in ionic compounds. 

Iodide ions reduce Cu to Cu , and attempts to prepare copper(ll) iodide therefore result in the formation of Cul. (In a quite analogous way attempts to prepare copper(ll) cyanide yield CuCN instead.) In fact it is the electronegative fluorine which fails to form a salt with copper(l), the other 3 halides being white insoluble compounds precipitated from aqueous solutions by the reduction of the Cu halide. By contrast, silver(l) provides (for the only time in this triad) 4 well-characterized halides. All except Agl have the rock-salt structure (p. 242). Increasing covalency from chloride to iodide is reflected in the deepening colour white yellow, as the... 

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. 

The compound Li4Nb04F crystallizes in cubic syngony, with a cell parameter of 4.192 A and a Rock Salt (NaCl) structure. The compound s X-ray diffraction pattern and cell parameter are very similar to those of nickel oxide, NiO. 

In order to construct an MeX-type compound (X Me = 1) using only octahedral elements, the octahedrons must be linked via their faces i.e. by sharing of three angles. This arrangement of the octahedral polyhedrons yields a rock-salt type structure (NaCl type structure). 

Fig. 43 shows fragments of X-ray powder diffraction patterns of compounds with rock-salt-type structures. 

Fig. 43. Fragments of X-ray powder diffraction patterns of compounds with rock-salt structures that underwent modification to a state of disordered ionic arrangement. 1 - Li3Ta04 2 - LiflbO 3 - Li4Ta04F 4 - Li3Ti03F 5 -LiiFeOiF 6 - LiNiOF (Reflections attributed to LiF are marked by an asterisk).

In all cases, broad diffuse reflections are observed in the high interface distance range of X-ray powder diffraction patterns. The presence of such diffuse reflection is related to a high-order distortion in the crystal structure. The intensity of the diffuse reflections drops, the closer the valencies of the cations contained in the compound are. Such compounds characterizing by similar type of crystal structure also have approximately the same type of IR absorption spectra [261]. Compounds with rock-salt-type structures with disordered ion distributions display a practically continuous absorption in the range of 900-400 cm 1 (see Fig. 44, curves 1 - 4). However, the transition into a tetragonal phase or cubic modification, characterized by the entry of the ions into certain positions in the compound, generates discrete bands in the IR absorption spectra (see Fig. 44, curves 5 - 8). 

Figure 3. The lattice parameter for the family of rock-salt structure actinide-antimonide compounds is shown where the line is for the corresponding lanthanide compounds. The metallic radii for the light actinide elements are plotted. The smooth line simply connects Ac to the heavy actinides. In both cases the smooth line represents the ideal tri-valent behavior.

Antimonide-actinide compounds, lattice parameter for rock-salt... 

Rock-salt structure of actinide-anti-monide compounds, lattice... 

Two factors that make sodium compounds important are their low cost and their high solubility in water. Sodium chloride is readily mined as rock salt, which... 

Most monochalcogenides of the Group 3 metals adopt the rock salt (NaCl) structure. Note that the crystal chemistry of divalent europium is very similar to that of the alkaline earths, particularly strontium, as the radius of Eu is almost the same as that of Sr ". For the Yb compounds, the cell dimensions are practically identical with those of the Ca compounds. 

---