Systems containing divalent cations

As it follows from numerous experimental measurements, some pure halides of divalent metals partially dissociate in the molten state according to the scheme 

Other divalent cations have the tendency to form complex anions of the type [MeX3] and/or [MeX4] not only in the presence of the alkali metal halides, but also in the pure state. 

Some pure halides of divalent metals show a tendency to form auto-complexes. The ability to form auto-complexes according to the scheme 

A typical example of a salt that forms auto-complexes is ZnCl2. The formation of complexes at melting takes place according to the scheme 

According to Mackenzie and Murphy (1960), the high viscosity of about 500Pa-s and the low conductivity of about 10 S cm of liquid ZnCl2 near its melting point. 

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It needs to be noted that when the ligand system contains extensive unsaturation, then oxidation of the corresponding complex may yield a product containing a stabilized cation radical (rather than one in which the metal oxidation state has been altered). For example, such a situation has a tendency to occur on oxidation of divalent metal complexes [including Ni(n)] of the tetraphenyl-substituted porphyrin macrocycle. 

To our knowledge, there are less than 30 compounds based on radical-cations and M(dmit)2 systems (Table 2). Most of them contain divalent or monovalent M(dmit)2 units, and only a few of them have been structurally and magnetically characterized. Since they are not in a fractional oxidation state, they behave as insulators with low room-temperature conductivity. 

In this section we are concerned with the properties of intrinsic Schottky and Frenkel disorder in pure ionic conducting crystals and with the same systems doped with aliovalent cations. As already remarked in Section I, the properties of uni-univalent crystals, e.g. sodium choride and silver bromide which contain Schottky and cationic Frenkel disorder respectively, doped with divalent cation impurities are of particular interest. At low concentrations the impurity is incorporated substitutionally together with an additional cation vacancy to preserve electrical neutrality. At sufficiently low temperatures the concentration of intrinsic defects in a doped crystal is negligible compared with the concentration of added defects. We shall first mention briefly the theoretical methods used for such systems and then review the use of the cluster formalism. 

Solid solutions within the apatite family are readily synthesized in the laboratory. Some examples include (Ca,Zn,Pb)5(P04)30H (Panda et al. 1991), (Ca,Cd,Pb)5(P04)30H (Mahapatra et al. 1995), (Ca,Sr,Cu)5(P04)30H (Pujari Patel 1989), or other apatite solid solutions containing various quantities of Cd, Mg, Zn, Cd or Y (Ergun etal. 2001). They are also found naturally (Botto etal. 1997). Unlike well-ordered naturally occurring minerals, these solid solutions may actually be the more typical form of the mineral in stabilized ash systems given the system complexity, rapid precipitation kinetics, and wide prevalence of available divalent cations. 

It should be realized that Equation 11 was chosen because of the unavailability of a solution for the electrostatic potential p(x) for a system of two interacting bodies in contact with solutions containing both monovalent and divalent cations. At the same time, we have solutions for the surface charge and surface potential for isolated plates, in contact with both monovalent and divalent ions, which bind to the surface to some degree. Our solution for the isolated plates also gives the distance dependence of j/(x) (30). The potential j/(x) falls off with x, the distance from the surface, more steeply than according to the linear approximation. Therefore, the linear approximation in Equation 11 is regarded as an overestimate of Ve. 

The thiomolybdites are a class of molybdenum-sulfur compounds which contain molybdenum in a low oxidation state, usually +3. Two main types of such materials exist. The first type has the formula MMoS2 where M is a monovalent cation, usually an alkali metal. The second type has the formula MMo2S4 where M is a divalent cation, usually a transition metal. There are other thiomolybdite species, of composition other than that described above, which have been identified in ternary phase studies involving the M-Mo-S system (M = a transition element), but these have not been well characterized. 

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