Alkali cations, halides

Due to the analytical problems described we expect that some of the disagreements in the literature (concerning mainly the physicochemical data of some tetrafiuoroborate ionic liquids) may originate firom differing amounts of alkali cation, halide anion and residual water impurities in the ionic liquids analyzed. 

The compounds which most nearly fit the clas-sicial conception of ionic bonding are the alkali metal halides. However, even here, one must ask to what extent it is reasonable to maintain that positively charged cations M+ with favourably... 

Oxygen is the more usual donor atom for the other alkali metals. A solid isolated with sodium bromide but not with other alkali metal halides provides an example of a trigonal prism. The complex crystallises as NaBr, [(PhO)2POCH2PO(OPh)2]3, H20 and consists of discrete complex cations shown in Fig. 3, bromide ions, and water molecules (27). 

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

Similar substantially constant differences are obtained with other pairs of alkali halides of B 1 structure, having either a cation or an anion in common. As a result, the conclusion was reached that each ion makes a specific contribution toward an experimentally observed r0, well-nigh irrespective of the nature of the other ion with which it is associated in the lattice. In other words, characteristic radii should be attributable to the ions (1,2). However, a knowledge of the internuclear distances in the crystals is not sufficient by itself to determine absolute values for crystal radii of ions, and various criteria have been used to assign the size of a particular ion or the relative sizes of a pair of alkali and halide ions. 

Curves plotted for AH° conv. hyd. and ionic radii given in Tables 5 and 6, and in col. 2 of Table 1 are shown in Fig. 4 for both alkali cations and halide anions. The uncertainty in the thermochemical data is taken as 0.5 kcal and the uncertainty in ionic radii is based on deviations from additivity of r0 values. From these curves A(AH° conv. hyd.) can be estimated and in Fig. 5 these values halved are plotted against (R- -a) 3. This curve becomes linear for large (i a)-3 values and this supports the use of the model based on charge-quadrupole interactions and assumptions concerning differences in kinetic contributions to the internal energy (39). 

Intrinsic point defects are deviations from the ideal structure caused by displacement or removal of lattice atoms [106,107], Possible intrinsic defects are vacancies, interstitials, and antisites. In ZnO these are denoted as Vzn and Vo, Zn and 0 , and as Zno and Ozn, respectively. There are also combinations of defects like neutral Schottky (cation and anion vacancy) and Frenkel (cation vacancy and cation interstitial) pairs, which are abundant in ionic compounds like alkali-metal halides [106,107], As a rule of thumb, the energy to create a defect depends on the difference in charge between the defect and the lattice site occupied by the defect, e.g., in ZnO a vacancy or an interstitial can carry a charge of 2 while an antisite can have a charge of 4. This makes vacancies and interstitials more likely in polar compounds and antisite defects less important [108-110]. On the contrary, antisite defects are more important in more covalently bonded compounds like the III-V semiconductors (see e.g., [Ill] and references therein). 

Water clusters containing simple ions are another area of current experimental and theoretical interest. Accordingly, they are also the subject of EA studies. Chaudhury et al. [113] have used EA methods on empirical potentials to obtain optimized structures of halide ions in water clusters, which they then subjected to AMI calculations for simulation of spectra. EA applications to alkali cations in TIP4P water clusters [114,115] have led to explanations of experimental mass-spectroscopic signatures of these systems, in particular the lack of magic numbers for the sodium case and some of the typical magic numbers of the potassium and cesium cases, and the role of dodecahedral clathrate structures in these species. 

A major difference between crystals and fluids refers to the necessity of distinguishing between different sites. So the autoprotolysis in water could, just from a mass balance point of view, also be considered e.g. as a formation of a OH vacancy and a IT vacancy. In solids such a disorder is called Schottky disorder (S) and has to be well discerned from the Frenkel disorder (F). In the densely packed alkali metal halides in which the cations are not as polarizable as the Ag+, the formation of interstitial defects requires an unrealistically high energy and the dominating disorder is thus the Schottky reaction... 

The hydration energy for the outer shell turns out to be 15% of the whole for cations and about 30% for anions. Thus, in hydration of the alkali and halide ions. 

Compounds with dithiolates have been characterized. The only binary halide known is the unstable MnF4 that decomposes to MnFs and F2. Room temperature syntheses of MnF4 and hexafluoromanganate(IV) of alkali cations A2MnF6 (A = Li-Cs) have been reported. ... 

Salts of the Agp4 ion were first described by Hoppe [3] and Hoppe and Homann [4]. These were alkali-cation salts and were typically prepared by higher temperature (300-400°C) fluorination of alkali and silver nitrates, mixed halides, or oxides. The products of these syntheses were sufficiently free of silver difluoride and other contaminants to provide X-ray powder diffraction data [4] to establish the unit cell character for the salts of the heavier alkali metals, and to show the diamagnetism (low spin tf ) of AgF4 in these salts. The LiAgp4, however, was not structurally defined. 

Because the [R2AI] cation can be stabilized by neutral Lewis bases, we assume that this is also true of ionogenic Lewis base alkali metal alkyls R-M. Thus, 1 2 complexes of alkali metal halides with aluminum trialkyls, especially fluorides, may dissociate (3a) into M and [R3AI-F-AIR3] , as shown by the moleeular structure in the crystalline state. Yet there is another possibility, shown in Scheme 3b, i. e., dissociation into aluminum-containing cations and anions. 

G. Toth, Ab initio pair potential parameter set for the interaction of a rigid and a flexible water model and the complete series of the halides and alkali cations, J. Chem. Phys. 105, 5518-5524 (1996). 

---