Molten salts coordination number

Bismuthides. Many intermetaUic compounds of bismuth with alkafl metals and alkaline earth metals have the expected formulas M Bi and M Bi, respectively. These compounds ate not saltlike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi . Both the alkafl and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particularly noteworthy as having extremely short bond distances between the alkafl metal atoms. 

There is difficulty in defining the absolute mobilities of the constituent ions in a molten salt, since it does not contain fixed particles that could serve as a coordinate reference. Experimental means for measuring external transport numbers or external mobilities are scarce, although the zone electromigration method (layer method) and the improved Hittorf method may be used. In addition, external mobilities in molten salts cannot be easily calculated, even from molecular dynamics simulation. 

Proof for the structure of the triphosphate anion comes from its composition and the cryoscopic molecular weight in molten Glauber s salt [values found, 372 (19) and 364 (317) calculated, 367.9]. Assuming that phosphorus has a coordination number of four with respect to oxygen, the anion structure can only be represented by the formula... 

When an ionic salt such as NaCl melts, the ionic lattice (see Figure 5.15) collapses, but some order is stiU retained. Evidence for this comes from X-ray diffraction patterns, from which radial distribution functions reveal that the average coordination number (with respect to cation-anion interactions) of each ion in liquid NaCl is 4, compared with 6 in the crystalline lattice. For cation-cation or anion-anion interactions, the coordination number is higher, although, as in the solid state, the intemuclear distances are larger than for cation-anion separations. The solid-to-liquid transition is accompanied by an increase in volume of il0-15%. The number of ions in the melt can be determined in a similar way to that described in Section 8.8 for H2SO4 systems in molten NaCl, v = 2. 

Figure 8.12. Two models for the structure of a molten salt. When the lattice (a) melts the ions form roughly equal cells according to schematic (b) as expected to minimize electrostatic energy. In model (c) the average interionic distance and the coordination number of the ions are lower than in (b). This model also has more and larger vacancies than the cellular model.

The data obtained allows supposing that the true characteristic of the OS composition of electroactive species is a thermodynamic characteristic, such as the OS shell formation energy, rather than the coordination number (from the crystal chemical standpoint). However, for obtaining of quantitative estimations the influence of the molten salt medium needs a study in extended systems. 

In molten salts (typically, alkali metal halides) the cation-anion distances are 5 % shorter and the anion-anion distances are 5 % longer than in salt crystals at the melting point, but are very near the distances in the crystals at room temperature, the coordination numbers being 10 % lower in the molten salts than in the... 

Table 3.4 Interionic distances and coordination numbers for the first and second peak of x-ray diffraction from molten salts...

Many authors have since then applied Monte Carlo (MC) and molecular dynamics (MD) simulations to molten salts. The simulations yielded the partial pair correlation functions, from which the inter-ionic distances and coordination numbers were deduced, as shown in Table 3.7. Generally the interionic distances were... 

The observed absorption bands of Eu(II) in the vapor complexes are caused by a 4f 5d <— 4f ( S7/2) transition and are known from spectra of Eu(II)-doped crystals and of Eu(II) dissolved in water and molten salts (see Sorlie and 0ye 1978 and references therein). Small temperature-dependent variations in the molar absorptivity were also observed. The splitting of the two bands, 6300 cm , is equal to the ligand field splitting, 10 Dq, of the 5d energy level. This splitting is not sensitive to the ligand field and therefore no conclusions could be drawn on the coordination number. The spectra of the Sm(II)-Al-Cl complex(es) appear more complicated, and transitions to 6s and/or 6p levels may... 

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