Systems containing trivalent cations

Among the systems containing a trivalent cation, technologically most important are the cryolite-based melts used as electrolytes for aluminum production. 

Very important also are systems containing BF3, which are used as electrolytes for boriding of steels and as heat-bearing medium in secondary circuits of nuclear power plants. 

To the melts containing a trivalent cation belong a great number of systems of alkali metal halide-rare earth metal halide systems. Lanthanide halides play an important role in the production of lanthanide metals by molten salt electrolysis and they are also used in a number of applications ranging from lighting to catalysis, through pyrochemical reprocessing of nuclear fuel. 

The above-mentioned three kinds of systems containing a trivalent cation will be described in the following chapters. 

In spite of extensive investigations over six decades, the stmcture of these melts is still not sufficiently understood. The most accepted electrolytic dissociation scheme of molten cryolite is as follows  

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Jensen, M.R, Yaita, T., Chiarizia, R. 2007. Reverse micelle formation in the partitioning of trivalent f-element cations by biphasic systems containing a tetraalkyldiglycolamide. Langmuir 23 (9) 4765 1774. 

Finally, the dissociation model of electrical conductivity was also successfully used in pseudobinary systems containing a trivalent cation MX-AIX3 (M = Li, Na, K ... 

A number of fluorite-based systems show anion interstitials. For example, fluorite itself, CaFj, exhibits a small solid solution range withtrivalent fluorides such as YFj, to give a solid solution Ca, xYxF2+x which contains interstitial F ions. The interstitial ions are accommodated in the vacant cubic sites (see Figure 3.24c). Similarly, the fluorite-based polymorphs of PbFj and BaFj may also be doped with suitable trivalent cations to give interstitial solid solutions. 

These factors are important considerations when interpreting data, as the mobile species may vary from electrolyte to electrolyte. In addition to the species, the morphology of the polymer electrolyte is important when determining transference number data. Many of the electrolytes containing divalent or trivalent cations comprise complexes with high melting temperatures (180°C and upward).In many instances, measurements may have been made on heterogeneous systems. 

Figure 130 Absorption (A), photoluminescence (PL) and electroluminescence (EL) spectra of a Langmuir-Bloddgett (LB) film (a) containing a donor-conjugated Ti-electron system acceptor (D-T-A) molecular cation coupled to a monovalent anion with a trivalent rare-earth (Nd3+) cation surrounded by four organic singly charged anionic ligands (b). The two EL spectra have been taken from a device being run for the first time (ELI), and the emission from a device that has been cycled several times (EL2). Reprinted from Ref. 512. Copyright 1996 with permission from Elsevier.

C.Y. Zhu and R.M. Izatt, Macrocyclic-mediated separation of Eu2+ from trivalent lanthanide cations in a modified thin-sheet-supported liquid membrane system, J. Membr. Sci., 1990, 50, 319 P.R. Brown, J.L. Hallman, L.W. Whaley, D.H. Desai, M.J. Pugia and R.A. Bartsch, Competitive, proton-coupled, alkali metal cation transport across polymer-supported liquid membranes containing s>yn(decyl-dibenzo-16-crown-5-oxyacetic acid) Variation of the alkyl 2-nitrophenyl ether membrane, ibid., 1991, 56, 195. 

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