Molten sublattice

A fast ion conductor with a molten sublattice contains ... 

The conducting ion sublattice in FICs is generally considered molten . The molten sublattice model for fast ion conduction was first proposed by Strock (1936) on the basis of structural and thermodynamic data for Agl. In most FICs, the entropy of the phase transition to the FIC state is larger than the entropy of melting. For example, in Agl the entropy of the transition at 420 K from the -form to the a-form (FIC state) is 14.7 J deg mol , whereas the entropy of melting at 861 K is only 11 J deg mol . ... 

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

In mixtures of molten salts it is necessary to take into account the fact that the interaction forces are very strong and that the nearest neighbors of the cations are anions and the nearest neighbors of the anions are always cations. Therefore, random distributions of anions and cations cannot be conceived. On the basis of these considerations Temkin [18] proposed a model for ideal mixtures of molten salts which assumes the existence of two interlocking sublattices, one of cations and the other of anions. In the case of mixing of two salts, the cations mix on the cation sublattice and the anions mix on the anion sublattice. 

Tay] carried out a reahstic thermodynamic assessment using an ionic liquid model, a sublattice model for the solid phases, with 4, 3 and 2 sublattices for flie spinel, corundum and wuestite phases, respectively. Thermodynamic models were also developed for flie oxidation of alloys in air [2004Gon], and for the desoxidation equilibria in molten steels [2004Jun]. 

Ionic conductors may be divided into three classes depending on their defect concentrations (i) dilute point defects, dpd ( 10 defects cm ), (ii) concentrated point defects, cpd, ( 10 cm ) and (iii) liquid-like or molten salt sublattice materials, mss, ( 10 cm ). 

In molten salt sublattice materials, practically all the ions in the sublattice are available for motion with an excess of available sites per cation as in e.g. Na -alumina, (a25 c = 1 4 x 10 Q cm S E = 0.16 eV). This leads to a high degree of disorder of these cations. The site occupancies and the conductivity characteristics for some of these salts are given in reference 12, pp. 49 and 53. Conduction is favoured by a levelling of the energy profile along the conduction pathway... 

Solid electrolytes have also been variously described as Fast Ionic Conductors or Superionic conductors and may cover ionic conductivities within the range of 10 to 1 S/cm with activation energies of 0.1 to 2eV/atom. The levels of ionic conductivity achieved in many of these solid electrolytes are well below their melting points and the values are more typical of liquids than solids. In contrast to liquid electrolytes such as the aqueous electrolytic solutions or molten salts, the mobile ions in a solid are limited to one sublattice such that one ionic component can move through a rigid framework provided by the other components. 

While certain preliminary ideas about the origin of ionic conduction in crystalline solids were derived from the crystal structure investigations of a-AgI by Stiock, and relate to a molten sublattice, i.e., no distinction can be made between a regular Ag lattice site and an Ag on an interstitial site, our knowledge of point defects is based maiidy on the woik of Frenkel, Wagner and Schotlky, Schotlky, and lost, and was developed primarily by studying electrical and optical properties. 

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