The Stable Complex Species in Melts of Alkali Metal Halides Quantum-Chemical Approach

The Stable Complex Species in Melts of Alkali Metal Halides Quantum-Chemical Approach 

In describing the properties of complex species in alkali metal halide melts and the electrochemical processes involving these species, the anionic complex should be considered integral with its outer-sphere (OS) cationic shell [1-4], In model calculations, the composition of this shell is chosen rather arbitrarily. However, calculations show that, in many cases, variations in the composition of the second coordination sphere in the model system radically changes the resulting correlations. Therefore, the task is to search for criteria that permit determining the composition of the dominant complex species in alkali metal halide. 

The approach we suggest is based on the fact that the computed energies describing the stability of complex species are an extreme function of the number of OS cations. In other words, quantum chemical calculations show that the composition of the second coordination sphere of the most stable particles is not at all the same as or close to the limiting crystal chemical composition. 

This approach was verified by comparing the calculated activation energies of charge transfer (E ) or, more precisely, the ratio of these values in the series of OS cations Na-K-Cs with the ratio of the experimental standard rate constants of charge transfer (k ) in the same series. 

The choice of such experimental data for comparison with calculations is based on a successful prediction of an anomalous ratio of charge transfer constants in the series of the Na-K-Cs OS cations obtained for the systems of the wM NbF7 type [5]. It turned out that, for the most stable particles, the correct ratio of the calculated activation charge transfer energies is fulfilled, that is, corresponding to the ratio of the charge transfer constants in the series of OS cations of Na-K-Cs. Therefore, one may assume that the composition of the most thermodynamically stable particles also characterizes the most probable dynamic composition of the electroactive complex in the melt, which imder certain conditions takes a predominant part in the electrochemical charge transfer. 

---