Cation polarizability

Schematic diagrams appropriate to NMP/TCNQ and TTF/TCNQ are shown in Fig. 30 and are based on experimental studies. Application of the one-dimensional Hubbard model to analyse low and high temperature data for NMP/TCNQ yielded consistent values of U and t. For TTF/TCNQ and HMTSF/TCNQ, the increased cation polarizability is believed to have successfully reduced the strength of the effective electron-electron interaction with the result that a true metal-semiconductor transition is observed at 58 K for TTF/TCNQ which disappears completely for HMTSF/TCNQ. At present the advantages of using complex salts as against simple salts of charge-transfer systems to produce organic metals are not clear, particularly since the...

Again we digress to discuss soft and hard (11). It has been said that polarizability measures softness and that the permanent charge-cloud distortion is responsible for (b)-class behaviour. Unfortunately polarization energies of this kind work in two ways — they generate (i) (b)-class behaviour due to anion polarizability but (ii) (a)-class behaviour due to the cation polarizability. Thus if softness and polarizability are related they are not necessarily related to (b)-class behaviour. Moreover on the basis of polarizability of this kind there can be no preferential soft-soft or hard-hard interaction. 

Most nonferroelectric oxides and fluorides fall in this category of well-behaved compounds, but some materials exhibit larger deviations. Such deviations between observed and calculated dielectric constants can frequently be traced to factors which are known to increase k ionic or electronic conductivity, ferroelectric behavior, the presence of H2O or CO2 in chaimels or large cavities, or the presence of dipolar impurities other than H2O. A further cause of large deviations from additivity is the presence of rattling or compressed cations having unusually large or small cation polarizabilities, respectively. 

Brzezinski, B., and Zundel, G., Collective H, Lff, andNa motions and cation polarizabilities of the cation-bonded systems within 1,11,12,13,14-pentahydroxypentacene salts A FTIR study, J. Phys. Chem. 98, 2271-2274 (1994). 

Zundel, G., Proton polarizability and proton transfer processes in hydrogen bonds and cation polarizabilities of other cation bonds—their importance to understand molecular processes in electrochemistry and biology. Trends Phys. Chem. 3, 129-156 (1992). 

Cation polarizability is also a factor in selectivity. Kodama et al studied the complexation of aza-18-crown-6 with alkali, alkaline earth, and some d-block metals. This ligand showed higher selectivity toward the more polarizable, soft d-block metal ions than the hard, nonpolarizable alkali and alkaline earth metal cations. 

The time-averaged anion density and two-dimensional diffusion within a-PbSnF4 have recently been successfully reproduced within MD simulations, which essentially treat the lone-pairs as an extreme manifestation of cation polarizability. As a consequence, the structural behaviour of Q -PbSnF4 can be interpreted in terms of a higher effective polarizability of the smaller Sn ion [6]. The authors of [30] recently proposed a detailed mechanism for conductivity of this compound. 

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