ENHANCEMENT OF CATIONIC CONDUCTIVITY IN SOME HEPTAFLUOROZIRCONATES DUE TO A PADDLE-WHEEL MECHANISM

The ionic conductivity of members of K(3-x)M(x)ZrF(7) (M=NH4, Tl and 0 less than or equal to x less than or equal to 3) solid solutions was investigated using complex impedance spectroscopy. Transient NMR measu rements were carried out on F-19, H-1 and Tl-205 in order to specify t he mobile species. The structure of KTl2ZrF7 has been refined from sin gle crystal diffraction data. Correlations between all these experimen tal results allow to conclude that a long-range motion of the monovale nt cations enhanced by the rotational/reorientational motion of the (Z rF7)(3-) complex anions is responsible for the ionic conductivity of t hese phases. Comparison of the ionic conductivities of Tl, NH4 and K c ompounds as well as that of some compositions among their solid soluti ons supports the conclusion that the higher performances of the pure t hallium compound can be related to the high polarisability of this ion .