Ion-Ion Pairs in the Superionic Conductors
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ION-ION PAIRS
TN THE SUPERIONIC CONDUCTORS
SERGEI E.SIGARYOV Institute of Crystallography USSR Academy of Sciences, Leninsky pr.59, Moscow, 117333 USSR
ABSTRACT On the base of numerous experimental data and theoretical approaches it is shown that ion-ion pairs with the intrapair distance about a sum of the corresponding ionic radii is the feature of charge transfer in the superionic conductors. INTRODUCTION The problem what the type of ion motion, individual or collective, rules by a charge transfer in the solid electrolytes remains one of the questions of most interest in the physics of superionic conductors. The existence of a ion-ion interaction in the mobile ion sublatice has no needs of confirmation now. But from the other hand have been done no real studies of the character of these interaction yet. This paper presents the results of an re-analysis of both the known and obtained by author experimental data and theoretical approaches on ion transport in crystals that leads to evidences of possibility of charge transport in solid electrolytes by motion of ion-ion pairs with intrapair distance about a sum of the corresponding ionic radii. The analysis is based on the consideration of data of diffraction studies, nuclear magnetic resonance and electrophysical measurements. I.
THE MODEL
Let us take into account the fact that ions in the solid electrolytes are in a lattice. It means that each site of ion of one sign (for example, cation) is surrounded by sites of ions of
Fig.1
Coordination
polyhedron of a cation
Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
144
opposite sign (anions). In other words there is so-called coordination polyhedron of each ion (see fig.1). It is clear that an effective negative charge induced by anion surrounding in the centre of the polyhedron may be more than magnitude of a positive charge of the ion situated there. So, the Coulomb interaction between sited ion and moving to this site other cation will be avoided (because effective charge of the site in question remains negative). Such a situation can be considered as a some kind of screening of the Coulomb interaction. When the "guest" ion is approached to the "host" one an atom -atom repulsion will increase. So, an equilibrium state appears when these two ions are closely situated. Let us call such ion configuration as ion-ion pair. It is clear that in a short-time period one of the ions consisting the pair will leave this polyhedron for another one due to effectively decreased potential barrier is caused by an unscreened Coulomb interaction and/or atom-atom repulsion (see fig.2).
Fig.2
Schematic magnitude
representation of potential barrier change due to ion-ion pair creation
Thus the pair in question created (see fig.3). This so the charge transfer in "effective" motion of the
o O4
is disintegrated while other pair is process will be repeated once and once the crystal can be described by an ion-ion pair (see fig.3).
o k
to+ a t to+2 At to+3 A t
Fig.3
Schematic representation of a charge
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