Superionic conductors: Transitions, structures, dynamics

The diffusive part and the oscillatory part (optically active lattice vibrations) of the conductivity in an ionic solid are related by an oscillator strength sum rule. In a superionic conductor a non negligible fraction of the total oscillator strength is in the diffusive part. A simple model is introduced which includes both diffusive and oscillatory part, takes account of the sum rule and is able to describe the full range from an insulating ionic solid to a free ion system. The model is applied to the frequency dependent conductivity σ(ω) of AgI and β-alumina at various temperatures. σ(ω) is determined from far infrared (FIR) reflectivity experiments. The importance is shown of such measurements for screening of new potential superionic conductors. FIR experiments are not hampered by contact, polarization, and grain boundary problems. A high conductivity at frequencies below the reststrahlen bands is thus a necessary condition for a high dc conductivity.



Der diffusive und der oszillatorische Anteil (infrarotaktive Gitterschwingungen) der Leitfahigkeit eines Ionenleiters sind durch eine Oszillator-Summenregel miteinander verknupft. In einem Supra-Ionenleiter ist ein nicht vernachlassigbarer Bruchteil der totalen Oszillatorstarke im diffusiven Anteil enthalten. Ein einfaches Modell wird eingefuhrt, welches sowohl das diffusive als auch das oszillatorische Verhalten enthalt, die Summenregel erfullt, und den ganzen Bereich vom isolierenden Ionenkristall bis zum freien Ionensystem beschreibt. Das Model wird auf die frequenzabhangige Ionenleitfahigkeit σ(ω) wurde durch Reflexionsmessungen im fernen Infrarot (FIR) bestimmt. Auf die Wichtigkeit solcher Messungen fur das Aufsuchen von neuen potentiellen Supraionenleitern wird hingewiesen. Bei FIR-Experimenten an Einkristallen treten keine Kontakt-, Polarisations- und Korngrenzen-Probleme auf. Eine hohe Leitfahigkeit bei Frequenzen unterhalb der Reststrahlen-Banden ist eine notwendige Bedingung fur eine hohe Gleichstrom-Leitfahigkeit.

Frequency-dependent conductivity and dielectric function of superionic conductors

The crystal structures of the β-alumina compositions have been described and used to explain the fast ion transport for which these materials are renowned. Measured values of both the single crystal and polycrystalline ionic conductivity show a wide variation; this is explained in terms of the range of chemical compositions of the β-alumina system and also the variety of measuring techniques used. Dopants or impurity ions can have a significant effect on the physical properties of the β-aluminas. The ionic conductivity, the stability of the material and the densification during sintering have been considered in relation to the nature and level of a range of dopants described in the literature. The optimization of the ionic and mechanical properties has been achieved by development of the fabrication techniques and it is this which accounts for much of the present research. Thus the many different methods of producing both single and polycrystalline material have been described, including the range of sintering routes currently available. The advantages and disadvantages of each production route in terms of the resulting properties have also been discussed.

Structure, properties and production of ?-alumina

Structural determinations of single-crystal K β-alumina and cobalt-doped K β-alumina

Structural investigation of K + and Tl + β-aluminas

The organization of the conducting ions is studied by means of X-ray diffuse scattering as a function of temperature. While at high temperature (750°K) the conducting ions are organized in a two-dimensional 'quasi-liquid' structure perpendicular to the c axis, at low temperature (77°K) they form micro-domains corresponding to a two-dimensional superstructure with a lattice constant of a√3 = 9.68 A (a = lattice constant of β alumina). The size of these micro-domains, which is about 50 A at 77 °K, decreases progressively with increasing temperature.

Y. Le Cars

Papers

Organisation des cations assurant la conduction dans l'alumine β