Abstract :
Frequency-dependent conductivities of the
two-dimensional sodium-ion conductor Na-b¢¢-alumina,
of composition Na1.70Li0.32Al10.66O17, have been taken
between 100 MHz and several THz, at different temperatures.
In their low-frequency sections, the spectra
are affected by the polycrystallinity of the sample, i.e.
by inter-grain polarization, while the vibrational component
prevails in the far infrared. In a first step, these
two contributions are identified and removed from the
spectra. In a second step, the remaining frequencydependent
conductivities are explained in terms of the
displacive movements performed by the sodium ions
within the crystal volume. Here, two kinds of motion
are distinguished. One is translational via vacant local
residence regimes, establishing macroscopic transport.
The other is localized within a residence regime. The
experimental conductivities are well reproduced by
model spectra. The values of the parameters used agree
with the available structural information.
