Polarization in LaF3

We report the results of measurements of the low-frequency dielectric behavior and of persistent polarization in single-crystal LaF3. This ionic conductor exhibits unusually large polarization effects. The measurements distinguish at least two components of polarization. One component, with a time constant of the order of milliseconds, is linear in the applied voltage; the long-term polarization is proportional to its square. The phenomena are evidently associated with surface effects, but they cannot be described by the accepted space-charge theory of ionic conductors. They appear, however, to be qualitatively consistent with an extension of the notion of ionic surface states introduced by Frenkel.