Using a special lock-in amplifier, the surface and volume conductivities of three kinds of high insulators are measured from -20℃ to 180℃. Hopping and band conductions of conductive polarons are observed in LiNbO3, the transition temperature between the two- conductive states is about 38℃. A conductive polaron can be captured by a trap and becomes a bound polaron. The bound polaron is not an electric carrier, but under the action of external field, its position may be changed to give an ultralow frequency polarization effect. There are many ferroelectric screenning charges on the surface of LiNbO3 and LiTaO3, however, their conductive currents are not so large as those of glass. But the screenning charges supply very large displacement currents. The fact shows that these screenning charges are in bound polaron state. The phenomenon of that the displacement current shows anomalous at about 45, 75, and 130℃ is observed in LiNbO3.
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