Freezing of the Nb⁵+ ion dynamics in AgNbO₃ studied by linear and nonlinear dielectric response.

Linear and nonlinear dielectric measurements of AgNbO₃ ceramics and single crystals were carried out for the M phases (77-673 K). The linear dielectric response is dominated by the contribution of the submillimetre relaxational mode related to the Nb⁵+ ion dynamics (M₂-M₃). On the other hand, nonlinear dielectric χ₃' susceptibility revealed anomalies at three characteristic temperatures: 90, 325 and 448 K. Two later ones are connected with changes of the Nb⁵+ ion dynamics. At T(f) = 448 K a partial freezing of the Nb⁵+ ion displacement to the anti-polar, antiferroelectric array takes place. At 325 K further freezing of Nb and Ag displacements to the polar weak relaxor ferroelectric or dipolar glass transition occurs. This polar state coexists with the ground antiferroelectric one.