Flattening of the electric permittivity curve ε(T) of NaNbO3:yMn single crystals caused by stress application

A DTA test, performed within the 300-700 K range, showed that the series of NaNbO3:yMn single crystals exhibit first-order phase transition known from pure sodium niobate. The introduced Mn dopant lowers the temperature of this phase transition at a rate DeltaT(P-R)/Delta%(Mn) approximate to -15 K wt%(-1). The electric permittivity measured at ambient pressure showed a clear Curie-Weiss maximum at this antiferroelectric phase transition. The applied axial compression caused gradual suppression of this anomaly towards step-like characteristics. Flat epsilon(T, X, y) curves appeared when the axial pressure X exceeded a few hundreds bars. The stress field was calculated around the Mn ions built into the host. It was pointed out that due to this the stress field and external compression are lower than misfit stress estimated for the P-R structural phase transition and any new phase was not induced with dopant manganese ions at the obtained real concentration y level. The recorded epsilon(T, X, y) characteristics were fitted with a generalized Curie-Weiss formula. It was shown that the molecular field approximation remains valid. Both linear and nonlinear elastic effects were necessary to describe the experimental epsilon(T, X, y) characteristics.