The field-induced ferroelectric phase of sodium niobate

Abstract

Antiferroelectric sodium niobate (a=5.57, b = 15.54, c=5.51 A) can be changed into ferroelectric sodium niobate (a =5.57, b = 7.77, c =5.51 /~) by the application of an electric field perpendicular to the b axis. The 'transition field' is about 50 kV. per cm. When the field is removed, the field-induced structure may then coexist in a single crystal with the unaltered part, not subjected to the field. With crystals of a particular orientation, changes in orientation of the field-induced area can be observed both by X-ray diffraction techniques and with the polarizing microscope. The combination of two related types of twinning in the ferroelectric crystal makes possible the optical detection of the ferroelectric direction. In the potassium niobate-sodium niobate system there are three phases, most readily distinguished by the length of the b axis which is successively 15.54, 7"77 and 3.89 /~ (4 ×, 2 x and 1 x the edge of the perovskite-type unit) with increasing potassium content. The relation of these phases to the field induced phase in potassium-free sodium niobate is discussed.