Ferroelectric and Optical Properties of Na(K-NH4)-tartrate Mixed Crystals

The phase diagram of the mixed crystals composed of NaK-tartrate and its isomorph NaNH 4 -tartrate has been investigated over the whole range of the concentration. The properties of each phase have been studied by dielectric and pyroelectric measurements and by optical observations. In 90.5 per cent NaNH 4 -tartrate crystal three kinds of polymorphic modification have been found, that is, successively, a ferroelectric phase with spontaneous polarization along the crystallographic b axis, another ferroelectric phase with spontaneous polarization along the crystallographic a axis and a paraelectric phase. The phase of compositions near the pure NaNH 4 -tartrate has been confirmed to be ferroelectric with spontaneous polarization along the b axis.