The phase diagram of (NH 4 ) 2 SO 4 -K 2 SO 4 mixed crystal system was obtained from dielectric and pyroelectric study. It was found that the ferroelectric phase transition temperature T c decreases with the increase of concentration of K + ions, and ferroelectric phase disappears if mixed crystals contain more than 70% of K 2 SO 4 . It was also found that the spontaneous polarization of mixed crystals with low concentration of K + ion (up to 30%) increases first to maximum, and then gradually decreases as the temperature is lowered. These experimental observations suggest that in this system single homogeneous polarization may not be an order parameter and the dipole arrangements of this system may be not ferroelectric but ferrielectric as was first pointed out by Unruh.
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Sawada et al. (1975) studied this question.
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