The ferroelectric phase transition of mixed crystal systems of Rb 2 SO 4 –(NH 4 ) 2 SO 4 and CS 2 SO 4 –(NH 4 ) 2 SO 4 was studied by measuring the lattice constants, the dielectric constant, the spontaneous polarization and the proton spin-lattice relaxation time with functions of temperature and concentration of Rb + ion and Cs + ion. With the increase in the concentration of the ions in both mixed crystal systems, the following changes occur. (1) The ferroelectric phase transition temperature increases a few degrees until it reaches a certain concentration and decreases sharply. (2) The Curie-Weiss constant increases. (3) The phase transition changes from the first order to the second order at the certain concentration. (4) The maximum value of the spontaneous polarization increases about 10% until it reaches the certain concentration and decreases gradually. The results are discussed with a two-nonequivalent sublattice model.
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Ohi et al. (1978) studied this question.
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