Temperature dependence of the order parameter in NaK_1−x(NH₄)_xC₄H₄O₆.4H₂O (x=0.28)

Abstract

Long range order parameter of Rochelle salt-ammonium Rochelle salt mixed crystal NaK_1-K(NH₄)_xC₄H₄O₆.4H₂O, x=0.28 is studied from measuring X-ray Bragg intensity. Additionally the mixed crystal of x=0.27 are studied from observing ferroelectric hysteresys loop. The (050) X-ray Bragg intensity is proportional to (T_c-T) or P² _s. The results give sufficiently good agreement. The critical index β of P_s is obtained to be 0.47 from the hysteresis loop observation, which is almost equal to the value in the mean-field theory. The saturated value of the spontaneous polarization (P_s at T=0 K) is estimated to be 0.57 F. The spontaneous polarization of other mixed crystals with x=0.56, 0.83 are also measured. The results show that the saturated value of the spontaneous polarization of the mixed crystal increases with increment of x in the range about 0.28 < x < 0.90.