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The phase transition of glycine phosphite (GPI) and partially deuterated glycine phosphite (DGPI) single crystals has been characterized by applying the effective mean field theory. Temperature dependence of the spontaneous polarization P s(T) has been analyzed from hysteresis loops for these crystals. With the increment of deuteration ratio, saturated spontaneous polarization P s 0 is also increased. As a result, the quantitative phase transition indicator g according to the mean field theory is being increased but the maximum increment is underlying the decisive tricritical point g = 1/3. The phase transition character of GPI crystal is proved to be second order but very near to the tricritical point.

In order to investigate the effect of deuteration on the ferroelectric properties of glycine phosphite, single crystals with various deuteration ratio x (0 ∼ 0.78) have been prepared. The crystal form is not affected by the deuteration. Internal bias fields influenced hysteresis loops in all samples cut from crystals. Temperature dependence of permittivity of all samples shows a dielectric anomaly characteristic of the second order ferroelectric phase transition. The reciprocal permittivity strictly fulfills the Curie-Weiss low in each paraelectric phase. The transition temperature T C increases with the ratio x from 224K ( x = 0) to 320K ( x = 0.78). The transition temperature of fully deuterated crystal ( x = 1) is roughly estimated to be about 350K. The Curie constant is slightly affected by the deuteration.