Antiferroelectric films of deuterated betaine phosphate

Thin films of partially deuterated betaine phosphate have been grown by the evaporation on Al$_{2}$O$_{3}$(110) and NdGaO$_{3}$(001) substrates with a preliminarily deposited structure of interdigitated electrodes. The grown films have a polycrystalline block structure with characteristic dimensions of blocks of the order of 0.1–1.5 mm. The degree of deuteration of the films $D$ varies in the range of 20–50%. It has been found that, at the antiferroelectric phase transition temperature $T_{c}^{\operatorname{afe}}$ = 100–114 K, the fabricated structures exhibit an anomaly of the electrical capacitance $C$, which is not accompanied by a change in the dielectric loss tangent $\operatorname{tg}\delta$. The strong-signal dielectric response is characterized by the appearance of a ferroelectric nonlinearity at temperatures $T>T^{\operatorname{afe}}_{c}$, which is transformed into an antiferroelectric nonlinearity at $T<T^{\operatorname{afe}}_{c}$. With a further decrease in the temperature, double dielectric hysteresis loops appear in the antiferroelectric phase. The dielectric properties of the films have been described within the framework of the Landau-type thermodynamic model taking into account the biquadratic coupling $\xi P^{2}\eta^{2}$ between the polar order parameter $P$ and the nonpolar order parameter $\eta$ with a positive coefficient $\xi$. The $E$–$T$ phase diagram has been constructed.