Phase Transitions in (CH$\text{}_{3}$NH$\text{}_{3}$)$\text{}_{5}$Bi$\text{}_{2}$X$\text{}_{11}$ (X=Cl, Br) Crystals

Electric properties of (CH$\text{}_{3}$NH$\text{}_{3}$)$\text{}_{5}$Bi$\text{}_{2}$Cl$\text{}_{11}$ and (CH$\text{}_{3}$NH$\text{}_{3}$)$\text{}_{5}$Bi$\text{}_{2}$Br$\text{}_{11}$ single crystals in the direction of the crystallographic b-axis were studied. The samples tested were natural crystal seeds in the form of thin plates, whose crystallographic b-axis coincides with the normal to the plate. Measurements of the pyroelectric coefficient were performed near the high temperature phase transition and spontaneous polarization from the paraelectric phase down to helium temperature. The attempt of explanation of the anomalous electric properties of these crystals has been presented