Pyroelectric and Dilatometric Studies on the Low Temperature Ferroelectric Phase Transition of (CH$\text{}_{3}$NH$\text{}_{3}$)$\text{}_{5}$Bi$\text{}_{2}$Br$\text{}_{11}$

The results of the linear thermal expansion studies on (CH$\text{}_{3}$NH$\text{}_{3}$)$\text{}_{5}$Bi$\text{}_{2}$Br$\text{}_{11}$ single crystals in the temperature range 60-340 K are presented. The thermal anomaly in the vicinity of 77 K is assigned to the phase transition classified as close to first order type. The pyroelectric measurements of (CH$\text{}_{3}$NH$\text{}_{3}$)$\text{}_{5}$Bi$\text{}_{2}$Br$\text{}_{11}$ were carried out in the range of 45-320 K along the a- and c-axis. The step-like change in the spontaneous polarization along the a-axis at 77 K indicates a discontinuous character of ferroelectric transition