THERMAL, ELECTRIC, AND DIELECTRIC-PROPERTIES OF CH3NH3SNBR3 AT LOW-TEMPERATURES

The heat capacity of CH3NH3SnBr3 was measured at temperatures between 13 K and 300 K. Four anomalies were found. Their temperatures and entropy changes are: 46.0 K, 0.37 J·K-1·mol-1, (H); 188.2K, 6.02 J·K-1·mol-1, (F); 213.0 K, (H), and 229.4 K, 18.98 J·K-1·mol-1, (F), where H and F mean higher-order and first-order transitions, respectively. The last entropy value is the sum of the entropies of the two highest-temperature transitions. The transition entropies indicate that the transition at 46.0 K is of the displacive type and those at 188.2 K and 229.4 K of the order-disorder type. The total of the transition entropies is consistent with the orientational disorder of the methylammonium ion in a cubic environment. The electric properties changed from insulating to semiconducting at the 229.4 K transition where the conductivity increased by a factor of 104 as the transition temperature was traversed from below. A thermal activation energy of 29.1 kJ·mol-1 was determined for the conductivity of the high-temperature phase. A dielectric dispersion was found below 150 K for frequencies between 20 Hz and 1 MHz. A Cole-Cole analysis of the dielectric permittivities showed that the relaxation is characterized by a strong deviation from a single Debye process. © 1991 Academic Press Limited All rights reserved.