Phase Transitions and Ferroelectricity in {N(CH3)4}2XBr4 (X: Zn, Co, Cu)

The phase transitions in {N(CH 3 ) 4 } 2 XBr 4 (X: Zn, Co, Cu) were stuided by dielectric measurements. A phase transition which accompanied with a break on the dielectric constant vs temperature curve was found at 13.5°C and 13.8°C for {N(CH 3 ) 4 } 2 ZnBr 4 and {N(CH 3 ) 4 } 2 CoBr 4 , respectively. In {N(CH 3 ) 4 } 2 CuBr 4 three phase transitions were found at -1°C, -31°C, and -36°C on cooling. In the temperature range between -31°C and -36°C the crystal shows ferroelectricity along the pseudo-hexagonal c -direction. The spontaneous polarization is about 7×10 -8 C cm -2 .