Successive phase transitions in single-crystalline (C2H5NH3)(2)CuCl4 and potential of multiferroicity

Various experiments, such as X-ray diffraction, polarization microscopy, relative permittivity, spontaneous electric polarization and ferroelectric loop measurements, have been performed on single crystals of a perovskite-type layered compound (C2H5NH3)2CuCl4. The crystal structure of this compound at 273 K and 100 K is determined as Abma and Pbca, respectively. In the polarization microscopy experiments, disappearance of the ferroelastic domain under a stress has been found at 370 K, suggesting that the crystal structure above 370 K is tetragonal (I4∕mmm). We have found, for the first time, a remarkable anomaly at T C = 25.8 K in the temperature dependence of the relative permittivity. In D-E hysteresis measurements, the ferroelectric loop opens below T C, where D is the electric displacement field and E is the electric field, respectively. The remnant polarization reaches 8 μC/m2 at 5 K. These results indicate that there is a weak ferroelectric ordering at T C. The clear D-E hysteresis curve is detected below T N = 10 K, which is a weak ferromagnetic transition temperature, suggesting that the weak ferroelectric and the weak ferromagnetic states coexist below T N.