Magnetic field and external-pressure effect on ferroelectricity in manganites: Comparison between GdMnO₃ and TbMnO₃

We have investigated dielectric properties in Mott insulators GdMnO₃ and TbMnO₃ under magnetic fields and external quasihydrostatic pressures. In the case of GdMnO₃, thermal hysteresis for dielectric constant ε and discontinuous lattice distortion were observed at ferroelectric transition temperature (T_C), and ferroelectric spontaneous polarization was suppressed by the application of external pressure. These results indicate that the ferroelectric transition in GdMnO₃ is a first-order displacive-type one. On the other hand, the thermal hysteresis and discontinuous lattice striction were not observed at T_C in TbMnO₃. The peak of ε corresponding to the ferroelectric transition was shifted toward higher temperatures by the application of external pressure in TbMnO₃. The ferroelectric transition of TbMnO₃ was thought to be a second-order order–disorder-type one.