  We have investigated dielectric properties in Mott insulators GdMnO$_{3}$ and
TbMnO$_{3}$ under magnetic fields and external quasihydrostatic pressures. In
the case of GdMnO$_{3}$, thermal hysteresis for dielectric constant $\epsilon$
and discontinuous lattice distortion were observed at ferroelectric transition
temperature ($T_{C}$), and ferroelectric spontaneous polarization was
suppressed by application of external pressure. These results indicate that the
ferroelectric transition in GdMnO$_{3}$ 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 TbMnO$_{3}$. The peak of $\epsilon$
corresponding to ferroelectric transition was shifted toward higher
temperatures by application of external pressure in TbMnO$_{3}$. The
ferroelectric transition of TbMnO$_{3}$ was thought to be a second-order
order-disorder-type one.
