  We have investigated the dielectric properties of Gd$_{1-x}$Tb$_{x}$MnO$_{3}$
crystals in magnetic fields to clarify the crossover between two distinct
ferroelectric phases, GdMnO$_{3}$-type (first-order displacive) and
TbMnO$_{3}$-type (second-order order-disorder). We have found that the
compositional phase boundary between the phases exists around 0.3$<$$x$$<$0.4.
For the $x$=0.1 sample exposed to magnetic fields above 3T, we have discovered
successive ferroelectric phase transitions that occur at two different
temperatures. We have determined the electric and magnetic phase diagram for
$x$$=$0, 0.1, and 0.3 in the plane of temperature and field in order to
demonstrate the change in the ferroelectric phases.
