A second ferroelectric transition induced by pressure in multiferroic GdMn$_{2}$O$_{5}$*

In multiferroic materials different magnetic and ferroelectric states compete for the ground state and are usually close in energy. This results in a high sensitivity of the multiferroic state to external perturbations, such as magnetic fields or pressure, which has been demonstrated, e.g. in Ni$_{3}$V$_{2}$O$_{8}$, MnWO$_{4}$, and RMn$_{2}$O$_{5}$ (R $=$ Tb, Ho, Dy, Y). Here we report the results of a high-pressure study of the multiferroic and ferroelectric properties of GdMn$_{2}$O$_{5}$, the RMn$_{2}$O$_{5}$ compound with the highest value of the polarization. The ferroelectric polarization below 29 K is enhanced upon application of pressure. Above a critical pressure, a second ferroelectric transition at even higher temperature is detected through a sizable increase of the polarization and a second peak of the dielectric constant. The ferroelectric polarization at high pressures exhibits two step-like increases upon decreasing temperature.