Pressure effect on ferroelectric properties of multiferroics RMn$_{2}$O$_{5}$, (R = Gd, Tm)

The pressure effect on the ferroelectric properties of the multiferroics GdMn$_{2}$O$_5$ and TmMn$_{2}$O$_5$ is studied up to 18.2 kbar. Unlike in RMn$_{2}$O$_5$ (R= Tb, Ho, Y), no significant change in polarization is observed in TmMn$_{2}$O$_5$ up to 16.6 kbar . However, a new ferroelectric phase is observed in GdMn$_{2}$O$_5$ above a critical pressure, $P_c$ = 10 kbar at higher temperature. Our result indicates that pressure decouples the Gd moment from the Mn spin system and splits the ferroelectric phase. Thermal expansion data shows a large increase of the c axis at the ambient-pressure ferroelectric transition. The pressure-induced contraction of the $c$ lattice parameter is found to be the cause for splitting of ferroelectric phase by decoupling of two spin systems above $P_c$. The pressure-temperature phase diagram is derived based on dielectric and ferroelectric properties.