Electric-field poling effect on thermal stability of monoclinic phase in a (110)-oriented Pb(Mg$_{1/3}$Nb$_{2/3})_{0.74}$Ti$_{0.26}$O$_{3}$ crystal

Temperature-dependent phase transformations in unpoled and poled samples cut from a (110)-cut Pb(Mg$_{1/3}$Nb$_{2/3})_{0.74}$Ti$_{0.26}$O$_{3}$ (PMNT26{\%}) single crystal have been investigated by polarizing microscopy and dielectric permittivity. X-ray diffraction (XRD) at room temperature (RT) was also measured. Poling was done at RT at various fields below and above the coercive field ($\sim $3 kV/cm). XRD and polarizing microscopy at RT indicate that the unpoled sample has a dominant rhombohedral ($R)$ phase coexisting with some monoclinic ($M)$ phase, i.e. $R(M)$, whereas the poled sample has coexistence of $R$ and$ M$ phases, i.e. $R/M$, in which the fraction of the induced $M$ phase increases with poling strength. In both unpoled and poled samples, continuous polarization rotation with increasing temperature via a monoclinic phase was revealed by polarizing microscopy from $\sim $360 K to 375 K, the same range in which a dielectric anomaly occurs in the poled sample. The crystal phase becomes cubic ($C)$ at $\sim $395 K. Thus, the phase transition sequence is $R(M)\to M\to C$ in the unpoled sample and $R$/$M\to M\to C$ in the poled sample.