Intermediate phases in [111]- and [001]-oriented PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$–29PbTiO$_{3}$ single crystals

Phase transformations in [111]- and [001]-oriented PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$–29PbTiO$_{3}$ single crystals have been studied using dielectric and optical measurements before and after applying an electric field. It is shown that the subsequence of phase transitions rhombohedral $(R)$–tetragonal $(T)$–cubic $(C)$ phases is observed in nonpolarized samples of both orientations as temperature increases. In the [111]-oriented crystal, an additional intermediate monoclinic phase (it is possible, $M_a$ ) is induced after preliminary polarization at room temperature and the $R$–$M_{a}$–$T$–$C$ phase transitions are observed on heating. In the [001]-oriented crystal, after its polarization, the monoclinic phase forms instead of the rhombohedral phase even at room temperature and the $M_a$–$T$–$C$ transitions occur on heating. The results are discussed from the point of view of the existence polar nanoregions with different local symmetries in a glasslike matrix.