Phase transitions in the ferroelectric relaxor (1−x)Pb(Mg1/3Nb2/3

Structural transformations of unpoled $(1\ensuremath{-}x)\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3\ensuremath{-}x}\mathrm{PbTi}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{PMN}\text{\ensuremath{-}}\mathrm{PT})$ single crystals ($x=0.28$ and $x=0.32$) were analyzed by calorimetry and acoustic spectroscopy. The compositions are close to the morphotropic boundary (MPB). We find extended two-phase regions where cubic and rhombohedral ($x=0.28$), cubic and tetragonal, tetragonal and monoclinic phases ($x=0.32$) coexist during cooling and heating. The observed structural transitions in PMN-PT are first order, despite their proximity to the MPB. The predicted tricritical points, if they exist, are possibly located extremely close to the MPB.