Electric-field-, temperature-, and stress-induced phase transitions in relaxor ferroelectric single crystals

Electric-field-induced phase transitions have been evidenced by macroscopic strain measurements at temperatures between $25\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and $100\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in ${[001]}_{C}$-poled $(1\ensuremath{-}x)\mathrm{Pb}({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}){\mathrm{O}}_{3}\ensuremath{-}x\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}\phantom{\rule{0.3em}{0ex}}[(\mathrm{PMN}\text{\ensuremath{-}}x\mathrm{PT});x=0.25,0.305,0.31]$ and $(1\ensuremath{-}x)\mathrm{Pb}({\mathrm{Zn}}_{1∕3}{\mathrm{Nb}}_{2∕3}){\mathrm{O}}_{3}\ensuremath{-}x\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}\phantom{\rule{0.3em}{0ex}}[(\mathrm{PZN}\text{\ensuremath{-}}x\mathrm{PT});x=0.05,0.065,0.085]$ single crystals. Such measurements provide a convenient way of ascertaining thermal and electrical phase stabilities over a range of compositions and give direct evidence for first-order phase transitions. A pseudorhombohedral $({M}_{A})$--pseudo-orthorhombic $({M}_{C})$--tetragonal $(T)$ polarization rotation path is evidenced by two first-order-like, hysteretic discontinuities in strain within the same unipolar electric field cycle for PZN-5PT, PMN-30.5PT, and PMN-31PT whereas, in PMN-25PT, a single first-order-like ${M}_{A}\text{\ensuremath{-}}T$ transition is observed. This agrees well with in situ structural studies reported elsewhere. Electric-field-temperature (E-T) phase diagrams are constructed showing general trends for ${M}_{A}$, ${M}_{C}$, and $T$ phase stabilities for varying temperatures and electric fields in poled samples over the given range of compositions. The complex question of whether the ${M}_{A}$ and ${M}_{C}$ states constitute true phases, or rather piezoelectrically distorted versions of their rhombohedral $(R)$ and orthorhombic $(O)$ parents, is discussed. Finally, stress-induced phase transitions are evidenced in ${[001]}_{C}$-poled PZN-4.5PT by application of a moderate compressive stress $(<100\phantom{\rule{0.3em}{0ex}}\mathrm{MPa})$ both along and perpendicularly to the poling direction (longitudinal and transverse modes, respectively). The rotation path is likely $R\text{\ensuremath{-}}{M}_{B}\text{\ensuremath{-}}O$, via a first-order, hysteretic rotation within the ${M}_{B}$ monoclinic plane. The results are presented alongside a thorough review of previously reported electric-field-induced and stress-induced phase transitions in $\mathrm{PMN}\text{\ensuremath{-}}x\mathrm{PT}$ and $\mathrm{PZN}\text{\ensuremath{-}}x\mathrm{PT}$.