Pressure dependence of the monoclinic phase in(1−x)Pb(Mg1/3Nb

We combine high-pressure x-ray diffraction, high-pressure Raman scattering, and optical microscopy to investigate a series of ($1\ensuremath{-}x$)Pb(Mg${}_{1/3}$Nb${}_{2/3}$)O${}_{3}$-$x$PbTiO${}_{3}$ (PMN-$x$PT) solid solutions ($x=0.2$, 0.3, 0.33, 0.35, 0.37, 0.4) in diamond anvil cells up to 20 GPa at 300 K. The Raman spectra show a peak centered at 380 cm${}^{\ensuremath{-}1}$ starting above 6 GPa for all samples, in agreement with previous observations. X-ray diffraction measurements are consistent with this spectral change indicating a structural phase transition; we find that the triplet at the pseudocubic (220) Bragg peak merges into a doublet above 6 GPa. Our results indicate that the morphotropic phase boundary region ($x=0.33\ensuremath{-}0.37$) with the presence of monoclinic symmetry persists up to 7 GPa. The pressure dependence of ferroelectric domains in PMN-0.32PT single crystals was observed using a polarizing optical microscope. The domain wall density decreases with pressure and the domains disappear at a modest pressure of 3 GPa. We propose a pressure-composition phase diagram for PMN-$x$PT solid solutions.