  We combine high-pressure x-ray diffraction, high-pressure Raman scattering,
and optical microscope to investigate a series of PMN-xPT 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-1 above 6 GPa for all
samples, in agreement with previous observations. X-ray diffraction
measurements are consistent with this spectral change indicating structural
phase transition; we find that the triplet at the pseudo cubic (220) Bragg peak
merges into a doublet above 6 GPa. Our results indicate that the morphotropic
phase boundary region (x=0.33 to 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 domains disappear at a
modest pressure of 3 GPa. We propose a pressure-composition phase diagram for
PMN-xPT solid solutions.
