The high-pressure structure of (1-x)Na Bi TiO -xBaTiO at the morphotropic phase boundary

The pressure-induced structural changes in the perovskite-type (ABO $$ ) ferroelectric solid solution (1-x)Na $$ Bi $$ TiO $$ -xBaTiO $$ (NBT-xBT) at the morphotropic phase boundary (MPB) ( $$ ) have been analyzed up to 12.3 GPa by single-crystal x-ray diffraction with synchrotron radiation. A pressure-induced phase transition takes place between 4.4 and 5.0 GPa, where the pseudocubic low-pressure phase transforms into an orthorhombic high-pressure phase with space group Pnma. The high-pressure phase is comprised of mixed BO $$ tilts and anti-polar A-cation displacements, without exhibiting coherent off-centered shifts of the B-site Ti $$ cations that can be detected by synchrotron x-ray diffraction. Our results reveal that at ambient pressure and room temperature the NBT- $$ BT structure possesses anti-phase BO $$ tilts with a relatively large correlation length and the same type of polar distortions as those present in pure NBT, but with strongly violated correlation length due to Ba $$ -induced local elastic-stress fields. For $$ the effect of Ba on the mesoscopic-scale structure is compensated by a mild external pressure of only 0.7 GPa, resulting in structural features resembling those of pure NBT at ambient conditions.