The high-pressure structure of (1-x)Na0.5Bi0.5TiO₃-xBaTiO₃ at the morphotropic phase boundary

The pressure-induced structural changes in the perovskite-type (ABO ₃ 3 ) ferroelectric solid solution (1- x )Na 0.5 0.5 Bi 0.5 0.5 TiO ₃ 3 - x BaTiO ₃ 3 (NBT- x BT) at the morphotropic phase boundary (MPB) ( xMPB=0.048 x MPB = 0.048 ) 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 ₆ 6 tilts and anti-polar A-cation displacements, without exhibiting coherent off-centered shifts of the B-site Ti ⁴⁺ 4 + cations that can be detected by synchrotron x-ray diffraction. Our results reveal that at ambient pressure and room temperature the NBT- x_MPB x MPB BT structure possesses anti-phase BO ₆ 6 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 ²⁺ 2 + -induced local elastic-stress fields. For xMPB x MPB 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.