The high-pressure structure of (1-x)Na\(_{0.5}\)Bi\(_{0.5}\)TiO\(_3\)-xBaTiO\(_3\) 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 \(_3\) $_3^{}$ -xBaTiO \(_3\) $_3^{}$ (NBT-xBT) at the morphotropic phase boundary (MPB) ( \(x_{\text {MPB}}=0.048\) $x_{\text{MPB}}=0.048$ ) 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- \(x_\mathrm{{MPB}}\) $x_{\mathrm{MPB}}$ 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 \(x_{\text {MPB}}\) $x_{\text{MPB}}$ 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.