Structural Transformations in the (1-X)na0.5bi0.5tio3-xbatio3 Solid Solution under High Pressure: Comparison Between X below and above the Morphotropic Phase Boundary

Single crystals of (1-x)Na0.5Bi0.5TiO3-xBaTiO3 (NBT-xBT) with a composition below (x = 0.013) and above (x = 0.074) the morphotropic phase boundary (MPB) were studied by in situ high-pressure x-ray diffraction with synchrotron radiation and Raman spectroscopy. We show that both compounds undergo a reversible pressure-induced phase transition to structures with Pnma symmetry, regardless of their symmetry at ambient conditions. The high-pressure phase resembles that recently reported for x(MPB) = 0.048 [Sci. Rep. 14, 18799 (2024)], as the critical pressure pc gradually increases with x. However, the pathway of developing the highpressure phase varies with the composition, revealing distinct ways of Ba incorporation into the NBT matrix for the pseudorhombohedral (x = 0.013) and tetragonal (x = 0.074) phases. For x < x(MPB) the Ba2+ cations are preferentially accommodated in the tetragonal-type nanodomains existing in the dominant pseudorhombohedral NBT matrix, whereas for x > x(MPB), Ba2+ is dispersed in both the rhombohedral nanodomains and the dominant tetragonal matrix. Furthermore, for x = 0.074 there are incommensurate structural modulations at ambient pressure, which up to p(c) remain unaffected by pressure, but vanish above p(c).