Single crystals of $(1\text{−}x){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_3\text{−}x{\mathrm{BaTiO}}_3$ $\left(\mathrm{NBT}\text{−}x\mathrm{BT}\right)$ 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 $x_{\mathrm{MPB}}=0.048$ [Sci. Rep. 14, 18799 (2024)], as the critical pressure $p_{\mathrm{c}}$ gradually increases with $x$. However, the pathway of developing the high-pressure 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 $x<x_{\mathrm{MPB}}$ the ${\mathrm{Ba}}^{2+}$ cations are preferentially accommodated in the tetragonal-type nanodomains existing in the dominant pseudorhombohedral NBT matrix, whereas for $x>x_{\mathrm{MPB}}$, ${\mathrm{Ba}}^{2+}$ 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 $p_c$.

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• Crystal structure
• Doping effects
• Ferroelectricity
• High-pressure studies
• Structural phase transition
• Ferroelectrics
• Crystal structures
• Pressure effects
• Raman spectroscopy
• X-ray diffraction
• X-ray diffuse scattering