Pressure-driven Structural Transformation in the 9R Rhombohedral Phase of BaMn0.85Ti0.15O3

The high-pressure behavior of the 9R rhombohedral polymorph in BaMn0.85Ti0.15O3 was investigated using synchrotron X-ray diffraction and Raman spectroscopy under compression up to 20.7 and 32.6 GPa, respectively. The 9R phase remains stable under pressure but exhibits anisotropic lattice compression, with the c-axis being less compressible due to intrinsic structural anisotropy. An isostructural transformation occurs around 9 GPa, as evidenced by anomalies in the pressure dependence of the lattice parameters, bulk modulus, and Raman vibrational modes. This transformation is attributed to pressure-driven rearrangements in the local bonding environment within the Mn/Ti–O octahedral framework, primarily involving Mn/Ti vibrations.