Pressure and chemical doping induced phase transition and phonon dynamics of multiferroic Bi₅Ti₃FeO₁₅ ceramics

The external pressure and doping effect have been studied on the lattice structure of multiferroic Bi₅Ti_3−2xFe_1+xNb_xO₁₅ (BTFNx) ceramics. Based on the x-ray diffraction and the Raman spectroscopic evidence, it is confirmed that the doping-induced lattice deformation is closely related to the concentrations of the Nb⁵⁺ and Fe³⁺ ions. The Bi₅Ti₃FeO₁₅ (BTF) ceramic transits from orthorhombic (O, A2₁am) - O and tetragonal (T) - T (I4/mmm) with pressure, showing the intermediate phase coexistence at a wide pressure range of 4.5-14.8 GPa, while for BTFN0.6 ceramics, it is O (Fmm2) - O and T - T(I4/mmm) having the phase coexistence at 6-18 GPa. The ionic substitution, phase diagram and the evolution of phonon, lattice symmetry, and stability of BTFNx ceramics have been systematically observed and studied under the internal and external factors. Our work provides a comprehensive understanding of the structural and physical functionality for the development and optimization of multiferroic four-layer compounds BTF material systems.