Pressure- and temperature-induced transformation processes in PbTiO$_3$ − Bi$Me$O$_3$ ferroelectrics

The structural response to high pressures as well as to high temperatures of (1−𝑥)⁢PbTiO$_3$−𝑥⁢Bi⁡(Zn$_{0.5}$⁢Ti$_{0.5}$⁢)⁢O$_3$ with 𝑥=0.08, (1−𝑥)⁢PbTiO$_3$−𝑥⁢Bi⁡(Mg$_{0.5}$⁢Ti$_{0.5}$)⁢O$_3$ with 𝑥=0.17, and PbTiO$_3$ single crystals was studied by in situ pressure-/temperature-dependent polarized Raman spectroscopy, complemented by synchrotron x-ray diffraction analysis at ambient pressure and different temperatures. The compositional and pressure dependencies of phonon anomalies indicate the existence of local-scale antiferrodistortive structural entities inside the perovskite (ABO$_3$) polar tetragonal matrix, the fraction of which is enhanced by the substitution of Bi⁢𝑀⁢𝑒⁢O$_3$ for PbTiO$_3$. The type of cation (Zn$^{2+}$ versus Mg$^{2+}$) replacing Ti$^{4+}$ at the B site affects the coherence within the dominant single-perovskite polar tetragonal matrix and the coupling between the “defect” double-perovskite antiferrodistortive entities. As a result, on increasing pressure, the partial substitution of Bi⁡(Mg$_{0.5}$⁢Ti$_{0.5}$)⁢O$_3$ triggers a structural instability at lower pressure values as compared to pure PbTiO$_3$, whereas that of Bi⁡(Zn$_{0.5}$⁢Ti$_{0.5}$)⁢O$_3$ preserves the polar tetragonal phase over a wider pressure range. Furthermore, at ambient pressure B-site Zn$^{2+}$ favors the coherence between local BO$_6$ tetragonal distortions already above the Curie temperature 𝑇$_𝐶$, causing a larger unit-cell tetragonality below 𝑇$_𝐶$; in contrast, B-site Mg$^{2+}$ reduces the local BO$_6$ anisotropy and disturbs the correlation between polar AO$_{12}$ tetragonal distortions below 𝑇$_𝐶$, which results in a considerable reduction of both the unit-cell tetragonality and accumulated volume strain on cooling between 𝑇$_𝐶$ and room temperature.