Relationship between negative thermal expansion and lattice dynamics in a tetragonal PbTiO₃-Bi(Mg_1/2Ti_1/2)O₃ perovskite single crystal

A range of compelling information on the thermal expansion behavior and lattice dynamics of a novel ferroelectric perovskite-type 0.62PbTiO₃-0.38Bi(Mg_1/2Ti_1/2)O₃ single crystal has been revealed by means of temperature-dependent X-ray diffraction and polarized Raman scattering. X-ray diffraction analysis suggests a strong ferroelectricity and structural change (tetragonal to cubic) upon heating. The A₁(1TO) soft mode, associated with the order parameter spontaneous polarization displacement, abnormally shifts downward and vanished, indicating the change in polar nanoregions and phase transition. Both the temperature dependent unit cell volume and frequency of the soft mode confirm the correlation between spontaneous polarization and negative thermal expansion. The experimental evidence of spontaneous volume ferroelectrostriction will further help in understanding the relationship between the structure and optical properties of perovskite-type ferroelectric oxides.