Phonon studies of the phase transition sequence in antiferroelectric single crystal of Pb(Hf0.83Sn0.17)O3

The sequence of phase transitions in PbHf0.83Sn0.17O3 has been studied by THz, far infrared and Raman spectroscopies, revealing the complementary behaviour of both, polar and non-polar phonons and their impact on the transition lattice dynamics. Pb atom is sensitive to all phase transitions, changing its dynamics with temperature. As temperature decreases, the crystal undergoes a sequence of three phase transitions. The first one to an intermediate (IM) phase, in which polar fluctuations are detected by THz and IR spectroscopy at frequencies below 100 cm-1 contributing to the maximum of permittivity and revealing important softening. Additional antipolar Pb fluctuations and softening were detected by Raman spectroscopy. At lower temperature, another transition to an antiferroelectric (AFE2) phase is revealed by nonpolar soft modes (antipolar and antiferrodistortive ones), and by an important drop of the dielectric strength of polar phonons. The final transition to the antiferroelectric phase (AFE1) is revealed by the appearance of new modes and a sudden change in the frequency of the soft modes when the antipolar shifts become larger. Using symmetry analysis and optical observation to study how the domain pattern changes with temperature, we identified a path for the cubic-AFE2 transition throughout the IM phase, of plausible tetragonal symmetry, driven by an instability from the center of the Brillouin zone. This mechanism coexists with antiferrodistortive instabilities that eventually drive the material into the AFE2 phase. The final phase transition to the AFE1 phase naturally follows from a mode outside the center of the Brillouin zone and a further doubling of the unit cell.