Order–disorder behavior of ferroelectric phase transition of KTa1−xNbxO3 probed by Brillouin scattering

Abstract

The precursor dynamics of a cubic-tetragonal ferroelectric phase transition of potassium tantalate niobate ͑KTa 1−x Nb x O 3 with x = 0.32͒ crystals is studied by Brillouin scattering. The appearance of the central peak ͑CP͒ and marked softening of the elastic constant c 11 are clearly observed above the Curie temperature, T C-T , owing to the interaction between the LA mode and local polarization fluctuations of polar nanoregions ͑PNRs͒. The relaxation time determined by the CP width clearly shows a critical slowing down above T C-T , indicating an order-disorder feature of the ferroelectric phase transition. The size of a dynamic PNR is evaluated, and it increases toward T C-T .

Fano resonance of Li-doped KTa1−xNbxO3 single crystals studied by Raman scattering

“…As can be seen in Fig. 3b, the A CP begins to increase below the intermediate temperature, T * ∼ 100 °C, upon cooling, reflecting the sudden growth of the volume fraction of PNRs101121. In Fig.…”

“…In contrast, the PNRs can be induced in KTN owing to the off-center displacements Nb ions at the B-site. In KTN and Li-doped KTN, the local polarization fluctuations of PNRs itself are much slower than that of the TO 2 phonon frequency, and appear as the CP in a GHz frequency range101121. Furthermore, it is significant that the value of the q gradually increased towards the T C-T and became the maximum in the ferroelectric phase, where the local polarization fluctuations were totally absent.…”

“…A light scattering study also explained the off-center displacements of Nb ions in KTN by eight site model, in which Nb ions displace among the equivalent [111] directions9. As widely reported by various kinds of measurements1011121314151617, one off-center displacement interacts with the neighboring off-center displacements, leading to the local polar structure called PNRs in the paraelectric cubic phase of KTN crystals. The PNRs are characterized by various temperatures in the cubic phase.…”

“…First, the Burns temperature, T B 18, below which the dynamic PNRs appear. Second, the intermediate temperature, T *, at which the dynamic PNRs start to transform into static PNRs101112.…”

“…In KTN, the dynamical aspect of PNRs was extensively studied by inelastic light scattering1011131422. However, the microscopic origin of the Fano resonance at around 196 cm −1 in Li-doped KTN still remains fuzzy.…”