Probing the para-to-ferroelectric phase-transition mechanism in (1-x)PbTiO₃-xBiB’_0.5B”_0.5O₃

Lead-/bismuth-based perovskite-type (ABO₃) solid solutions have gained much of attention as appealing alternatives of the famous PbZr_1-xTi_xO3 due to their reduced content of Pb and yet high Curie temperature (TC) and piezoelectric coefficients near the morphotropic phase boundary (MPB). The fundamental understanding of the mechanism of formation of polar order and its dependence on the degree of doping as well as on the type of B-site dopants is critical for the effective design of novel functional materials with desired properties. To elucidate the relationship between the local polar distortions and macroscopic polarization evolving with the temperature decrease, we have studied three different (1-_x)PbTiO₃-xBiB’0.5B”0.5O3 compounds having almost the same tolerance factor and room-temperature unit-cell tetragonality as well as two solid solutions with MPB, (1-x)PbTiO₃-xBiNi_0.5Ti_0.5O₃ (PT-xBNT) and (1-x)PbTiO₃-xBiMg_0.5Ti_0.5O₃ (PT-xBMT), by Raman spectroscopy and second-harmonic generation. The results reveal the importance of B-site divalent dopants with an isotropic electron configuration for the enhancement of local symmetry breaking at the A site and a consequent increase in the Curie temperature at small x, regardless of the macroscopic tetragonality with respect to PbTiO₃ [1]. The temperature evolution of atomic dynamics in PT-xBNT [2] shows that the paraelectric-to-ferroelectric phase transition at the MPB does not involve any dynamical instability driven by a soft mode but it should be a result of thermodynamic instability in a heavily inhomogeneous system comprising competing types of ferroic distortions and a high level of orientation disorder. Comparison between PT-xBNT and PT-xBMT with respect to the temperature evolution of both macroscopic and microscopic order parameters will be presented.

Financial support by DFG (MI 1127/8-2, BA4020) is acknowledged.

[1] B. Mihailova, L. Bayarjargal, I. Margaritescu, K. Datta, J. Chen, Appl. Phys. Lett., 117, 022904 (2020).

[2] I. Margaritescu, K. Datta, J. Chen, B. Mihalova, J. Raman Spectrosc., 51, 1200-1209 (2020).