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Mesoscopic-scale transformation processes in Bi-based perovskite-type ferroelectric solid solutions

Subject Area Mineralogy, Petrology and Geochemistry
Experimental Condensed Matter Physics
Term from 2015 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 269878195
 
Final Report Year 2020

Final Report Abstract

The goal of project was to explore the mesoscopicscale aspects of the mechanism of formation of polar order in Bi-based solid solutions with morphotropic phase boundary (MPB) and its dependence on the degree and type of doping. For this purpose a pair-distribution-function analysis along with reverse Monte-Carlo modeling as well as Raman spectroscopy was applied to (1-x)PbTiO3-xBiB’0.5B”0.5O3 (PTxBB’B”) with B’0,5B”0.5=Mg0.5Ti0.5, Ni0.5Ti0.5, Ni0.5Zr0.5 and (1-x)Na0.5Bi0.5TiO3-xBaTiO3 (NBTxBT) with x < 0.1. The compositional dependence of the intermediate-range atomic structure and dynamics at room temperature revealed the key role of three main factors for the enhanced piezoelectric response at the MPB of both PT-xBB’B” and NBT-xBT: (i) suppressed anisotropy in the local potentials at both A and B sites; (ii) high degree of orientation disorder, consisting in a reduction of the correlation length of the local dipoles within the same cationic sublattice; (iii) dynamical synchronization of adjacent A-site and B-site dipoles. The in situ studies of the temperature-driven transformation processes of PT-xBB’B” revealed that: (i) the room-temperature structure of PT-based solid solutions at the MPB is a chemically-stabilized analogue of the high-temperature state of PT, which comprises local ferroic distortions; (ii) the incorporation of BB’B” into the PT host matrix introduces a new energetic state of the A-site cations and strongly enhances the order-disorder phenomena above and across TC; (iii) The MPB is distinct by the unchanged correlation length among coherent off-centred displacements of the A-site cations at TC and competing local-scale polar and antiferodistortive orders related to the B-site sublattice below TC, i.e. the paraelectric-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. The in situ studies of NBT-xBT single crystals at different temperatures and electric fields E showed that: (i) the formation of ferroelectric state on cooling is a pure manifestation of increasing correlation length among coupled local polar shifts; (ii) the structural state between the paraelectric and ferroelectric zones in the x-T diagram is weak ferrielectric in nature; (iii) the enhanced longitudinal piezoelectric coefficient in alkali-Bi based solid solutions with a pseudocubic structure near MPB is related to the composition-driven reduction of the local strains and the consequent enhancement of the structural flexibility under external stimuli.

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