Phase transitions in advanced relaxor-ferroelectric materials with a perovskite-type structure

Abstract

Perovskite-type compounds of the general formula ABO3 play a key role in modern materials science. In particular, relaxor ferroelectrics are of great technological importance due to their outstanding dielectric, optoelectric, and electromechanical properties related to the complex nanoscale structure of these materials, i.e. polar nanoregions (PNR) dispersed in a paraelectric matrix. Relaxor behaviour is most commonly found in lead-based compounds with partial or complete chemical B-site disorder. Although relaxors have been extensively studied over the last decades, the relationship between chemistry, polar nanostructure, local and macroscopic properties is still not clarified and requires further structural analysis.

In order to elucidate the impact of A-site doping on the structure of relaxors, a comparative X-ray/neutron diffraction study on selected A-site doped model relaxor systems lead scandium tantalate (PST) and lead scandium niobate (PSN) was performed for the first time in this thesis. In particular, the effect of disturbed orientational order of lone-pair electrons (LPE) associated with A-site doping is analysed in regard to local elastic and local electric fields.

The detailed analysis of temperature-induced structural phase transformations highlights the importance of an undisturbed LPE system for the formation of ferroelectric long-range order (LRO). Substitution of Bi3+ for Pb2+ enhances the fraction of ferroelectrically ordered domains in PSN due to its affinity to form LPE, regardless of the additional local electric fields resulting from the aliovalent doping. In contrast, the isovalent substitution of the larger Ba2+, without LPE, for Pb2+ disturbs the LPE system of Pb2+. At the same time local elastic fields are introduced which lead to suppression of the ferroelectric LRO.

Pressure as a thermodynamical variable is a much stronger driving force as compared to temperature. Therefore, complementary high-pressure structural analysis on the same model compounds can reveal better structural peculiarities of perovskite-type relaxors. In the limited number of published high-pressure studies rearrangement and/or suppression of PNR was suggested, without however to specify the structure of the high-pressure phase in detail. This comparative study of the pure and doped PST and PSN reveals a pressure-induced structural phase transition involving elastic softening of both phases in all the samples. For the first time the high-pressure structural state of lead-based perovskite-type relaxor ferroelectrics was revealed to consist of anti-phase tilts of BO6 octahedra that continuously evolve with increasing pressure. Local elastic fields due to isovalent doping free of LPE hamper the development of structural distortions associated with the high-pressure phase whereas orientational order of LPE even enhances the distortions and lowers the critical pressure. Hence, the effect of A-dopants on the pressure-driven phase transition displays their impact on the temperature-induced structural transformations.