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

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Our modern life is unthinkable without ferroelectrics:materials with spontaneous polarization switchable under an external electric field, which exhibit large dielectric permittivity as well as strong piezoelectric, pyroelectric and optoelectric responses. The majority of advanced ferroelectrics are perovskite-type (ABO3) solid solutions possessing a morphotropic phase boundary (MPB): a critical composition x at which the symmetry of the ferroelectric long-range order changes and the properties are strongly amplified. It is well known that in the vicinity of MPB there are abundant nanoscale structural inhomogeneities. However, although the enormous efforts over the past decades to establish a generalized atomistic model of the relation composition - structure – properties, the driving mechanism of chemically induced structural instability and enhanced macroproperties near MPB is still not clarified. The quest to find environmentally friendly Pb-reduced or Pb-free alternatives of the famous PbZrxTi1-xO3 has led to an increasing interest in Bi-based perovskite-type ferroelectrics. A thorough study of the composition dependence of the local structure and atomic dynamics in novel Bibased solid solutions using Raman spectroscopy and total neutron scattering was the subject of project MI 1127/8-1. The results obtained so far reveal that the larger piezoelectric response at the MPB is due to the increased synchronization between the A- and B-site cation vibrations. The superior piezoelectric properties in specific ABO3-type solid solutions result from to the absolute flattening of the local potentials for all ferroelectrically active cations preserving relatively high polar displacements at MPB. The major goal of this continuation project is to complete the comparative study on the mesoscopic-scale transformation processes in (1-x)PbTiO3-xBiMeO3 solid solutions with Me = Mg0.5Ti0.5, Ni0.5Ti0.5, Ni0.5Zr0.5. For each series four selected compounds across the MPB are going to be analyzed by Raman spectroscopy and pair-distribution-function analyses complemented by reverse Monte Carlo simulations in a wide temperature range, including high temperatures well above the Curie temperature. The results are expected to clarify the atomistic driving forces of formation of highly disordered structural state at the MPB, as revealed from our analyses at room temperature. The comparative analyses of the renomalization phenomena will considerably improve our understanding of the microscopic origin of the dependence of the Curie temperature and piezoelectric coefficients on the type of the dopant and thus, may have critical effect on the strategy of designing novel functional materials.