Temperature- and pressure-induced structural transformations in PbTiO3-BiMeO3 ferroelectric single crystals

Abstract

Perovskite-type ferroelectric materials with the general formula ABO3 are essential for a wide range of technological applications. Several industries, including aerospace, automotive, and energy sectors, are experiencing a rising demand for ferroelectric materials that exhibit high piezoelectricity and maintain their stability under extreme conditions. PbTiO3-BiMeO3 ferroelectric systems, where Me can be either a single trivalent cation or a mixture of cations with an average valence of 3+, have attracted increasing attention over the past years due to their outstanding piezoelectric properties and high Curie temperatures. However, the influence of the substitutional disorder on the processes occurring at different length scales in PbTiO3-BiMeO3 solid solutions is still not fully understood. Particularly high-pressure studies, which can provide a more comprehensive insight into the chemically-induced local elastic strains and the associated ferroic distortions, are very limited for these systems. To better understand the effect of partial substitution at the A and B sites on the crystal structure and atomic dynamics of the PbTiO3-BiMeO3 systems, two single-crystal compounds, 0.92PbTiO3-0.08Bi(Zn0.5Ti0.5)O3 and 0.83PbTiO3-0.17Bi(Mg0.5Ti0.5)O3, were thoroughly analyzed by temperature- and pressure-dependent polarized Raman scattering experiments as well as by synchrotron x-ray diffraction experiments conducted at ambient conditions. PbTiO3 was used as a reference end-member material. This thesis demonstrates for the first time that local non-tetragonal polar distortions play a major role in both the pressure- and temperature-induced phase transitions of PbTiO3-BiMeO3 compounds. Furthermore, the Raman scattering data reveal that the chemical nature of the Me cation significantly influences the structural transformations induced by variations in temperature and pressure. For 0.92PbTiO3-0.08Bi(Zn0.5Ti0.5)O3, the partial substitution of Zn2+ for Ti4+ increases the resistance of the tetragonal matrix to rising pressure and temperature, preserving the tetragonal phase over a broader pressure and temperature range relative to PbTiO3. In contrast, for 0.83PbTiO3-0.17Bi(Mg0.5Ti0.5)O3, Mg2+ disturbs the coupling of the tetragonally distorted TiO6 octahedra and promotes the expansion of local non-tetragonal polar distortions in the tetragonal matrix. The partial substitution of 0.17Bi(Mg0.5Ti0.5)O3 for PbTiO3 induces a structural instability at lower pressures relative to PbTiO3. In addition, both the temperature and pressure dependences of the Raman modes reveal an enhancement of order-disorder phenomena for this compound. Both solid solutions exhibit a local-scale doubling of the perovskite structure, which is enhanced with increasing pressure and temperature.