Abstract: SrTiO₃-based compounds have been attracting considerable interest both for a wide range of applications and from a fundamental point of view [1,2]. Fundamentally, perovskite-structured SrTiO₃ undergoes a cubic (Pm3m) to tetragonal (I4/mcm) phase transition at ~108 K (T_a) associated with rotations of the O octahedra in antiphase around the [001] direction. This phase transition gives rise to modes at the R-point of the Brillouin zone. Here, inelastic light scattering as well as electron diffraction are used to study the lattice dynamics of SrTiO₃ ceramics with isovalent and heterovalent dopants, substituting for Sr²⁺ or Ti⁴⁺ ions [3-6]. Even for a small percentage of Mn or other dopants, T_a is significantly altered, varying oppositely to the tolerance factor (t) [3,4]. In the case of sintering SrTi_0.95Mn_0.05O_3-δ ceramics in N₂ or substituting Ti⁴⁺ with acceptor Mg²⁺ dopant, Ta is strongly suppressed due to a creation of oxygen vacancies, disrupting the octahedra connectivity needed for their cooperative rotation [3]. On the other hand, for Sr-site donor dopants (La³⁺, Gd³⁺, Y³⁺), a common linear dependence of T_a versus t is obtained, if strontium vacancies with a size ~7% larger than Sr²⁺ radius are taken into account [5,6].

Keywords: Perovkites, Phase transitions, Doping, Vacancies

References
[1] A. Tkach, P.M. Vilarinho, Strontium Titanate: Synthesis, Properties and Uses, Nova Science Publishers, New York (2019).
[2] W. Kleemann, J. Dec, A. Tkach, P.M. Vilarinho, Condensed Matter, 5, 58 (2020).
[3] A. Tkach, P.M. Vilarinho, A. Kholkin, I.M. Reaney, J. Pokorny, and J. Petzelt, Chem. Mater., 19, 6471 (2007).
[4] A. Tkach, P.M. Vilarinho, D. Nuzhnyy, J. Petzelt, Acta Mater., 58, 577 (2010).
[5] A. Tkach, A. Almeida, J. Agostinho Moreira, T.M. Correia, M.R. Chaves, O. Okhay, P.M. Vilarinho, I. Gregora, J. Petzelt, Appl. Phys. Lett., 98, 052903 (2011).
[6] A. Tkach, T.M. Correia, A. Almeida, J. Agostinho Moreira, M.R. Chaves, O. Okhay, P.M. Vilarinho, I. Gregora, J. Petzelt, Acta Mater. 59, 5388 (2011).

Biography: Dr. Oleksandr Tkach is a researcher of the University of Aveiro and CICECO – Aveiro Institute of Materials, Portugal. He graduated in Microelectronics and Semiconductor Devices by the National Technical University of Ukraine “Kyiv Polytechnic Institute” in 1999. In 2005 he got Ph.D. in Materials Science and Engineering from the University of Aveiro, Portugal. Present research interests of Dr. Tkach include the development of thermoelectric and piezoelecric oxide materials and composites for energy harvesting technologies; giant permittivity dielectric oxides for energy storage; nanostructured magnetoelectrics for sensor and electronic applications; and alternative energy efficient concepts for current assisted ceramic sintering. Dr. Tkach publication profile contains 1 edited book, 6 book chapters and 86 articles published in journals, including high impact factor Nature Physics, Adv. Funct. Mater., Chem. Mater., Phys. Rev. Lett. and Appl. Mater. Today, with h-index of 24 to date.