Dr. Oleksandr TkachResearcher, CICECO – Aveiro Institute of Materials,
Department of Materials and Ceramic Engineering, University of Aveiro, Portugal
Speech Title: A Role of the Perovskite Lattice Vacancies in Antiferrodistortive Phase Transition of Doped Strontium Titanate Ceramics
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 (Ta) associated with rotations of the O octahedra in antiphase around the  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, Ta is significantly altered, varying oppositely to the tolerance factor (t) [3,4]. In the case of sintering SrTi0.95Mn0.05O3-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 . On the other hand, for Sr-site donor dopants (La3+, Gd3+, Y3+), a common linear dependence of Ta 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
 A. Tkach, P.M. Vilarinho, Strontium Titanate: Synthesis, Properties and Uses, Nova Science Publishers, New York (2019).
 W. Kleemann, J. Dec, A. Tkach, P.M. Vilarinho, Condensed Matter, 5, 58 (2020).
 A. Tkach, P.M. Vilarinho, A. Kholkin, I.M. Reaney, J. Pokorny, and J. Petzelt, Chem. Mater., 19, 6471 (2007).
 A. Tkach, P.M. Vilarinho, D. Nuzhnyy, J. Petzelt, Acta Mater., 58, 577 (2010).
 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).
 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.