Available online at www.sciencedirect.com Journal of the European Ceramic Society 30 (2010) 941–946 Influence of point defects in KTaO 3 on low-temperature dielectric relaxation Anna-Karin Axelsson a , Matjaz Valant a,b,∗ , Neil McN. Alford a a Department of Materials, Imperial College London, Exhibition Road, London SW7 2AZ, UK b Materials Research Laboratory, University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia Received 4 August 2009; received in revised form 23 September 2009; accepted 30 September 2009 Available online 4 November 2009 Abstract Substituted KTaO 3 ceramics were synthesized, sintered and studied using low-temperature microwave dielectric analysis and Raman spectroscopy. Because of a fundamentally different nature of aliovalent Mn- and isovalent Na-substitution mechanisms, significant differences in processing and dielectric properties were identified. The properties were correlated to the defect structure of the substituted KTaO 3 lattices. Characteristics of the induced polar domains were clearly different for the two substitutional mechanisms, which further reflects in a significantly different dielectric behavior. Linear response of changes in the Raman spectra corresponds to evidence of the formation of symmetry-breaking regions. © 2009 Elsevier Ltd. All rights reserved. Keywords: Defects; Dielectric properties; Ferroelectric properties; Tantalates; Polar domains 1. Introduction KTaO 3 is an incipient ferroelectric and displays a continuous increase in the dielectric constant with decreasing temperature. KTaO 3 exhibits many technologically interesting properties. It can be used in optoelectronics for optical waveguides. In combi- nation with low-loss superconductors it has been considered for applications in tunable microwave components and fatigue-free nonvolatile memories. 1 Additionally, it possesses a combina- tion of a high permittivity and relatively low dielectric losses at room temperature, which makes it an attractive candidate for application in microwave devices. 2 It exhibits semiconduc- tor properties with a band gap suitable for photocatalytic water splitting. 3 In many applications it is used in a form of powders and single crystals but ceramic elements are frequently required to reduce production and machining cost, increase design flex- ibility and functionality. Sintering of KTaO 3 powder is very difficult mainly due to the high covalent bonding within this crystal structure. 4,5 More effective sintering has been obtained with an aid of dopants. 6,7 However, the dopants can significantly change functional properties (e.g. optical and dielectric) and can- ∗ Corresponding author at: University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia. Tel.: +386 53653502; fax: +386 53653527. E-mail address: matjaz.valant@ung.si (M. Valant). not be tolerated for particular applications. Especially sensitive to the presence of dopants are dielectric properties. The presence of additional polarization mechanisms can significantly alter the dielectric relaxation processes in such a material. Consequently, this can result in a significant increase in dielectric losses and a reduced technological value of the material. One type of defect that occurs at aliovalent substitutions is dipoles, generated by e.g. a Columbic attraction of oppositely charged species. This type of random lattice disorders would cause a dielectric anomaly and pronounced relaxation when exposed to an electromagnetic field. Most commonly, this type of local ferroelectric phase transition is induced by different types of defect such as local symmetry-breaking defects (SB) or the soft non-symmetry-breaking (NSB) defects that increase the local ferroelectric transition temperature. 8 Raman spectroscopy and inelastic neutron scattering studies have revealed the effect of the oxygen vacancies on the ferroelectric soft mode in reduced SrTiO 3 . Similar results have been obtained for KTaO 3 . 9 In these cases, the oxygen vacancies act as very hard local NSB defects, which affect the local ferroelectric transition temperature. In practice this may mean that for polycrystalline substituted KTaO 3 systems, the physics, which is governing the behavior of the single crystals, is changed. By solid state synthesis, devia- tion of the phonon fluctuations comes from a number of sources including grain boundaries, impurities, vacancies. Therefore, the dielectric loss, especially in low temperatures, is linked to 0955-2219/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2009.09.034