Dielectric Relaxation, Local Structure and Lattice Dynamics in Mn-Doped Potassium Tantalate Ceramics
TKACH A. 1, LEVIN I. 2, WOICIK J. 2, ALMEIDA A. 3, VILARINHO P. 1
1 Department of Materials and Ceramic Engineering, CICECOľAveiro Institute of Materials, University of Aveiro, Aveiro, Portugal; 2 Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, MD, United States; 3 Department of Physics of Science Faculty, IFIMUP, University of Porto, Porto, Portugal
Alkali tantalates, such as KTaO3 (KT) and LiTaO3, are members of the ferroic family of lead-free compounds with potential applications for electronic components and tissue engineering, respectively . Their solid solution K1–xLixTaO3 has also been intensively studied, wherein off-centre displacements of small Li+ ions on cuboctahedral K+ sites generate strong local dipole moments, which couple electrostatically to the KT polar soft mode . As a result, both dielectric relaxations and ferroelectric phase transition were reported for heavily Li-doped KT. Similar to Li-doped KT, Mn-doped KT exhibit a dielectric relaxation [3,4]. Moreover, the K0.97Mn0.03TaO3±δ ceramics display both dielectric and magnetic anomalies , which resemble the “multi-glass” behaviour observed in Mn-doped SrTiO3 [5,6]. However, little research has been undertaken into the local structure and lattice dynamics in Mn-doped KT ceramics or single crystals, especially as a function of temperature.
Here, we combined variable-temperature dielectric measurements over a broad frequency range, room-temperature X-ray absorption fine structure (XAFS) measurements, and variable-temperature Raman spectroscopy to determine the site occupancy and coordination environments for the Mn dopant species in monophasic K0.985Mn0.015TaO3±δ ceramics . Our XAFS results support preferential Mn occupancy of the cuboctahedral sites as Mn2+, with these cations strongly off-centred in the oversized oxygen cages. Such disordered Mn displacements generate electric dipoles, which are proposed as the source of the observed dielectric relaxation. We show that in Mn-doped ceramics, the low-frequency polar TO1 mode softens on cooling and, at low temperatures, exhibits a higher frequency than in undoped KT. This mode displays no detectable splitting, which contrasts with Li-doped KT. Therefore, we conclude that the coupling between the Mn displacements and the lattice is weaker than in the Li case, and Mn-doped KT therefore exhibits a dielectric relaxation but no ferroelectric transition.
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