Abstract
Alkaline niobate and tantalate perovskites have attracted attention as polar dielectrics for electronics and telecommunications. Here, we studied the polar behaviour, lattice dynamics, and local structure in conventionally processed K0.985Mn0.015TaO3±δ ceramics using a combination of variable-temperature dielectric and Raman spectroscopies, and X-ray absorption fine structure (XAFS) measurements, respectively. Mn doping induces a low-frequency dielectric relaxation in KTaO3 (KT), which follows the Arrhenius law with an activation energy U ≈ 105 meV and the characteristic relaxation time τ0 ≈ 4.6 × 10−14 s. 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 that also contains off-centred Li+ species on the cuboctahedral sites. 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.
Highlights
Perovskite-structured compounds SrTiO3 (ST), KTaO3 (KT) and CaTiO3 (CT) stand out as incipient ferroelectrics, with their dielectric permittivity increasing continuously on cooling due to the polar-mode softening but without a ferroelectric phase transition [1].Incipient ferroelectrics exhibit strong dependence of the real part of the dielectric permittivity, ε0, on electric field and small values of the dissipation factor, tanδ, which makes them attractive for applications in tunable electronic components [2,3]
Among KT-based compounds, K1−x Lix TaO3 solid solutions have been studied intensively, wherein displacements of small Li+ ions on K sites generate strong local dipole moments that couple electrostatically to the KT’s polar soft mode [8,9]. Both dielectric relaxations and a ferroelectric phase transition were reported for the K1−x Lix TaO3 system [8,9,11,12,13]
In addition to this apparent controversy regarding the Mn solubility limit in the loose powders [20,24], K1 − 2x Mnx TaO3 ceramics exhibited a non-monotonic trend for the dielectric relaxation, with the relaxation strength being the strongest for x = 0.01, significantly diminished for x = 0.02 and 0.03, and partially restored for x = 0.05 [19,24]
Summary
Perovskite-structured compounds SrTiO3 (ST), KTaO3 (KT) and CaTiO3 (CT) stand out as incipient ferroelectrics, with their dielectric permittivity increasing continuously on cooling due to the polar-mode softening but without a ferroelectric phase transition [1]. Among KT-based compounds, K1−x Lix TaO3 solid solutions have been studied intensively, wherein displacements of small Li+ ions on K sites generate strong local dipole moments that couple electrostatically to the KT’s polar soft mode [8,9] As a result, both dielectric relaxations and a ferroelectric phase transition were reported for the K1−x Lix TaO3 system [8,9,11,12,13]. For x = 0.04, a Ta-rich tungsten bronze structure was detected as the main extra phase [20] In addition to this apparent controversy regarding the Mn solubility limit in the loose powders [20,24], K1 − 2x Mnx TaO3 ceramics exhibited a non-monotonic trend for the dielectric relaxation, with the relaxation strength being the strongest for x = 0.01, significantly diminished for x = 0.02 and 0.03, and partially restored for x = 0.05 [19,24]. 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 KT ceramics that have been confirmed as monophasic [23]
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