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. 2016 Apr 6:6:23898.
doi: 10.1038/srep23898.

Fano resonance of Li-doped KTa(1-x)NbxO3 single crystals studied by Raman scattering

Affiliations

Fano resonance of Li-doped KTa(1-x)NbxO3 single crystals studied by Raman scattering

M M Rahaman et al. Sci Rep. .

Abstract

The enhancement of functionality of perovskite ferroelectrics by local structure is one of current interests. By the Li-doping to KTa(1-x)NbxO3 (KTN), the large piezoelectric and electro-optic effects were reported. In order to give new insights into the mechanism of doping, the microscopic origin of the Fano resonance induced by the local structure was investigated in 5%Li-doped KTN single crystals by Raman scattering. The coupling between the continuum states and the transverse optical phonon near 196 cm(-1) (Slater mode) caused a Fano resonance. In the vicinity of the cubic-tetragonal phase transition temperature, TC-T = 31 °C, the almost disappearance of the Fano resonance and the remarkable change of the central peak (CP) intensity were observed upon heating. The local symmetry of the polar nanoregions (PNRs), which was responsible for the symmetry breaking in the cubic phase, was determined to E(x, y) symmetry by the angular dependence of Raman scattering. The electric field induced the significant change in the intensity of both CP and Fano resonance. From these experimental results, it is concluded that the origin of the Fano resonance in Li-doped KTN crystals is the coupling between polarization fluctuations of PNRs and the Slater mode, both belong to the E(x, y) symmetry.

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Figures

Figure 1
Figure 1. A typical sketch of the experimental scattering geometry with a half-wave plate.
The orthogonal coordinates (a–c) represent the experimental coordinates, where the incident light propagating along ā had a polarization direction along c axis. When the polarization direction of incident light is inclined θ/2 with respect to the optical axis of the waveplate, the polarization plane of incident is rotated by θ while that of the scattered light rotates by −θ.
Figure 2
Figure 2
Reduced (a) ā(cc)a (VV) and (b) ā(bc)a (VH) Raman scattering spectra at some selected temperatures of the KLTN/0.05/0.27 single crystal.
Figure 3
Figure 3
(a) The example of a fit of Raman spectrum using the equation (2). (b) Temperature dependence of the integrated reduced intensity and FWHM of the CP of the KLTN/0.05/0.27 single crystal at the VV scattering. The dotted and dash-dotted lines in (b) are guide to eyes. (c) The frequency shift of the first order TO1 and TO2 modes (upper half) and the variation of the intensity of the 2TA mode (lower half) at the VV scattering of the KLTN/0.05/0.27 crystal as a function of temperature.
Figure 4
Figure 4
The temperature dependence of reduced intensity, I0, line shape parameter, q, and FWHM, ГTO2 of the Fano resonance in (a) VV and (b) VH scattering spectra of the KLTN/0.05/0.27 crystal fitted by equation (2). The example of a fitted curve is shown in (c). (d) The temperature dependence of q−1 and ΓTO2 observed at the VV scattering geometry in the ferroelectric phase. The solid lines in (d) are guide to eyes.
Figure 5
Figure 5. A schematic illustration of the coupling between slow-relaxing CP (a broad spectrum indicates continuum states) and TO2 phonon (a sharp spectrum indicates discrete state).
The ΓCP and ΓTO2 are the relaxation rates of the CP and TO2 phonon, respectively, (ΓCP > ΓTO2). The displacement pattern of Slater mode is depicted in the right side, whereas dynamic PNRs is displayed in the left side. The local polarization of PNRs fluctuates along the equivalent [111] directions indicated by an arrow.
Figure 6
Figure 6
Angular dependence of (a) VV and (b) VH Raman scattering spectra of the KLTN/0.05/0.27 single crystal measured at 40 °C. The observed intensity of the CP (upper half) and the Fano resonance (lower half) in (c) VV and (d) VH spectra as a function of rotation angle. The solid lines in (c) and (d) are the best fitted curves using the equations (5) and (6), respectively.
Figure 7
Figure 7
(a) Electric field dependence of both VV (upper half) and VH (lower half) Raman spectra of the KLTN/0.05/0.26 single crystal measured at 25 °C. (b) The reduced intensity of the Fano resonance as a function of the electric field in both VV (upper half) and VH (lower half) spectra measured at 25 °C. A schematic illustration of off-center displacements of Nb ions in the cubic phase of the Li-doped KTN crystals in (c) before applied field and (d) after applied field. The off-centering of Li ions among the equivalent symmetry related sites, albeit not shown in the figure, can be described in a similar way.
Figure 8
Figure 8. The electric field dependence of the reduced intensity of the Fano resonance (upper half) and the CP (lower half) measured at 2 °C.

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