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. 2024 Aug 24;17(17):4195.
doi: 10.3390/ma17174195.

Growth of Single Crystals of (K1-xNax)NbO3 by the Self-Flux Method and Characterization of Their Phase Transitions

Doan Thanh Trung  1 , Eugenie Uwiragiye  1 , Tran Thi Lan  1 , John G Fisher  1 , Jong-Sook Lee  1 , Jungwi Mok  2 , Junseong Lee  2 , Furqan Ul Hassan Naqvi  3 , Jae-Hyeon Ko  3
Affiliations

Growth of Single Crystals of (K1-xNax)NbO3 by the Self-Flux Method and Characterization of Their Phase Transitions

Doan Thanh Trung et al. Materials (Basel). .

Abstract

In this study, single crystals of (K1-xNax)NbO3 are grown by the self-flux crystal growth method and their phase transitions are studied using a combination of Raman scattering and impedance spectroscopy. X-ray diffraction shows that single crystals have a perovskite structure with monoclinic symmetry. Single crystal X-ray diffraction shows that single crystals have monoclinic symmetry at room temperature with space group P1211. Electron probe microanalysis shows that single crystals are Na-rich and A-site deficient. Temperature-controlled Raman scattering shows that low temperature monoclinic-monoclinic, monoclinic-tetragonal and tetragonal-cubic phase transitions take place at -20 °C, 220 °C and 440 °C. Dielectric property measurements show that single crystals behave as a normal ferroelectric material. Relative or inverse relative permittivity peaks at ~-10 °C, ~230 °C and ~450 °C with hysteresis correspond to the low temperature monoclinic-monoclinic, monoclinic-tetragonal and tetragonal-cubic phase transitions, respectively, consistent with the Raman scattering results. A conduction mechanism with activation energies of about 0.5-0.7 eV was found in the paraelectric phase. Single crystals show polarization-electric field hysteresis loops of a lossy normal ferroelectric. The combination of Raman scattering and impedance spectroscopy is effective in determining the phase transition temperatures of (K1-xNax)NbO3.

Keywords: (K0.5Na0.5)NbO3; Raman scattering; impedance spectroscopy; lead-free piezoelectric; self-flux crystal growth; single crystal.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
(a) Schematic arrangement of the self-flux single crystal growth experiment (reproduced from [51]) (b) Heat treatment schedule for single crystal growth.
Figure 2
Figure 2
(K1−xNax)NbO3 single crystals (a) in the Pt crucible and (b) after removal from the crucible and cleaning.
Figure 3
Figure 3
(a) XRD patterns of a bulk (K1−xNax)NbO3 single crystal and (K1−xNax)NbO3 single crystals crushed into powder; (b) the patterns in the 2θ range 45–47°.
Figure 4
Figure 4
Single crystal XRD pattern of a bulk (K1−xNax)NbO3 single crystal.
Figure 5
Figure 5
SEM micrographs of as-grown surfaces of (K1−xNax)NbO3 single crystals: (a,b) single crystal 1; (c,d) single crystal 2 in Figure 2.
Figure 6
Figure 6
(a) Raman spectrum of a single crystal of (K1−xNax)NbO3 taken at 20 °C; (b) Normalized intensity contour plot of Raman spectra of a (K1−xNax)NbO3 single crystal taken at different temperatures.
Figure 7
Figure 7
Normalized Raman spectra of a (K1−xNax)NbO3 single crystal taken at different temperatures. Representative spectra are shown with fitted Lorentzian peaks in blue. The red curves are the sum of the fitted peaks.
Figure 8
Figure 8
Position of the Raman modes vs. temperature. Each mode is labelled with the value of its wavenumber at −196 °C or the temperature at which it first appeared.
Figure 9
Figure 9
(a) ε, (b) ε−1, (c) tan δ, (d), (e) σT vs. temperature of a (K1−xNax)NbO3 single crystal on 1st heating and cooling: (g) ε, (h) ε−1, (i) tan δ, (j), (k) σT on 2nd heating and cooling. (f,l) are the respective impedance spectra at selected temperatures as indicated by stars in (e,k). Phase transition temperatures from the dielectric peaks are indicated for all plots.
Figure 10
Figure 10
Polarization vs. electric field hysteresis loops of a (K1−xNax)NbO3 single crystal.
See this image and copyright information in PMC

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