Structure and properties of Fe-modified Na${}_{0.5}$Bi${}_{0.5}$TiO${}_3$ at ambient and elevated temperature

Sodium bismuth titanate (NBT) ceramics are among the most promising lead-free materials for piezoelectric applications. This work reports the crystal structure and phase evolution of NBT and Fe-modified NBT (from 0-2 at.$\%$ Fe) using synchrotron x-ray diffraction and Raman spectroscopy, at both ambient and elevated temperatures. The crystallographic results are discussed with reference to permittivity and piezoelectric thermal depolarization measurements of the same compositions. Changes in the depolarization temperature due to Fe substitution were detected by Raman spectroscopy and were found to correlate closely with depolarization temperatures obtained from converse piezoelectric coefficient and permittivity measured in situ. The depolarization temperatures obtained from direct piezoelectric coefficient measured ex situ as well as the phase transition temperatures obtained from synchrotron x-ray diffraction were found to be at higher temperatures. The mechanisms underlying the relationship between permittivity and piezoelectric depolarization to structural transitions observed in Raman spectroscopy and x-ray diffraction are discussed.

Figure 1

Longitudinal piezoelectric coefficient $d_{33}$ measured as a function of temperature for various compositions of Fe-modified NBT. The converse $d_{33}$ was measured in situ (a), and the direct $d_{33}$ was measured ex situ (b).

Figure 2

Permittivity and loss measured as a function of temperature at 0.1, 1, 10, 100, and 1000 kHz for (a) unmodified NBT poled, (b) unmodified NBT unpoled, (c) 0.5 at.$\%$ Fe modified poled, and (d) 0.5 at.$\%$ Fe modified unpoled.

Figure 3

Excerpts of high-resolution XRD patterns of NBT with varying Fe concentrations. For simplicity, peaks are labled relative to the pseudocubic planes.

Figure 4

Rietveld refinement of a high-resolution XRD pattern of 1$\%$ Fe-modified NBT showing the observed pattern (x) and the calculated fit (―). The line below is the difference between the observed and calculated intensities.

Figure 5

Excerpts of high-resolution XRD patterns of 1$\%$ Fe-modified NBT at various temperatures.

Figure 6

Selected areas of XRD patterns of (a) unmodified and (b) 1$\%$ Fe modified NBT plotted as a function of temperature.

Figure 7

(a) Raman spectrum of unmodified NBT at room temperature. Spectral deconvolution was performed according to eight Gaussian-Lorentzian modes based on literature.[20, 21] The assignment of spectral modes to specific vibrations in the crystal lattice is superimposed on the graph. (b) Raman spectra of unmodified and modified NBT at room temperature. The spectral signature is not greatly affected by Fe modification. (c) Raman spectra of unmodified NBT as a function of temperature. The displayed temperatures refer to the setting of the heating stage at 20 °C steps. The apparent peak coalescence in the midfrequency region can be ascribed to intrinsic thermal broadening.

Figure 8

Variation of A${}_1$ phonon line center (∼270 cm${}^{-1}$) as a function of temperature for all compositions: (a) unmodified NBT, (b) NBT with 0.5 at.$\%$ Fe, (c) NBT with 1 at.$\%$ Fe, (d) NBT with 1.5 at.$\%$ Fe, and (e) NBT with 2 at.$\%$ Fe. Intersection (anomaly) points between two fitting data sets (red line) are highlighted by continued dashed lines, and are observed only in compositions up to 1 at.$\%$ Fe. The detected anomaly points can be associated with changes in the short-range structure occurring at $T_d$. For higher Fe content the higher experimental error prevents from detecting any anomaly point, and thus fitting in panels (d) and (e) is presented considering the $T_d$ values obtained from the depolarization study as first approximation.