Evidence of structural modifications in the region around the broad dielectric maxima in the 30% Sn-doped barium titanate relaxor

The structural and dielectric characterization of 30% Sn-doped ${\mathrm{BaTiO}}_3$ (${{\mathrm{BaSn}}_{0.3}{\mathrm{Ti}}_{0.7}\mathrm{O}}_3$) as a function of temperature is carried out combining, complementary probes to reveal structural modifications associated to the relaxor behavior. Dielectric data confirms the existence of relaxor-type behavior in ${{\mathrm{BaSn}}_{0.3}{\mathrm{Ti}}_{0.7}\mathrm{O}}_3$. The local and average structural changes as a function of temperature have been investigated in the region of broad changes of dielectric response. Local probes, such as x-ray absorption fine structure spectroscopy and Mössbauer spectroscopy reveal a special trend of the lattice related to the diffuse dielectric phase transition as observed from dielectric data. The analysis of Raman spectra as a function of temperature reveals a peculiar behavior in the temperature region corresponding to the dielectric transition. The analysis of x-ray diffraction patterns points out a pseudocubic structure throughout the temperature range while the volume exhibits a negative thermal expansion in the region of broad dielectric maximum. Our results suggest structural modifications occurring in the system throughout a wide temperature range, interestingly, associated with changes in macroscopic properties of relaxors such as dispersion in dielectric constants and raising of dipole moments.

Figure 1

Temperature-dependent dielectric constant and ${\varepsilon }^{\prime \prime }$ of ${{\mathrm{BaTi}}_{0.7}{\mathrm{Sn}}_{0.3}\mathrm{O}}_3$ at different frequencies. Inset shows the Vogel-Fulcher fit to the variation of $T_m$ as a function of frequency as discussed in the text.

Figure 2

Left: Ferroelectric loops (P-E) of ${{\mathrm{BaTi}}_{0.7}{\mathrm{Sn}}_{0.3}\mathrm{O}}_3$ at the indicated temperatures. Right: Remnant polarization variation with temperature with its derivative shown in the inset.

Figure 3

Left: Representative temperature-dependent x-ray diffraction patterns of ${{\mathrm{BaTi}}_{0.7}{\mathrm{Sn}}_{0.3}\mathrm{O}}_3$ measured with $\lambda =0.7530$ nm radiation. The experimental data is profile fitted with FullProf program. Right: The variation of unit-cell volume and the full width at half maximum (FWHM) of (111) reflection (red) and (110) reflection (black).

Figure 4

Representative temperature-dependent Raman spectra of ${{\mathrm{BaTi}}_{0.7}{\mathrm{Sn}}_{0.3}\mathrm{O}}_3$.

Figure 5

Top: Normalized temperature-dependent Raman spectra of ${{\mathrm{BaTi}}_{0.7}{\mathrm{Sn}}_{0.3}\mathrm{O}}_3$ from 10 K to 300 K. The first (${\mathrm{PC}}_1$) and second (${\mathrm{PC}}_2$) principal components are shown, the main changes in the Raman spectra as a function of temperature are highlighted (labels A–D). The bottom plots report the temperature evolution of the ${\mathrm{PC}}_1, {\mathrm{PC}}_2$ (a)Raman shift (b), and FWHM (c) for the isolated peak D. The vertical lines in the bottom frames point out the $T^{\prime }$ and $T$* temperatures.

Figure 6

Representative temperature-dependent x-ray absorption spectra (XAS) of ${{\mathrm{BaTi}}_{0.7}{\mathrm{Sn}}_{0.3}\mathrm{O}}_3$. The data is vertically shifted. Inset shows the representative fitting of different modes in pre-edge region for 300 K data. Right: Variation of integral area under peak b with temperature.

Figure 7

Sn $K$-edge EXAFS signals [$k^2\chi \left(k\right)$] and the moduli of their Fourier transforms ($\left|FT\right|$). The amplitude of the EXAFS oscillations decrease with raising temperature, causing attenuation of the peaks in the Fourier transform.

Figure 8

Analysis of Sn $K$-edge EXAFS data. Left: Data (green dots) and best fit line (black line) in $k$-space. Right: Modulus and imaginary part of the Fourier transform of the experimental data (green dots) and best fit line (black line). Below, shifted for clarity, are reported the partial contributions used in the analysis grouped according to the various shells. The refinement demonstrates a good agreement till around $\sim 4.5\text{Å}$.

Figure 9

Coordination distances as a function of temperature for all the shells involved in the analysis. They are in agreement with the crystallographic structure with only weak changes as a function of temperature. The average ${\mathrm{R}}_{\text{SnTi}}$ distances are shorter than ${\mathrm{R}}_{\text{SnSn}}$, accordingly, to the smaller ionic radius of Ti respect to Sn.

Figure 10

Temperature dependence of mean square relative displacement (MSRD) $[{\sigma }^2]$ parameters for all shells involved in the analysis. The insert shows a change at roughly 120–180 K in the ${\sigma }_{Sn-O}^2$ behavior.

Figure 11

Left: Representative temperature-dependent ${}^{119}\mathrm{Sn}$ Mössbauer spectra of ${{\mathrm{BaTi}}_{0.7}{\mathrm{Sn}}_{0.3}\mathrm{O}}_3$. Right: Temperature variation of width of ${}^{119}\mathrm{Sn}$ Mössbauer spectra, considered to be proportional to the electric field gradient.