Ferroelectric, dielectric and piezoelectric responses of BT/BBT

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Fig. 3.12 Raman spectra of (a) BT mesocrystal heat-treated at 800 °C, (b) BBT mesocrystal of BT/BBT-0.25-800 (BBT-800), BT/BBT nanocomposites of (c) BT/BBT-0.5-800 and (d) BT/BBT-0.75-800.

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smallest remanent polarizations (Pr), respectively, in these samples. The larger ferroelectric response of BT than that of BBT is consistent with the results of their normal ceramic samples reported.^{14, 37} It is noteworthy that although BT/BBT-0.5-1100 has a lower BT content than BT/BBT-0.75-1100, the remanent polarization of BT/BBT-0.5-1100 (Pr = 2.8 μC/cm²) is much larger than that of BT/BBT-0.75-1100 (Pr = 1.1 μC/cm²). The larger Pr value of BT/BBT-0.5-1100 than that of BT/BBT-0.75-1100 can be explained by higher density of the BT/BBT heteroepitaxial interface which can cause a lattice strain around the interface and enhance the ferroelectric response.^{16, 32, 34} According to the SEM-EDS results, the BT/BBT mole ratios are about 2 in BT/BBT-0.5-1100 and about 10 in BT/BBT-0.75-1100, respectively (Fig. 3.14). This result indicates that the chemical composition of BT/BBT-0.5-1100 closes to the optimum condition for the high density of the BT/BBT heteroepitaxial interface to enhance the ferroelectric response by using the lattice strain engineering.

Fig. 3.13 P-E hysteresis loops of the BT-1100, BBT-1100, BT/BBT-0.5-1100 and BT/BBT-0.75-1100 pellet samples.

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Fig. 3.14 SEM-EDS spectra for (a) BBT-1100, (b) BT/BBT-0.5-1100 and (c) BT/BBT-0.75-1100 samples.

The relative permittivities (εr) measurement was also employed to understand the influence of the BT/BBT heteroepitaxial interface on the dielectric behavior, as shown in Fig. 3.15. The εr values of the BT/BBT-0.5 nanocomposite are improved greately when the heating temeperature increases from 600 to 800 ^oC due to formation of the BT/BBT nanocomposite, and then improved slowly from 800 to 1100 ^oC maybe due to the enhancemences of dennsity and crystallinity (Fig. 3.16). The εr results for BT/BBT, BT, BBT samples exhibit the same tendency as their P-E hysteresis results, in which εr

value increases in an order of BBT-1100 < BT/BBT-0.75-1100 < BT/BBT-0.5-1100 <

BT-1100. Generally, the BT-based composites exhibit the enhancing ferroelectric and

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dielectric responses with increasing BT contents,14, 37, 50 whereas the εr value of BT/BBT-0.5-1100 is about 1.3 times larger than that of BT/BBT-0.75-1100, namely the opposite result in the case of the BT/BBT nanocomposite because the higher density of the BT/BBT heteroepitaxial interface in BT/BBT-0.5-1100 than that in the BT/BBT-0.75-1100, which corresponds the Raman spectrum results (Fig. 3.12) and the P-E hysteresis (Fig. 3.13) results.

Fig. 3.15 Variations of relative permittivities (εr) with frequency for BT-1100, BBT-1100,

BT/BBT-0.5-1100 and BT/BBT-0.75-1100 pellet samples.

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Fig. 3.16 Variations of relative permittivity (εr) with frequency for BT/BBT-0.5 samples obtained at different temperatures.

Fig. 3.17 shows the temperature dependences of the relatively permittivity (εr) for the BT-1100, BBT-1100, andBT/BBT-0.5-1100 pelletsamples. The BT-1100 pelletsample exhibits a phase transition peak around 130 ^°C that corresponds to the Curie temperature (Tc) of the normal ferroelectric BT phase (Fig. 3.17(a)).^{49, 51} The BBT-1100 pellet sample shows a phase transition peak around 400 ^°C that corresponds to the Tc of the normal ferroelectric BBT phase (Fig. 3.17(b)).⁵² It is interesting that two phase transition peaks are observed for the BT/BBT-0.5-1100 nanocomposite at around 40 and 635 ^°C, which can be assigned to the Tc of BT and BBT nanocrystals in the

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nanocomposite, respectively. We think the anomalous enhancement of the Tc for the BBT phase can be attributed to the introduced lattice stain at the BT/BBT heteroepitaxial interface due to their lattice mismatch.13, 14, 23, 34

Fig. 3.17 Temperature dependences of the relative permittivity (εr) for (a) BT-1100, (b) BT/BBT-0.25-1100 (BBT-1100) and (c) BT/BBT-0.5-1100 nanocomposite at measurement frequency of 10 kHz.

By considering the larger crystal lattice of BT phase than that of BBT phase (Fig.

3.11), at around the heteroepitaxial BT/BBT interface, BT lattice will bear an in-plane compressive strain and an out-of-plane tensile strain, while BBT lattice will bear an in-planetensile strain and an out-of-plane compressive strain. Haeni et al. have firstly demonstrated hundreds of degrees increasing in the Tc of SrTiO3 (ST) thin film deposited on DyScO3 substrate by introducing the in-plane tensile strain with +1 %

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lattice mismatch.¹³ A BaTiO3/Sm2O3 composite thick film has been reported with a highly improved Tc of BT, where the BT lattice bears an in-plane tensile strain with +2.35 % lattice mismatch at the BT/Sm2O3 interface.¹² This result reveals that the in-plane tensile strain can result in the elevated Tc of the ferroelectric phase. Therefore, we think the elevated Tc of BBT phase in the BT/BBT nanocomposite can be assigned to the introduced in-plane tensile strain.

However, it is very interesting that the Tc for BT in the BT/BBT-0.5-1100 nanocomposite was lowered from 130 to 40 ^°C. We think the decreased Tc of BT in the BT/BNT nanocomposite could not be ascribed to the introduced in-plane compressive strain. Choi et al. have presented an elevated Tc forBT thin film epitaxially grown on the DyScO3 substrate where the BT film undergoes a compressive strain of -1.7% lattice mismatch because the lattice constant of BT is larger than that of DyScO3 substrate. ¹⁴ Suzuki et al. have reported a mesostructured BT/ST composite film with elevated Tc of BT by introducing an in-plane compressed strain with -4 % lattice mismatch to BT lattice.²³ In our former research, the mesocrystalline ferroelectric BaTiO3/Bi0.5Na0.5TiO3

(BT/BNT) nanocomposite exhibits an elevated Tc for both BT and BNT in the mesocrystalline nanocomposite, where BNT has a smaller lattice constant than that of BT, namely, BNT lattice bears an in-plane tensile strain and BT lattice bears an in-plane compressive strain.³⁴

The above results suggest that both tensile and compressed strains can result in the elevated Tc of ferroelectric phases, which may be due to that an in-plane tensile strain accompanies an out-of-plane compressed strain and an in-plane compressed strain accompanies also an out-of-plane tensile strain. The lattice strain situation in the 3D system should be quite complicated. To understand lattice strain situation and its effect

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on ferroelectric phase in the 3D system, a further detail study is necessary, and the phase field calculation can be an effective approach.⁵³ Another possible reason for the lowered Tc of BT is due to its small crystal size in the BT/BBT nanocomposite (Table 3.1).

Sánchez-Jiménez and co-workers have reported a lowered Tc of BT to around 80 ^°C in a BT-Ni nanocomposite with a crystal size of BT about 45 nm.⁵⁴

Table 3.1 Sizes of BT nanocrystals in BT/BBT-0.5 nanocomposites obtained at different temperatures, and in BT/BBT-0.75-800 nanocomposite and BT-800 mesocrystal.

Sample Size of BT nanocrystal*

800 °C 900 °C 1000 °C 1100 °C

BT/BBT-0.5 40 nm 50 nm 55 nm 70 nm

BT/BBT-0.75 70 nm - - -

BT mesocrystal 75 nm - - -

* Scherrer equation (also referred to as the Debye–Scherrer equation) was applied to estimate the sizes of BT nanocrystals in BT/BNT nanocomposites and BT mesocrystal, as shown followed:

D(2θ) 110 = Kλ/(Bcosθ110)

Where D is the average nanocrystal size, K is a constant (K = 0.89), λ is the wavelength of the X-ray source (λ = 0.154056 nm), B is the value of the full width at half maximum (FWHW) of the diffraction peak of plane (110) and θ is the Bragg angle.

The Curie temperatures of the BT and BBT phases in the BT/BBT-0.5 nanocomposite are dependent on the preparation temperature, as shown in Fig. 3.18. The BT/BBT-0.5-800 exhibits a BBT phase transition peak at around 700 ^oC, and an unclear peak of the BT phase transition at around 40 ^°C. With increasing the heating temperature, the Tc of BBT shifts slightly to the lower temperature, while a sharp and clear phase transition peak of BT is observed at around 40 ^°C. The low-temperature-shifting Tc of

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BBT may be ascribed to the destruction of the BT/BBT heteroepitaxial interface due to the crystal growth of the BBT nanocrystals with increasing heating temperature.⁶ The appearance of the sharp peak for the BT phase can be ascribed to the transition from the pseudo-cubic structure to the tetragonal one with increasing heating temperature from 800 to above 900 ^°C due to the lattice mismatch stress at the BT/BBT heteroepitaxial interface.^{34, 49} The Tc of BT is almost constant in the temperature rang of 900 to 1100 ^°C because the crystal size of the BT phase is almost constant in this temperature range (Table 3.1), namely the crystal growth of the BT nanocrystals is limited by the neighboring BBT nanocrystals.

Fig. 3.18 Temperature dependences of relativie permittivities (εr) for (a) BT/BBT-0.5-800, (b) BT/BBT-0.5-900, and (c) BT/BBT-0.5-1100 at measurement frequency of 10 kHz.

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The piezoelectric responses of the mesocrystalline BT/BBT-0.5 nanocomposites obtained at different temperatures were investigated by using piezoresponse force microscopy (PFM). The Displacement-applied voltage (D-V) loops and d^*33-applied voltage (d^*33-V) loops are presented in Fig. 3.19, where converse piezoelectric constant (d^*33) is calculated from D/V value of the D-V loop. The d^*33 value of 130 pm/V at 10 V of applied voltage for BT/BBT-0.5-800 is much larger than those of 40 and 60 pm/V for BT/BBT-0.5-600 and BT/BBT-0.5-700, respectively. The sudden increase of the d^*33

value with the increase of heating temeprature from 700 to 800 ^°C can be ascribed to the formation of the mesocrystalline BT/BBT nanocomposite above 800 ^°C (Fig. 3.3), which introduces the lattice stain at the BT/BBT heteroepitaxial interface. The d^*33

value of BT/BBT-0.5-800 nanocomposite is about 5 and 6 times larger than d^*33 values of a nanostructured BT (28 pm/V)⁵⁵ and BBT ceramic (23 pm/V).³⁷ The result reveals that the lattice strain engineering is an effective aproach for enhancing the piezoelectric response.

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Fig. 3.19 Displacement-applied voltage loops and d^*33-applied voltage loops for BT/BBT-0.5 nanocomposites obtained by heat-treatement at (a) 600, (b) 700 and (c) 800 ^°C, respectively.