Microstructure, Dielectric and Piezoelectric Properties of Nd-Doped Bi₄Ti₃O₁₂ Ceramics

Abstract

In this paper, a series of Aurivillius phase (AP) Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) ceramics, prepared in a solid reaction process and sintered at different temperatures (1000 ~ 1050 ℃), are investigated. Their dielectric and piezo properties are studied. The piezoelectric modulus, crystal lattice parameters and Curie temperature were measured for all of studied compounds The results of the study of the microstructure show that the average size of BNT ceramic grains remains with increasing sintering temperature. It was found that the relative permittivity increased by almost 1.5 times with the increasing of sintering temperature to 1050 ℃.

1 Introduction

In 1949, while studying the Bi₂O₃ – TiO₂ system, V. Aurivillius established the formation of Bi₄Ti₃O₁₂ (BTO) oxide with a perovskite type structure [1]. Only ten years later, after the discovery by G. Smolensky, V. Isupov and A. Agranovskaya of the ferroelectric properties of Bi₂PbNbO₉, which belongs to this class of compounds [2], a new stage of study began. Aurivillius phases (APs) form a large family of bismuth-containing layered compounds of the perovskite type, the chemical composition of which is described by the general formula: A_m–1Bi₂B_mO_3m+3. The crystal structure of AP consists of alternating [Bi₂O₂]²⁺ layers, separated by m perovskite-like layers [A_m−1B_mO_3m+1]^2–, where A are the ions with large radii have dodecahedral coordination, and B-positions inside oxygen octahedra are occupied by highly charged (≥ 3 +) cations with a small radius. The value of m is determined by the number of perovskite layers [A_m−1B_mO_3m+1]²⁻, located between the fluorite-like [Bi₂O₂]²⁺ layers, and can take an integer or half-integer values in the range of 1 – 6 (see Fig. 1, m = 3).

Fig. 1

Aurivillius phase structure: m = 3

The layered ferroelectrics of the Aurivillius phase family (APF) attract much attention due to their high Curie temperature (T_C) [3, 4]. The structure of APF Bi₂A_m−1B_mO_3m+3 compounds is tetragonal above the Curie point T_C and belongs to the space group I4/mmm. The type of space group below the Curie point T_C depends on the value of m: B2cb or Pca2₁ for odd m; A2₁am for even m; Cmm2 or I2cm for half-integer m.

In the AP, much attention was paid to the BTO compound. It is believed to have potential applications in high-temperature piezoelectric devices and ferroelectric memory devices. It has large spontaneous polarization (P_s) ~ 50 µC/cm² [5,6,7], high piezoelectric modulus [8] and high Curie temperature (T_C) ~ 675 ℃ [9]. In this work, bismuth ions Bi³⁺ in the A-position were doped with neodymium ions Nd³⁺. It was indicated in early studies of Bi₃Ti_1.5W_0.5O₉ ceramics that doping of A-position with the Nd³⁺ rare earth ion improves the piezoelectric activity and reduces the number of oxygen vacancies [10].

Ceramics of Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) was synthesized by the solid-phase reaction method. The crystal structure, microstructure, dielectric properties, piezoelectric properties of ceramics have been studied.

2 Experiment

Polycrystalline samples Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) with APF structure were synthesized by the solid-state reaction of the Bi₂O₃, Nd₂O₃, TiO₂ corresponding oxides. All initial compounds were analytical grade. After weighing in accordance with the stoichiometric composition and thorough grinding of the initial oxides with the addition of ethanol, the pressed samples were calcined at a temperature of 750 ℃ for 4 h. The samples were fired in a laboratory muffle furnace in air. Then the sample was crushed, repeatedly crushed and pressed into tablets of 10 mm in diameter and 1.0 – 1.5 mm thickness, followed by final synthesis at a temperature of 1000 ~ 1050 ℃ (for 2 h).

The X-ray pattern was recorded on a Rigaku Ultima IV diffractometer with a Cu X-ray tube. The Cu_Kα1,α2 radiation was separated from the total spectrum using a Ni-filter. The X-ray pattern was measured in the range of 2θ angles from 10° to 80° with a scan step of 0.02° and an exposure (intensity registration time) of 4 s per point. Analysis of the X-ray pattern profile, determining the position of the lines, their indexing (hkl), and refinement of the unit cell parameters were carried out using the PCW 2.3 program [11].

To measure the permittivity and electrical conductivity, electrodes were deposited on flat surfaces of APF samples in the form of disks of 10 mm in diameter and about 1.5 mm thick using Ag paste, annealed at a temperature of 650 ℃ (20 min.). The temperature and frequency dependences of the dielectric characteristics were measured using an E7–20 immittance meter in the frequency range from 100 kHz to 1 MHz and in the temperature range from room temperature to 750 ℃. The sample was subjected to polarization in an oil bath at 145 ℃ at a voltage of 35 kV/cm for 30 min. Microstructure images of Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) were obtained using a Carl Zeiss EVO 40 scanning electron microscope (SEM) (Germany) at the Center for Collective Use of the SSC RAS (No 501994). The study was carried out on mechanical cleavages of the fabricated ceramics. In the absence of an additional conductive layer, grain smearing and multiple charge accumulation effects were observed. Therefore, to analyze the cleavage surface, the conductive layer was deposited using an SC7620 “MiniSputterCoater” magnetron sputtering setup. Before sputtering, the samples were not preliminarily subjected to mechanical treatment. The study was carried out in the regime of high accelerating voltage (EHT = 20 kV). To increase the resolution, the probe current was I_probe = 55 pA, and the work distance was WD = 7 – 9 mm.

3 Results and Discussion

Powder X-ray diffraction patterns of all studied Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) solid solutions correspond to single-phase APF with m = 3 and do not contain additional reflections; they are isostructural to the known perovskite-like APF Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) oxide. It was found that all synthesized APs crystallize in an orthorhombic system with unit cell space group B2cb. The X-ray diffraction patterns of all compounds correspond to APF with m = 3. Figure 2 shows the experimental powder X-ray diffraction patterns of the compounds under study. All the diffraction peaks were indexed to the three-layered Aurivillius-type structure (JCPDS No. 72–1019).

The strongest diffraction peak was found as (117), and this agreed with the fact that the most intensive reflection of layered perovskites belonged to the type of (112m + 1) [12]. Based on X-ray diffraction data, the parameters and volume of the unit cell were determined, they are shown in Table 1.

Fig. 2

XRD patterns of Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) ceramics in the 2θ range of 10 – 80 ℃

Table 1 Unit cell parameters a, b, c and volume V, tolerance-factor t, relative permittivity ε'/ε₀ at a frequency of 100 kHz, Curie temperature T_C, and activation energy E_a in various temperature regions

The obtained unit cell parameters of the studied Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) samples are close to those determined earlier for APF Bi₄Ti₃O₁₂: a = 5.411 Å, b = 5.448 Å, c = 32.83 Å. [13]. To obtain the degree of distortion of the ideal perovskite structure, we determined the tolerance factor t, which is presented in Table 1. The tolerance factor t was introduced by Goldschmidt [14] as a geometric criterion that determines the degree of stability and distortion of the crystal structure:

$$ T = {{\left( {R_{A} + R_{{\text{O}}} } \right)} \mathord{\left/ {\vphantom {{\left( {R_{A} + R_{{\text{O}}} } \right)} {\left[ {\left( {R_{B} + R_{{\text{O}}} } \right)} \right]}}} \right. \kern-\nulldelimiterspace} {\left[ {\left( {R_{B} + R_{{\text{O}}} } \right)} \right]}}, $$

(1)

where R_A and R_B are the cation radii in positions A and B, respectively; R_O is the ionic radius of oxygen. In this work, the tolerance factor was calculated taking into account the Shannon ionic radii [15] for the corresponding coordination numbers (CN) (O^2– (CN = 6) R_O = 1.40 Å, Nd³⁺ (CN = 12) R_Nd3+  = 1.27 Å, Ti⁴⁺ (CN = 6) R_Ti4+  = 0.605 Å). Shannon did not give the Bi³⁺ ionic radius for coordination with CN = 12. Therefore, its value was determined from the ionic radius with CN = 8 (R_Bi3+  = 1.17 Å) multiplied by an approximation factor of 1.179, then for Bi³⁺ (CN = 12), we obtained R_Bi3+  = 1.38 Å. Table 2 shows the parameters of orthorhombic δb₀ and tetragonal δc’ deformation; average tetragonal period a_t and average thickness of one perovskite layer c’; c’ = 3c/(8 + 6m) is the thickness of a single perovskite-like layer, a_t = (a₀ + b₀)/(2 $\sqrt 2$) is the average value of the tetragonal period; a₀, b₀, c₀ are the lattice periods; δc’ = (c’ – a_t)/a_t is the cell deviation from the cubic form, that is, elongation or contraction from the cubic form; δb₀ = (b₀ – a₀)/a₀ is the orthorhombic deformation, c₀ = c/(2 + 1.5m), where c/(2 + 1.5m) is the bismuth-oxygen layer thickness [16].

Table 2 V is the unit cell volume, a_t is the parameter of the tetragonal period, c’ is the octahedron height along c-axis, δc’ is the deviation of the unit cell from the cubic shape, δb₀ is the rhombic distortion and c₀ is the bismuth – oxygen layer

SEM analysis is shown that all compounds sintered at temperatures of 1000 and 1050 ℃ have a similar microstructure. There are grains of both round and oblong shape, the grain boundaries are mostly distinct. This indicates that the cleavage occurred predominantly along the grain boundaries. It can be seen that there are conglomerates of strongly bonded grains in all studied ceramics.

Fig. 3

Microstructure of the cleavage surface of Bi₄Ti₃O₁₂ ceramics after synthesisat 1000 ℃ at different magnifications; both scale markers are 5 µm

In the case of sintering at 1000 ℃ (Fig. 3), the grains have rounded edges, and their average size is ~ 1.3 µm. Grain conglomerates can be up to 6 – 7 µm in size. The size distribution has a log-normal character with a maximum frequency of occurrence (87%) of 1 − 1.8 µm (Fig. 4).

Fig. 4

Grain size distribution in Bi₄Ti₃O₁₂ ceramics after synthesis at 1000 ℃

After synthesis at 1050 ℃ (Fig. 5), the grain size situation is identical. The average size remains within ~ 1.3 μm, however, their size distribution acquires a character close to Gaussian (Fig. 6). At the same time, the maximum frequency of occurrence (88%) also remains within 1 – 1.8 µm. The volume of conglomerated grains is noticeably higher, and their size in some cases reaches 8 – 9 µm.

Fig. 5

Microstructure of the cleavage surface of Bi₄Ti₃O₁₂ ceramics after synthesis at 1050 ℃ at different magnifications; both scale markers are 5 µm

Figure 7 shows images of the microstructure of Bi_3.8Nd_0.2Ti₃O₁₂. It can be seen that the cleavage surface is quite loose, the cleavage occurred mainly along the grain boundaries. Therefore, the grain boundaries are clear. There are small areas with densely grouped crystallites, in which the cleavage went directly through the grains. In these regions, one can see that the internal structure of the grains is homogeneous (Fig. 7, selected regions).

Fig. 6

Grain size distribution in Bi₄Ti₃O₁₂ ceramics after synthesis at 1050 ℃

Fig. 7

Microstructure of the cleavage surface of Bi_3.8Nd_0.2Ti₃O₁₂ ceramics after synthesisat 1000 ℃ at different magnifications; both scale markers are 5 µm

Fig. 8

Grain size distribution in Bi_3.8Nd_0.2Ti₃O₁₂ ceramics after synthesis at 1000 ℃

The grains mostly have rounded edges, and their average size is ~ 1.15 µm. The size distribution has a classical log-normal character with the maximum frequency of occurrence (80%) of grains within 0.75 – 1.75 µm (Fig. 8).

After synthesis at 1050 ℃ (Fig. 9), the following occurs. Obviously, here the cleavage surface is denser; therefore, the cleavage occurred both along the grains and along their boundaries. The average crystallite size increases to ~ 1.45 µm. The scatter in size retains its character of log-normal distribution (Fig. 10). The regions in which cleavage occurs along the grains are characterized by the presence of lamellar grains, similar to those that form in the structure of the Aurvilius phases (Fig. 9, highlighted regions). At the same time, it can be noted that the internal structure of these lamellar crystallites is slightly inhomogeneous, which most likely indicates their layering.

Fig. 9

Microstructure of the cleavage surface of Bi_3.8Nd_0.2Ti₃O₁₂ ceramics after synthesis at 1050 ℃ at different magnifications; both scale markers are 5 µm

Fig. 10

Grain size distribution in ceramics Bi_3.8Nd_0.2Ti₃O₁₂ after synthesis at 1050 ℃

Figure 11, 12 show the temperature dependences of the relative permittivity ε/ε₀(T) and the dielectric loss tangent for Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) at a frequency from 100 kHz to 1 MHz, sintered at temperatures of 1000 ℃ and 1050 ℃, respectively.

Fig. 11

Temperature dependences of relative permittivity ε’ = ε/ε₀ (a₁, a₂) and loss tangent tgσ (b₁, b₂) for APs Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) at a frequency from 100 kHz to 1 MHz at a sintering temperature of 1000 ℃

The electrical conductivity along a-axis is several orders of magnitude higher than along c-axis. Oxide ion conductivity, due to a large number of oxygen vacancies in the perovskite layers, leads to a high electrical conductivity along a-axis of about 700 ℃. As it is known, with an increase in the sintering temperature, the grain sizes in the ab-direction will increase (Fig. 10), which leads to an increase in conductivity and an increase in the dielectric loss tangent (Figs. 11, b1, 12, b3).

Calculations show that the formation of Bi³⁺ vacancies occur predominantly at the A-site of the perovskite layers, rather than in the Bi₂O₂ layers, leading to oxygen vacancies in the perovskite layers [17]. The temperature dependences of the permittivity (ε’) and dielectric loss tangent (tgσ) in the range from 100 kHz to 1 MHz for all compositions are shown in Figs. 11, 12. In Figs. 11, a1 – 12, a3 the T_C of the Bi₄Ti₃0₁₂ compositions is ~711 – 715 ℃, and a significant change in T_C is caused by doping with Nd³⁺ (Figs. 11, a2 – 12, a4) and the T_C of the Bi_3.8Nd_0.2Ti₃O₁₂ compositions is ~ 680 – 685 ℃. However, doping with Nd at the B-site does not significantly affect T_C and ε'_max (maximum of ε’). The value of ε'_max for Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) sintered at 1050 ℃ is ~ 2800 – 4000, which is much higher than that of Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2), sintered at temperature 1000 ℃ (~ 2200 – 2500). Figures 11, b1 – 12, b3 Bi₄Ti₃O₁₂ shows an increase in the dielectric loss tangent from 1 to 6 above ~ 500 ℃, and this increase is almost exponential. At the same time, Bi₄Ti₃O₁₂, doped with Nd (Figs. 11, b2 – 12, b4), demonstrates a decrease in the dielectric loss tangent, and its increase in the temperature range from 500 ℃ to 645 ℃ is ~ 1, and this increase has a linear dependence. Relaxation in the temperature range from 200 ℃ to 500 ℃, associated with a wide anomaly, and, is also associated probably with the movement of oxygen anions over short distances inside the crystal [18]. Moreover, Bi₄Ti₃O₁₂, doped with Nd, demonstrates a shift in the beginning of the relaxation “step” toward higher temperatures from 150 to 200 ℃ (see Figs. 11, 12). The low-temperature dielectric anomaly depends on frequency, while for the dielectric anomaly, associated with the phase transition, no dependence on frequency is observed.

Fig. 12

Temperature dependences of relative permittivity ε’ = ε/ε₀ (a₃, a₄) and loss tangent tgσ for APs Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) at a frequency from 100 kHz to 1 MHz, at sintering temperature 1050 ℃

The low temperature anomaly is probably associated with the formation of Schottky barriers between the ceramic and metal electrodes due to the difference in the Fermi level between them [19, 20].

A distinctive evolution like broadening is observed from Bi₄Ti₃O₁₂ to Bi_3.8Nd_0.2Ti₃O₁₂. This suggests that the broadening at x = 0.2 is not a microstructural effect, but a truly compositional effect. This is indicative of the presence of a Curie temperature distribution, and is not necessarily due to chemical inhomogeneities, and is commonly referred to as a diffuse phase transition.

Ceramics Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) have a low piezoelectric activity (7 pC/N ≤ d₃₃ ≤ 8 pC/N) and doping with Nd did not increase the piezoelectric activity. After the polarization of the samples Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2), we again measured the temperature dependence of the permittivity. It was found that the Curie temperature decreased by 10 ℃. Thus, we can say that the presence of the sample in an electric field leads to a decrease in the Curie temperature.

The activation energy (E_a) was determined from the Arrhenius equation:

$$ \sigma = \, \left( {A/T} \right){\text{exp}}[ - E_{a} /\left( {kT} \right)], $$

(3)

where σ is the electrical conductivity, k is a Boltzmann constant, A is a constant, E_a is the activation energy. A typical dependence of lnσ (σ-conductivity) on 1/T (at a frequency of 100 kHz), which was used to determine the activation energy E_a, is shown in Fig. 4 for APs Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2). The Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) compounds have two temperature ranges, in which the activation energy E_a differs significantly in value. In the area of low temperatures, electrical conductivity is determined mainly by impurity defects with very low activation energies of the order of several hundredths of an electron volt. For Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) compounds, we observe a region with pronounced impurity conductivity in the temperature range from 20 to 200 ℃.

Fig. 13

Dependence of lnσ on 10,000/T for the Bi_4-xNd_xTi₃O₁₂ sample (x = 0.2) at a frequency of 100 kHz

When Bi₄Ti₃O₁₂ is doped with neodymium, the activation energy in the low-temperature and high-temperature regions, as follows from Table 2, practically does not change. The invariance of the activation energy indicates the constancy of the nature of conductivity in the synthesized series of compounds.

4 Conclusions

In summary, Bi_4-xNd_xTi₃O₁₂ (x = 0.0, 0.2) ceramics, were successfully prepared by a conventional solid‐state reaction method. XRD patterns were identified as Aurivillius orthorhombic B2cb for all phases. As expected, Nd³⁺ doping reduced the electrical conductivity by about two orders of magnitude, compared to undoped Bi₄Ti₃O₁₂. The Curie temperature for Bi₄Ti₃O₁₂ was refined and it was T_C = 715 ℃. The polarization of the sample resulted in a decrease in the Curie point. Doping Bi₄Ti₃O₁₂ with neodymium Nd³⁺ lowered the Curie temperature by 30 ℃.