Trans. Nonferrous Met. Soc. China 27(2017) 2656-2662

Dielectric properties and point defect behavior of antimony oxide doped Ti deficient barium strontium titanate ceramics

Chen ZHANG, Zhi-xin LING, Gang JIAN, Fang-xu CHEN

Provincial Key Lab of Advanced Welding Technology, Jiangsu University of Science and Technology, Zhenjiang 212003, China

Received 27 September 2016; accepted 17 March 2017

Abstract: The microstructures and dielectric properties of Sb₂O₃-doped Ti deficient barium strontium titanate ceramics prepared by solid state method were investigated with non-stoichiometric level and Sb₂O₃ content by SEM, XRD and LCR measure system. It is found that with the increase of δ, (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics transform from single phase solid solutions with typical cubic perovskite structure to multiphase compounds while (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics remain to be single-phase with the increasing Sb₂O₃ content. The distortion of the ABO₃ perovskite lattice caused by V_Ti″″ and  induces the drop of Curie temperature and the rise of relative dielectric constant in (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics with increasing δ value. The orientation of  elastic dipoles results in the domain-wall pinning and thus the reduction of the dielectric loss. With increasing Sb₂O₃ content, the relative dielectric constant, dielectric constant maximum and Curie temperature of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics decrease dramatically while the dielectric loss increases.

Key words: barium strontium titanate; defects; dielectric properties; non-stoichiometric ceramics

1 Introduction

Barium strontium titanate (Ba_1-xSr_xTiO₃, BST) as a ferroelectric material with perovskite structure has drawn considerable attention in the microelectronic industries because of its outstanding properties such as adjustable phase transition temperature, high dielectric constant and low dielectric loss [1], which allows it to have a great potentiality for phase shifter, phased array antenna, resonator and capacitor applications. However, the conflict between the fine dielectric properties at room temperature and good dielectric temperature stability remains in the uniform BST ceramics [2]. In order to overcome the above mentioned drawback, the traditional doping method applied widely in electronic ceramics has been introduced into BST ceramics [3-5].

Due to the intermediate size and charge of trivalent ions compared with the Ba²⁺, Sr²⁺ and Ti⁴⁺ ions, the aliovalent doping mechanism and the substitution preference of trivalent ions in BST ceramics as well as in BaTiO₃ ceramics have been debated for many years [6]. However, it is no doubt that the dielectric properties of perovskite BST ceramics are related to the doping mechanism and correspondingly the point defect behavior. In many cases, the doping mechanisms are not only dependent on the oxygen partial pressure (p_O2) and sintering temperature/time but also the overall n_A/n_B  ratio [7]. Many literatures have been focused on the relationships between the point defect behavior and n_A/n_B ratio in BaTiO₃ ceramics [8,9]. Also, some influences of n_A/n_B ratio on the microstructures and dielectric properties in BST ceramics are gradually understood. DONG et al [2] concluded that adding excessive TiO₂ could remarkably inhibit grain growth, suppress and broaden the Curie peaks in compositionally inhomogeneous BST ceramics. SYAMAPRASAD et al [10] found that excess Ba led to high un-changed dielectric constant with a slight decrease of loss tangent in Ba_0.71Sr_0.29TiO₃ ceramics.

In our previous work, the dielectric properties of Sb₂O₃-doped stoichiometric BST ceramics such as (Ba_0.992-xSr_xY_0.008)TiO_3.004 ceramics (n_A/n_B=1) have been investigated and the Sb₂O₃ dopant showed extraordinary effects on the dielectric temperature stability improvement [11,12]. So, in this work, we report a systematic study of the microstructure, point defect behavior and dielectric properties of Ti deficient (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics (n_A/n_B>1) still taking the trivalent Sb³⁺ ions as dopant. The influences of δ value (namely the n_A/n_B ratio) and doping content on point defect behavior and the dielectric properties in barium strontium titanate ceramics are discussed.

2 Experimental

The chemical compositions of the Sb₂O₃-doped Ti deficient barium strontium titanate specimens were given by the formula (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ+0.6%Sb₂O₃ (δ= 0.002, 0.004, 0.006, 0.008) and (Ba_0.75Sr_0.25)Ti_0.998O_2.996 + xSb₂O₃ (x=0, 0.4%, 0.8%, 1.2%). High purity BaCO₃ (>99.0%), SrCO₃ (>99.0%) and TiO₂ (>98.0%)  powders used as starting raw materials were weighed according to the above specimens, ball-milled, dried and calcined at 1080 °C for 2 h. The calcined powders were mixed with Sb₂O₃ (>99.0%), reground, dried and added with 5% polyvinyl alcohol (PVA) as a binder for granulation. The mixture was sieved through 250 μm screen and then pressed into pellets of 10 mm in diameter and 2 mm in thickness under 250 MPa. Sintering was conducted in air at 1300-1320 °C for 2 h. For dielectric measurement, both the flat surfaces of the specimens were coated with BQ-5311 silver paste after ultrasonic bath cleaning and then fired at 800 °C for 10 min.

The crystal structures of the specimens were confirmed by X-ray diffraction analysis (XRD, Rigaku D/max 2500v/pc) with Cu K_α radiation. The surface morphologies of the specimens were observed using the SEM (JSM-6480 ESEM). The capacitance quantity (C) and dissipation factor (D) were measured with LCR-8101G Automatic LCR Meter at 1 kHz. The relative dielectric constant (ε_r) and the loss tangent (tan δ) were calculated as follows:

                                 (1)

                                (2)

where h is the thickness (cm), d is the diameter of the electrode (cm) and f is the test frequency (Hz). An automatic measuring system consisting of Automatic LCR Meter and THP-F-100 temperature control unit was used to record the capacitance quantity and dissipation factor from -20 °C to 50 °C at 1 kHz for measuring the temperature dependence of dielectric parameters.

3 Results and discussion

3.1 XRD and SEM analysis

The X-ray diffraction patterns of 0.6%Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ bulk ceramics are shown in Fig. 1. With the increase of δ value, (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ+ 0.6%Sb₂O₃ ceramics transform from single phase solid solutions with typical cubic perovskite structure to multiphase compounds. In other words, to maintain the ABO₃ perovskite single phase structure, the δ value should be restricted to a very narrow range which is no more than 0.006 in present samples. Also, a slight shift of diffraction peaks to higher 2θ values with the increasing δ value was observed, especially for the (110), (200) and (211) peaks, which indicated that the unit cell volumes of (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics decreased as the non-stoichiometric level of Ti ions in present ABO₃ perovskite structure increased. Similar phenomena have been previously reported in Ca substituted BST  ceramics [13] and in our previous work for (La, Sb)- doped (Ba_0.74Sr_0.26)TiO₃ ceramics [14]. Apparently, this shrinkage of unit cell volume is mainly attributed to the appearance of B-site vacancies V_Ti″″ and oxygen vacancies in (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics which is revealed by the following point defect reaction equation:

                 (3)

Fig. 1 XRD patterns for (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ+0.6%Sb₂O₃ ceramics

Fig. 2 XRD patterns for Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics with different Sb₂O₃ contents (1—0; 2—0.4%; 3—1.2%)

The X-ray diffraction patterns of sintered Sb₂O₃- doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996 bulk ceramics are shown in Fig. 2. All these polycrystals are single-phase compounds with perovskite structure, which implies that Sb³⁺ ions have incorporated into the lattice and thus maintain the perovskite structure of non-stoichiometric barium strontium titanate solid solution. The XRD profiles focusing on the (110) diffraction peaks are presented in Fig. 2(b). It shows that the diffraction peaks move towards lower 2θ values and then shift to higher 2θ values as the Sb₂O₃ doping content increases in (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics, which reveals a variation of the unit cell volume for Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics. To be specific, with the increase of Sb₂O₃ content, the unit cell volume increases and then decreases slightly. In terms of size, the ionic radii of Ba²⁺, Sr²⁺ in 12 coordination and Ti⁴⁺ in 6 coordination are 0.161, 0.144 and 0.061 nm, respectively. The radius of Sb³⁺ ion in 6 coordination is 0.076 nm which is bigger than that of Ti⁴⁺ ion but smaller than that of host A-site ion. Therefore, B-sites in the perovskite lattice are partially occupied by bigger Sb³⁺ ions for present low Sb₂O₃ doping content samples, consequently causing the increase of the unit cell volume. While the above shrinkage of unit cell suggests that the substitution of Sb³⁺ ions for the host A-site ions takes place in high Sb₂O₃ doping content samples. The substitution preference of Sb³⁺ ions in present system is opposite to that in (Ba_0.992-xSr_xY_0.008)TiO_3.004 ceramics [11]. This difference of Sb³⁺ substitution preference in perovskite lattice can be explained by the existence of B-site vacancies V_Ti″″ in non-stoichiometric (Ba_0.75Sr_0.25)- Ti_0.998O_2.996 ceramics as shown in Eq. (3). The incorporation of Sb³⁺ ions into the lattice also brings about some kinds of point defects which turn out to be the main factors changing the dielectric characteristics of non-stoichiometric barium strontium titanate ceramics. In (Ba_0.75Sr_0.25)Ti_0.998O_2.996 samples with low Sb₂O₃ doping concentration, Sb³⁺ ions enter B-sites in the perovskite structure and serve as an acceptor dopant. The defect reaction is as follows:

                    (4)

For high Sb₂O₃ doping concentration samples, Sb³⁺ ions tend to occupy the A-sites and serve as a donor dopant. The defect reaction is as follows:

                     (5)

Figure 3 shows the surface morphologies of 0.6%Sb₂O₃-doped Ti deficient (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics. And the surface morphologies of Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics are shown in Fig. 4. It appears that all samples exhibit dense microstructure and no abnormal grain growth is observed. There is   no  obvious  change  in  the  average  grain  size  of (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics as the δ value increases, presenting that the non-stoichiometric level makes little contribution to refine the grain size. Distinguishingly, the average grain size of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics decreases dramatically after applying the Sb₂O₃ dopant while remains almost the same when gradually increasing the Sb₂O₃ content. And the grain size distribution of sintered ceramics can be refined by Sb₂O₃ addition.

Fig. 3 SEM images of 0.6%Sb₂O₃-doped (Ba_0.75Sr_0.25)- Ti_1-δO_3-2δ ceramics

Fig. 4 SEM images of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics with various Sb₂O₃ contents

3.2 Dielectric characteristics

Table 1 shows the relative dielectric constant and dielectric loss of 0.6%Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_1-δ- O_3-2δ ceramics with different δ values at room temperature. It is obvious that all the non-stoichiometric (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics possess high relative dielectric constant (more than 4000) at room  temperature. With the increase of δ value which is in the range of 0.002-0.006, the relative dielectric constant increases notably while the dielectric loss decreases considerably. This demonstrates that a proper non-stoichiometric level in (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics helps to improve the dielectric properties at room temperature. n_Ba²⁺/n_Ti⁴⁺>1 in BaTiO₃ has been demonstrated by HYATT et al [15] to reduce the loss tangent particularly in the paraelectric region above the Curie temperature. In our present (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics satisfying the condition of (n_Ba²⁺+n_Sr²⁺)/n_Ti⁴⁺>1, the dielectric loss is now proved to decrease with the increasing non-stoichiometric level, which is in accord with Hyatt’s results.

Table 1 Dielectric properties of (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ+   0.6% Sb₂O₃ ceramics

Temperature dependence of relative dielectric constant and dielectric loss for 0.6%Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics is shown in Fig. 5. The relative dielectric constant first increases, achieves a maximum remarked as  and then decreases with increasing temperature. Contrarily, the dielectric loss decreases at the beginning, achieves a minimum and then increases with increasing temperature. The temperature corresponding to the relative dielectric constant maximum is taken as the Curie temperature T_C. It is obvious that the Curie temperature (see Table 1) decreases with increasing δ value. The B-site vacancies V_Ti″″ and oxygen vacancies  in (Ba_0.75Sr_0.25)Ti_1-δ- O_3-2δ ceramics resulting in the shrinkage of crystal lattice as mentioned above lead to a distortion of the ABO₃ perovskite structure thus inducing a drop in the Curie temperature. Also, the lattice deformation and inner stress were reported to cause the rise of dielectric constant [16], which exactly explains the increase of relative dielectric constant with the increasing δ value in present (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics. It is noteworthy that in the whole temperature range the higher the δ value is, the lower the dielectric loss of (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics becomes. This phenomenon is related to the existing point defects as well. Oxygen vacancies  residing at the corners of octahedra are well interconnected and therefore can be regarded as relatively mobile defects. The mobile defects migrate to domain boundaries. Orientation of the elastic dipoles caused by  results in the domain-wall pinning and thus the reduction of the dielectric loss.

Fig. 5 Temperature dependence of relative dielectric constant (a) and dielectric loss (b) for (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ+0.6% Sb₂O₃ ceramics

The relative dielectric constant and dielectric loss of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics with different Sb₂O₃ doping contents at room temperature are shown in  Table 2. It is clear that the relative dielectric constant of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics decreases dramatically with increasing Sb₂O₃ addition content. The substitution for A/B-site with any of the Sb³⁺ ions leads to a shorter distance between the B-site ion and its nearest neighbors of the octahedron, so the movement of B-site ion is restricted, which weakens the spontaneous polarization of grain lattice, and consequently, the dielectric constant decreases with increasing Sb₂O₃ content macroscopically. The dielectric loss of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics increases with the increase of Sb₂O₃ content. Orientation of the electric dipoles formed from -V″_A/Sb′_Ti- complexes shown in Eqs. (4) and (5) leads to domain-wall pinning and thus a reduction of the dielectric loss. In titanium-containing ceramics the dissipation factor is always related to the reduction of Ti⁴⁺ ion [4]. When the Sb³⁺ ions substitute for host ions in present samples, the electrons will be generated and then trapped by the Ti⁴⁺ ions. Hereby, the limited reduction of Ti⁴⁺ ion in (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics is sufficient to cause a severe deterioration in dielectric loss. Therefore, the electric dipoles related to the point defect and the reduction of Ti⁴⁺ ion caused by Sb³⁺ ion substitution for host ion become two conflicting factors for the dielectric loss. Apparently, the latter is the controlling factor for Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998-O_2.996 ceramics since the dielectric loss increases with the increasing Sb₂O₃ content.

Table 2 Dielectric properties of Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics

Temperature dependence of relative dielectric constant and dielectric loss for Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996  ceramics is shown in Fig. 6. It is obvious that the Curie temperature of Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics (see Table 2) shifts to lower value with increasing Sb₂O₃ doping content, which is attributed to the weakening of spontaneous polarization of the grain lattice caused by the substitution for host ion with any of the Sb³⁺ ions as mentioned before. The charged vacancies caused by A-site /B-site substitution give rise to the local deformation of the perovskite unit cells, which also causes the reduction of Curie temperature. In the whole temperature range relative dielectric constants of Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996  ceramics are significantly suppressed as the Sb₂O₃ content increases. At high Sb₂O₃ doping contents (>0.8%), the curves become flat, which implies the high thermal stability of Sb₂O₃-doped non-stoichiometric barium strontium titanate ceramics. Particularly, the peak value remarked as decreases with the increase of Sb₂O₃ content. The effects of Sb₂O₃ content on dielectric maximum are caused by the weakening of ferroelectricity, which is attributed to the replacing reaction of Sb³⁺ ions for the host ions in perovskite lattice. As shown in Fig. 6(b) the dielectric loss decreases dramatically with the increasing temperature in low temperature range and reaches the minimum at around 35 °C. Also, (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics with high Sb₂O₃ content exhibit better thermal stability of dielectric loss than that with low Sb₂O₃ content.

Fig. 6 Temperature dependence of relative dielectric constant (a) and dielectric loss (b) for Sb₂O₃-doped (Ba_0.75Sr_0.25)Ti_0.998- O_2.996 ceramics

4 Conclusions

1) (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics transform from single phase solid solutions with typical cubic perovskite structure to multiphase compounds with the increase of δ value while (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics remain to be single-phase solid solutions with the increasing Sb₂O₃ doping content.

2) The B-site vacancies V_Ti″″ as well as oxygen vacancies exist in (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics and the -V″_A/Sb′_Ti- complexes show in Sb₂O₃- doped (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics. Due to the existence of V_Ti″″ in Ti deficient (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics, the substitution preference of Sb³⁺ ions is opposite to that in stoichiometric barium strontium titanate ceramics.

3) The distortion of the ABO₃ perovskite structure caused by V_Ti″″ and  facilitates the drop of Curie temperature and the rise of relative dielectric constant in (Ba_0.75Sr_0.25)Ti_1-δO_3-2δ ceramics with increasing δ value. The orientation of the elastic dipoles caused by leads to the domain-wall pinning and thus the reduction of the dielectric loss. With increasing Sb₂O₃ content, the relative dielectric constant, dielectric constant maximum and Curie temperature of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics decrease dramatically. Contrarily, the dielectric loss increases with the increasing Sb₂O₃ content because of the reduction of Ti⁴⁺ ion induced by Sb³⁺ ion substitution for host ion.

4) The average grain size and grain size distribution of (Ba_0.75Sr_0.25)Ti_0.998O_2.996 ceramics can be refined by Sb₂O₃ addition.

Acknowledgements

This work is supported by the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions and also sponsored by Suzhou Pante Electric Ceramics Tech. Co., Ltd., China.

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氧化锑掺杂贫Ti钛酸锶钡陶瓷的介电性能及缺陷行为

张　晨，凌志新，简　刚，陈方旭

江苏科技大学 江苏省先进焊接技术重点实验室，镇江 212003

摘  要：采用固相法制备Sb₂O₃掺杂的贫Ti钛酸锶钡陶瓷，通过SEM、XRD和LCR测试系统研究其显微结构及介电性能随非化学计量比及Sb₂O₃含量的变化。结果表明：随着δ值增大，(Ba_0.75Sr_0.25)Ti_1-δO_3-2δ陶瓷由典型立方钙钛矿结构单相固溶体转变为多相化合物，而(Ba_0.75Sr_0.25)Ti_0.998O_2.996陶瓷随Sb₂O₃掺杂量增加始终为单相固溶体。由V_Ti″″及引起的ABO₃型钙钛矿晶胞畸变导致(Ba_0.75Sr_0.25)Ti_1-δO_3-2δ陶瓷随δ值增大居里温度降低且相对介电常数增高。弹性偶极子的定向引起畴壁钉扎而导致其介电损耗降低。(Ba_0.75Sr_0.25)Ti_0.998O_2.996陶瓷的相对介电常数、介电常数最大值及居里温度均随着Sb₂O₃掺杂量增加显著降低而其介电损耗却随Sb₂O₃掺杂量的增加而   增大。

关键词：钛酸锶钡；缺陷；介电性能；非化学计量陶瓷

(Edited by Xiang-qun LI)

Foundation item: Project (BK20140517) supported by the Natural Science Foundation of Jiangsu Province, China; Project (14KJB430011) supported by Jiangsu Provincial Natural Science Foundation for Colleges and Universities, China

Corresponding author: Chen ZHANG; Tel:+86-511-84401184; Fax: +86-511-84407381; E-mail: czhang1981@hotmail.com

DOI: 10.1016/S1003-6326(17)60294-2