Dielectric and Ferroelectric Properties of Ho2O3 Doped Barium Strontium Titanate Ceramics

The crystalline structure, surface morphology, dielectric and ferroelectric properties of 0~10wt% Ho2O3 doped (Ba0.75Sr0.25) TiO3 ceramics prepared by conventional solid state method were studied using X-ray diffractometer, scanning electron microscopy, LCR measuring system and ferroelectric property test systems aiming for ceramic capacitor applications. It is found that proper amount of Ho2O3 can refine grains of the system. With the increase of Ho2O3 doping content, the average grain size of (Ba0.75Sr0.25) TiO3 ceramics decreases. When Ho2O3>8 wt%, (Ba0.75Sr0.25) TiO3 based ceramic samples are multi-phase compounds with typical perovskite structure accompanied by the appearance of cylindrical grains. The Ho ions substitute the host A sites and B sites of (Ba0.75Sr0.25) TiO3 perovskite lattice, resulting in the lattice distortion of the system and the change of the relative dielectric constant and dielectric loss at room temperature. With the increase of Ho2O3 doping content, the relative dielectric constant at room temperature of the system increases first and then decreases. The maximum of relative dielectric constant at room temperature can be found in the 1 wt% Ho2O3 doped (Ba0.75Sr0.25) TiO3 ceramics. When Ho2O3>1 wt%, the maximum of relative dielectric constant εrmax decreases and the temperature corresponding to the maximum of relative dielectric constant Tm shifts toward lower temperature with the increase of Ho2O3 doping content. The (Ba0.75Sr0.25) TiO3 ceramics with high Ho2O3 content show relaxor-like behavior which is characterized by the typical diffuse phase transition and frequency dispersion of dielectric constant. However, the (Ba0.75Sr0.25) TiO3 ceramics with low Ho2O3 content do not exhibit permittivity frequency dispersion. According to the P-E hysteresis loops of Ho2O3 doped (Ba0.75Sr0.25) TiO3 ceramics, the ferroelectricity was increased and then decreased with the increase of Ho2O3 doping content. With the increase of Ho2O3 doping content, the P-E relationships turn out to be straight lines, implying the paraelectric phase for (Ba0.75Sr0.25) TiO3 ceramics with high Ho2O3 content.


Introduction
Barium strontium titanate ((Ba 1-x Sr x ) TiO 3 , BST), as an infinite solid solution of BaTiO 3 and SrTiO 3 , maintains perovskite structure (ABO 3 ) similar to BaTiO 3 , and has outstanding properties such as high dielectric constant, low dielectric loss and excellent ferroelectric properties [1][2]. In addition, its Curie temperature can be adjusted over a wide range of temperature by changing Ba/Sr ratio, making the BST systems become one of the basic ceramic materials for ceramic capacitors [3].
As an important component of electronic products, ceramic capacitors require superior and more stable performance in smaller sizes. In order to meet the needs in different applications, people add metal oxides or their derivatives to BST systems to improve their comprehensive properties. The rare earth metal oxides play important roles in the property modification for dielectric materials, which has aroused great interest of many researchers [4][5][6][7][8][9]. D. C. Sinclair et al. studied the rare earth metal ions RE 3+ doped barium titanate ceramics, and proposed that when the RE 3+ ions enter the A site of the perovskite lattice, charge imbalance is created which must be compensated by either cation vacancies on the A or B site (ionic compensation), or by electrons (electronic compensation) [10][11]. Three point defect reactions can be identified: When the RE 3+ ions enter the B site of the perovskite lattice, charge imbalance is created which must be compensated by the oxygen vacancies [10]. The point defect reaction can be seen as follows:  [14]. In addition, some researchers studied the effect of grain size on dielectric and ferroelectric properties of Ba 0.80 Sr 0.20 TiO 3 ceramics [15][16].
In our present work, 0~10 wt% Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics were prepared by solid state reaction method. The effects of Ho 2 O 3 doping content on crystalline structure, surface morphology, dielectric and ferroelectric properties of the system were investigated. The substitution characteristics of Ho 3+ ions in (Ba 0.75 Sr 0.25 ) TiO 3 perovskite lattice will be determined and the interrelationship between the macroscopic dielectric constant, dielectric loss, temperature-dependent properties and microscopic defect behavior will also be established.

Sample Preparation
In this paper, high purity BaCO 3 (>99.0%), SrCO 3 (>99.0%) and TiO 2 (>98.0%) powders used as starting raw materials were proportionally weighed according to the formula (Ba 0.75 Sr 0.25 ) TiO 3 and ball-milled for 24 h. After drying, the obtained powders were calcined at 1080°C for 2 h to form main crystalline phase. The calcined powders were mixed with 0.2 wt% MgO (≥98.5%)、0.2 wt% ZnO (≥99.0%) and 0~10 wt% Ho 2 O 3 (>99.0%), reground for 24 h, dried and added with 5 wt% polyvinyl alcohol (PVA) as a binder for granulation. The mixtures were sieved through 40-mesh screen and then pressed into pellets 10mm in diameter and 2~3 mm in thickness. Sintering was conducted in air at 1400°C for 2 h, and the sintering regime was illustrated on Figure 1. For dielectric properties measurement, both the flat surfaces of the sintered samples were coated with BQ-5311 silver paste after ultrasonic bath cleaning and then fired at 800°C for 10 min.

Equipment and Characterization
The crystal structures of the samples were confirmed by X-ray diffraction analysis (XRD, Rigaku D/max 2500v/pc) with Cu Kα radiation; The phase and plane index (hkl) were obtained by search/match using Jade 6.0. The surface morphologies of the gold-sprayed ceramic samples were observed using the SEM (JSM-6480 ESEM). The capacitance quantity (C) and dissipation factor (D) were measured with TZDM-200-300B testing system. The relative dielectric constant (ε r ) and dielectric loss (tanδ) were calculated by the following Equations: where C is the capacitance quantity (pF); h is the thickness (cm); Φ is the diameter of the electrode (cm); D is the dissipation factor. The temperature dependence of dielectric parameters was measured at 1~100 kHz from −150 to 150°C. The P-E hysteresis loops of Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics at room temperature was obtained using the ferroelectric property test system.

XRD Analysis
The XRD patterns of Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics are shown in Figure 2. It appears that all samples are single phase solid solutions with typical perovskite structure. No obvious secondary phase is found even for the 10 wt%   that Ho 3+ ions are easily segregated near the grain boundary, which hinders the further grain growth [17]. However, the cylindrical grains indicating the appearance of secondary phase (marked using red circles in Figure 3 (g) and (h)) which is not detected by XRD due to its small amount exist in the 9 and 10 wt% Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics.

Dielectric Properties at Room Temperature
In the ABO 3 type perovskite structure, the coordination numbers of A and B site are 12 and 6, respectively. The radius of Ho 3+ ion (1.23 Å, in 12 coordination) is smaller than that of Ba 2+ ion (1.61 Å, in 12 coordination) and Sr 2+ ion (1.44 Å, in 12 coordination); The radius of Ho 3+ ion (0.90 Å, in 6 coordination) is bigger than that of Ti 4+ ion (0.61 Å, in 6 coordination) [18]. Therefore, Ho 3+ ion can substitute the A or B site of (Ba 0.75 Sr 0.25 ) TiO 3 perovskite lattice. When Ho 3+ ion substitutes the A site, the point defect reaction is as follows: When Ho 3+ ion begins to substitute the B site, the point defect reaction can be expressed as follows: Figure 4 shows the relative dielectric constant and dielectric loss of (Ba 0.75 Sr 0.25 ) TiO 3 ceramics at room temperature with variation of Ho 2 O 3 content. It is obvious that all ceramics possess high relative dielectric constant (ε r ≥1.69×10 3 ) and low dielectric loss (tanδ≤6.4×10 -3 ) at room temperature. With the increase of Ho 2 O 3 doping content, the relative dielectric constant of the system increases first and then decreases. The maximum of relative dielectric constant at room temperature can be found in the 1 wt% Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics. The dielectric loss of the system increases first, then decreases, and finally increases with the increase of Ho 2 O 3 doping content. When Ho 2 O 3 ≤1 wt%, Ho 3+ ions tend to substitute the A site ions of the perovskite lattice. The difference of the ionic radius causes the shrinkage deformation of the perovskite unit cells and the increase of the internal stress which result in the increase of relative dielectric constant [19]. When Ho 2 O 3 =1~8 wt%, Ho 3+ ions gradually enter the B site. The bigger Ho 3+ ions in the B site restrict the activity of B site ions and thus weaken the spontaneous polarization, which causes the relative dielectric constant decrease significantly with the increase of Ho 2 O 3 doping content. When Ho 2 O 3 ＞8 wt%, non-ferroelectric secondary phase induced by the excessive Ho 2 O 3 addition dilutes the ferroelectric phase, which makes the relative dielectric constant further decrease with the increase of Ho 2 O 3 doping content [20].
As for the dielectric loss, When Ho 3+ ions enter the A site of the perovskite lattice, the electrons as shown in Equation (7) are trapped by Ti 4+ to form Ti 3+ , leading to the increase of the dielectric loss gradually [21]. When Ho 2 O 3 =1~5 wt%, the weakened spontaneous polarization results in the decrease of the dielectric loss significantly with the increase of Ho 2 O 3 doping content. On the other hand, the oxygen vacancies as shown in Equation (8) have pinning effect on the ferroelectric domains, which also causes the decrease of dielectric loss [22][23][24]. Figure 5 shows the temperature dependence of the relative dielectric constant and dielectric loss at 1kHz~100kHz for Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics. When Ho 2 O 3 content is higher than 5 wt%, the temperature corresponding to the maximum of relative dielectric constant (T m ) for (Ba 0.75 Sr 0.25 ) TiO 3 ceramics shifts toward higher temperature obviously and the relative dielectric constant (in the T<T m range) decreases with the increase of test frequency, which is known as the frequency dispersion [25]. However, as indicated in Figure 5 Figure 5 (c)~(h), the tanδ increases significantly with the increase of test frequency at low temperature; the tanδ is almost unaffected by the test frequency at high temperature, and the tanδ remains at a low value over a wide temperature range (room temperature~150°C), indicating that Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics are promising for the application in capacitors as low dielectric loss dielectrics. The maximum of relative dielectric constant (ε rmax ) and the temperature corresponding to this maximum (T m ) of (Ba 0.75 Sr 0.25 ) TiO 3 ceramics with variation of Ho 2 O 3 content are shown in Figure 6. When Ho 2 O 3 ＞1 wt%, the T m shifts toward lower temperature and the ε rmax decreases with the increase of Ho 2 O 3 doping content. The local deformation caused by Ho 2 O 3 doping gives rise to the reduction of T m . On the other hand, the non-ferroelectric layer in grain boundary of ferroelectric ceramics makes the ferroelectricity decrease, which causes T m and ε rmax decrease. As mentioned previously, the average grain size of Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics decreases with the increase of Ho 2 O 3 doping content, which means that the grain boundary effect [26] is enhanced with the increase of Ho 2 O 3 doping content and consequently makes the T m and ε rmax decrease. Since the diffuse phase transition is generally characterized by broadening in the dielectric constant (ε) versus temperature (T) curve, the full-width of half-maximum (FWHW) of ε r -T curve is here calculated and listed in

Conclusions
The Ho 2 O 3 doped (Ba 0.75 Sr 0.25 ) TiO 3 ceramics were prepared by solid state reaction method and their crystalline structure, surface morphology, dielectric and ferroelectric properties were investigated. The results show that: (1) The proper Ho 2 O 3 doping content is beneficial to obtaining fine grain structure. When Ho 2 O 3 >8 wt%, ceramic samples are multi-phase compounds with typical perovskite structure. (2) Ho 3+ ions enter the A and B site of perovskite lattice, causing lattice distortion of the system and affecting the relative dielectric constant and dielectric loss at room temperature. In addition, the electrons and oxygen vacancies produced in the substitution process also have a certain effect on the dielectric loss.