Enhanced the tunability properties of pure (Ba,Sr)TiO 3 lead free ferroelectric by polar nano-region contributions

In the present work, a series compositions of (Ba 1-x Sr x )TiO 3 (0≤x≤0.4) lead-free ferroelectric (BST) were successfully prepared by sol-gel method. The effects of Sr 2+ on the optical properties, crystal structure and morphology of BaTiO 3 were systematically investigated. Compositions with x<0.4 exhibit single phase tetragonal perovskite at room temperature, while a cubic structure of BST has been detected in the ceramic with x=0.4. Phase transition temperature was shifted toward lower temperature by increasing Sr-doping. Moreover, detailed tunability analysis for the present ceramics confirmed an enhancement by Sr-addition and the maximum values were observed at Sr=0.2. The enhancement back to present multiple components contribute to the dc-field induced dielectric constant, where at low electric fields the Langevin theory indicate present polar nano-region (PNR) contributions into tunability data, however domain switching contributions at high electric field was described by Johnson equation. Ferroelectric properties appraised by P-E loop and positive up negative down (PUND) methods have been introduced. The results shown the remnant polarization appraised by PUND method is much lower than polarization estimated by P-E loop indicate the difference values of P r is mostly owing to leakage current contribution in P-E method.


Introduction
Such kind of non-linearity dielectric materials in ferroelectric phase such as BaTiO 3 (below phase transition) possess an interesting functional properties in different area of electronic applications such as tunable microwave, piezoelectricity, resonators, capacitors, oscillators, actuators and so on [1]. However, mostly of tunable applications required a materials with low relative permittivity, low dielectric loss and high response of permittivity with external applied field [2,3] at room temperature. Pure BT prepared by solid state reaction technique have large domain size in micro-meter with longe range order which imply it can possess high relative value of dielectric constant even at room temperature and this is not suitable for tunable applications [4,5]. Such kind of ferromagnetic materials have shown great performance when used as phase shifting in phased antennas applications, due to controlled the change of the magnetic permeability of the material under the application of a dc-magnetic field [6]. However, these materials are very costly and have to be usually implemented in large size devices, posing serious limitations in practical applications. Moreover, there are different models have been derived to understand the mechanism or the relation between permittivity and dc-electric field for different systems. For instance, permittivity dependent high electric field-normally refer to domain switching-, had been derived by Johnson from the Helmholts free energy when high electric field was applied into ferroelectric material [7]. Another model for ferroelectric domain switching of PZT was proposed by Uchida et al [8], and they suggested it is 90 o domain reorientation by applying high electric field. Diamond [9] proposed that the nonlinearity of dielectric constant of BST is attributed to field induced phase transition from non-ferroelectric to ferroelectric. For ferroelectric system with a single polarization mechanism, the dc-tunability data can be descriped by Landau-Ginzburg-Devonshire (LGD) model [10]. Another model has been derived by Langevin [11] for interpretation the nonlinearity of permittivity dependent low electric field (below domain switching) based on polar nanoregion (PNR), where PNR can define as nano-scale areas with spontaneous polarization and the dipoles are easily re-orientable under low external electric fields [12]. Recently, many researchers have been interested in (Ba,Sr)TiO 3 (BST) ferroelectric materials, which have shown superior tunable dielectric properties for potential applications in microwave devices, such as antennas, resonators, tunable filters and phase shifters in the microwave frequency range [13,14]. Pure (Ba 0.6 Sr 0.4 )TiO 3 ceramics prepared by solid state reaction technique and sintered at 1350°C for 2h showed high permittivity (~ 4400) and large dielectric loss (~ 0.1), which precluded their use in most tunable applications [15]. Adding non-ferroelectric materials with low permittivity values such as (MgO, Al 2 O 3 , BaWO 4 ) into BST ferroelectric compound shown effective enhancement the functional properties of BST including reducing the permittivity which can make the material a more suitable in tunable applications [16,17]. Addition of MgO, was reported suppress the dielectric constant of BST and as consequently enhanced their tunable properties. Low percentage of Al 2 O 3not excess 1%-shown a great effect on reducing the dielectric properties of BST and improved the tunable properties at room temperature. One of the appropriate method can pinched the dielectric properties of BST is reducing the domain size from Polar micro-region to polar nano-region. This can achieve by prepare the material in nano range of particles by physical method such as High-activation energy ball milling or chemical method such as Sol gel method [18,19].
In this study, we aim to enhancement the tunable properties of pure BST by reducing the domain size to be in nano range and as consequently pinched the dielectric properties by using Sol-gel method. Furthermore to understand the mechanism of the electric field dependence of the dielectric constant in (Ba 1-x Sr x )TiO 3 (0≤x≤0.4) with particular attention to the polar nanoregions contribution of polar nano-regions on the tunability behaviour. The interpretation is supported by the study of the P-E loops and the PUND curves.

Experimental
Perovskite (Ba 1-x Sr x )TiO 3 ceramics (0.0≤x≤ 0.4) were synthesized using the sol-gel technique. High purity of barium acetate Ba(CH 3 COO) 2  (Technochem 98%) was used as a solvent for titanium isopropoxide. The procedure for the preparation of BST ceramics is reported in the flowchart shown in Fig.1 and described in details in our previous work [20,21]. The calcined powders at 1000 o C/3h were mixed with a binder polyvinyl alcohol and pressed into discs of 10mm diameter and 1.5mm thickness. The discs were sintered at 1300 o C for 2h in air. Small amount of calcined powder was mixed with potassium bromide (KBr) and Fourier transform infrared-spectrometer (FT/IR-4100) was used to identify the infrared-active functional groups. The phase identification was carried out by analysing the X-ray diffraction (XRD) patterns obtained with PANalytical X'Pert PRO diffractometer. The morphology of BCT ceramics was examined by scanning electron microscopy (SEM) (JEOL JSM-840A). The sintered samples were electroded with fired silver paste for dielectric, tunability, piezoelectric and ferroelectric measurements. The dielectric properties were measured on unpoled ceramics using an LCR meter (HIOKI 3532-50 LCR HITESTER). The electric-field dependence of the dielectric response (tunability) and ferroelectric properties were measured at room temperature using a specific tester (RADIANT Precision II Multiferroic Ferroelectric Test System 10kV HVI-SC Model 609B).  The relative permittivity slightly decreases with increasing frequency at low temperature, while it becomes nearly frequency-independent at high temperature (from ~110 o C up to Curie temperature). The dielectric loss (tan δ) increase with increasing frequency in the whole temperature range and the maximum value of loss was detected at near the phase transition temperature.

Tunability Properties
The dc-electric field dependence of the permittivity at room temperature of (Ba 1-x Sr x )TiO 3 (0≤x≤0.4) ceramics is shown in Fig.(7a). It can be observed that all the compositions display similar behavior, characterized by a decrease of the dielectric permittivity with increasing applied field. The variation of the dielectric constant with an applied DC electric field is termed tunability, which can be defined by the following relationship: where (0) is the dielectric constant in absence of the DC electric field and ( ) is the dielectric constant in presence of an applied DC electric field of magnitude E. The tunability of the BST ceramics at different dc electric fields is displayed in Fig. ( The tunability phenomenon is not exclusively observed in ferroelectric materials, but it has also been reported in relaxors such as (Ba,Sn)TiO 3 [24], non-ferroelectric dielectric materials like BaWO 4 [25], and ferroelectric materials in their paraelectric phase such as SrTiO 3 [26]. It is worth to recall that the macroscopic tunability can reflect a number of bias field-induced effects occurring at different material length scales. These include: i) intrinsic mechanisms active at the unit cell scale, such as lattice phonons; ii) the response of polar nano-regions (PNR) at the nano-scale; iii) ferroelectric domain switching and domain wall movement at the submicron scale, and iv) interfacial grain-grain boundary effects at the micron scale [11].
The dc-tunability data can be described with appropriate models that are able to account the different underlying mechanisms active in specific ranges of temperature and DC electric field.
In polar dielectrics, the tunability is usually described by the multi-polarization mechanisms model proposed by Ang and Yu [27] ( ) = [1 + ( 0 (0) ) 3 2 where Ʃ indicates the sum over different cluster-polarization, α is the temperature-independent coefficient, 0 is the effective polarization of one cluster dependence on temperatures, and x is defined as = 0 , where V is the volume or size of the cluster and K B is Boltzmann's constant.
Also, by assume a small polarization and high dc-electric field able for domain switching was applied into ferroelectric material or in the state close to ferro-paraelectric phase transition, the tunability data can be described by Johnson scenario The dc-electric field dependence permittivity at room temperature of (Ba 1-x Sr x )TiO 3 (0.0<x<0.4) sintered ceramics shown in Fig.(6.a). As observed, all the compositions have shown the same behavior, where dielectric permittivity decreases with increasing the applied field which is attributed to the lattice deformation as an intrinsic contributions and as consequently variation the domain wall structure due to applying high external electric field [28]. Tunability of the present ceramics versus dc-electric field displayed in Fig. (6.b)  into the tunability such as polar nano-regions contributions [10]. As observed by SEM (Fig 4), the particles size were observed decreased by Sr-addition which lead to decrease the domain size, so that increasing the tunability by Sr-addition could be owing to increase the effect of PNR contributions, where PNR can define as nano-scale areas with spontaneous polarization and the dipoles are easily re-orientable under small or low external electric fields [19] which can give a contributions into permittivity and tunability as well at these range of fields. Decline slopes at high electric field can explain as the large electric fields can convert the PNR into micro or macro-scale of domains and consequently their contribution cannot detected at high fields. Decreasing of tunability data at x=0.4 can confirm by decline the slope of linear fitting at low electric field and this can interpret due to decay of PNR contribution due to form paraelectric phase at room temperature. It is worth to notice that, the tunability increase rapidly with electric field however at higher bias field it tend to be saturated. This could be owing to at high dc field, polar nano-domains would grow up accompanied by congelation of polar nanodomains which leads to a reduction of dielectric permittivity [17]. A similar behavior of tunability versus dc-field has been reported [19]. So, we can conclude that pure nano-BST with Sr=0.2 ferroelectric material has an acceptable and appropriate tunability properties which makes the material a promising candidate for tunable devices applications.

Ferroelectric properties
The polarization-electric field (P-E) hysteresis loops of BST ceramics are presented in Third (-P*) and Fourth (-P^) pulses were used for the same purpose of first and second pulses but in negative bias of electric field (pulses in down direction).
Pure BT shown high switching charged density value (Q sw = 1.239µC/cm 2 ) compared to the BT doped Sr. In both of Sr=0.1 and 0.385, both of P* and P^ are close to each other's, which confirm the switching charge density is close to zero value (Q sw ~ 0). These results confirm that, the remnant polarization estimated by P-E loop is due to the contribution of leakage current polarization with absent any contribution of switching charged density. Similar results for different composition have been reported [29]. Pure BT shown appropriate value of switching charged density (Q sw = 1.06µC/cm 2 ), however x=0.4 shown (Q sw ~ 0) due to form cubic phase at RT. Fig.1. Flowchart of synthesis BST by sol-gel method [15] .       Sr-content (mol %) Fig. (9). P-E hysteresis loops of the(Ba1-xSrx)TiO3 bipolar ceramics (0<x<0.4) at 1Hz at room temperature.