Revisiting the phase transitions in Ba x Sr 1-x TiO 3 at low doping range (x 0.1)

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Revisiting the phase transitions in Ba _x Sr _1-x TiO ₃ at low doping range (x 0.1)

J.X. Wang

^a^,^c^,^*

, C. Zhang

^a

, J.-M. Liu

^b^,^**

aSchool of Physics and Engineering, Henan University of Science and Technology, Luoyang 471003, China

bLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

cHenan Key Laboratory of Photoelectric Energy Storage Materials and Applications, Luoyang 471003, China

a r t i c l e i n f o

Article history:

Received 2 May 2017 Received in revised form 3 March 2018

Accepted 14 March 2018 Available online 15 March 2018

Keywords:

BaxSr1-xTiO3solid solution Dielectric property Pyroelectric performance Ferroelectricity

a b s t r a c t

Ba^2þ substituted SrTiO3, BaxSr1-xTiO3(BST) at low doping range (x0.1) is of particular interest for studying the phase transitions induced by a small amount of polar dopants in the paraelectric matrix SrTiO3. We have studied the crystal structure, dielectric and pyroelectric properties of BST (x0.1) in this paper. It is revealed that a few of polar dopants Ba^2þin quantum paraelectric matrix SrTiO3, not only enhance the phase transitions of SrTiO3 around 18 K and 37 K, which are suppressed by the high quantumﬂuctuations in pure SrTiO3, but bring about“local ferroelectric symmetry breaking”. However, no structural transition related to the ferroelectric phase in the BST (x0.1) at macroscopic scale.

1. Introduction

The hybridization between Ti-3d and O-2porbitals makes Ti⁴^þ ferroelectric active [1]. However, this is only one side of the matter, and not all perovskite ATiO3(A¼Ca, Sr, Ba) are ferroelectrics. In fact, whether the hybridization allows off-center ion displacements is subject to the A-site size. CaTiO3 (CTO) is a stable quantum paraelectric, while SrTiO3(STO) is a marginal quantum paraelectric, and in contrast, BaTiO₃(BTO) is a typical ferroelectric. The critical size of Sr²^þallows a ferroelectric phase transition occur at about 37 K associated with softening of the IR-active mode. However, another kind ofﬂuctuations, i.e., quantumﬂuctuations (QFs) are highlighted at this low temperatureTrange, and the smaller Ti⁴^þ ion displacement submerged into the higher QFs [2e4]. Just for this reason, STO is featured by a high dielectric plateau below 4 K, and absent of long range ferroelectric (FE) order even thoughTis down to the lowest limit available so far [5,6], and it is so easy to get STO polarized even by weak compositional and structural perturba- tions, such as isotope ¹⁸O substitution of ¹⁶O [7,8], Sr/Ti non- stoichiometry [9e13] and substitution of Sr²^þby Ca²^þ[14,15], Ba²^þ

[16e20], and Bi^3þ[21e23] etc.

Among them, barium strontium titanate (BaxSr1-xTiO3, BST) is one of the most documented system not only because of its various applications but also its interesting dielectric properties and phase transitions [16e20,24,25]. The matrix STO is highly polarizable and the implanted dopant BTO is strong polarized, so polarized BST with smallxis expected. Probably thefirst phase diagram of BST (x0.03) was produced by Bednorz with dielectric and birefri- gence tests, suggesting the existence of a ferroelectric phase transition atx¼0.005 in 1982 [26]. V. V. Lemanov gave us a full picture of phase transitions in BST in 1996 deriving from dielectric and ultrasonic measurements [16]. He claimed that BST is in a glass-like state as 0.005x0.035, based on the following experimental facts: the absence of any ferroelectric hysteresis loops below the dielectric peak temperature Tm, a broad T dependence of the dielectric anomaly and its strong frequency dispersion, as well as the dependence of the dielectric constant on time afterfield cooling. Their investigation on the thermal hysteresis of BST revealed that there is a tricritical point for the cubic-tetragonal phase transition when the first order phase transition transforms into the second order one withxdecrease. It is interesting to note here that for the critical Ba²^þconcentrationx_Cappearing ferroelectric phase transition for BST, related research groups have reported xC¼0.0002, 0.005, 0.01, 0.035, 0.094 different values [16e20], which should have to do with the purity of raw materials and

*Corresponding author. School of physics and engineering, Henan University of Science and Technology, Luoyang 471003, China.

**Corresponding author.

E-mail address:[email protected](J.X. Wang).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : h t t p : / / w w w . e ls e v i e r . c o m / l o c a t e / j a l c o m

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synthesis procedure. Subsequently, V. V. Lemanov research group derived the concentration (x-T) phase diagram of BST in the framework of phenomenological theory, which adequately agrees with the above experimental data and tells us that for a certain doping concentration, it may show two polar phase transitions asT decreases [27].

The main attention of present research is focused on BST solid solutions with smallx(x0.10), that is on the STO side, to revisit the impact of highly polarizable paraelectric host lattice STO on the polar impurity BTO induced ferroelectric phase transitions. We have combined dielectric and pyroelectric measurement techniques to investigate the evolution of electric polar state with temperature. Consequently, not only a wide-temperature range dielectric response, but also multi-peaks both in the temperature dependence of dielectric constantε׳(T) and pyro-electric currentI (T) spectra were observed.

The round wide-temperature range dielectric response can arise in two cases [28e36]. One common case is the relaxor ferroelectrics [34e36], in which there are microdomains with local compositions over length scales of 100 to 1000Å. Different microdomains are assumed to transform at different temperatures, and thus lead to what is commonly termed a ''diffused phase transition'' (DPT) due to the heterogeneous compositional distribution. Another case is the chemically homogeneous solid solution, in which the random quenched disorders lead to ''local ferroelectric symmetry breaking'' with smeared out dielectric response [28e33]. Due to the indirect dipole(s)-highly polarizable host lattice-dipole(s) interaction, some clusters of these dipoles undergo a ferroelectric domain freezing or a relaxational freezing below a transition temperatureTm. Thus a local ferroelectric phase transition or a relaxational glass transition occurs in the homogeneous composite system.

The seemingly incomprehensible point here is the actual source of the multi-peaks inε׳(T) andI(T) curves of BST with smallx0.10.

For this, it is necessary for us to make clear whether the multi- peaks are from chemical inhomogeneity or structural phase transitions at low temperature. Following this topic, the paper is organized as follows. Weﬁrst illustrate that the samples are of good quality and compositional homogeneous, the wide-temperature range dielectric response and multi-peaks feature both in the ε׳(T) and pyro-electric currentI (T) curves, take the sample BST x¼0.08 as an example, is the intrinsic behavior of BST solid solutions with low Ba content and by three different sample preparation routes. Then, the origins of these phenomena are analyzed, and ferroelectric phase evolution in series of BST at low Ba²^þdoping level (x0.1) is discussed. Finally, a summary of the work is presented.

2. Experimental details

2.1. Ba_xSr_1-xTiO₃ceramics samples synthesis

In order to investigate the intrinsic behavior of BST, the role of sample preparation and its proper characterization is most crucial.

We have fabricated the BST x¼0.08 ceramic sample by three different synthesis routes: conventional solid-state method, sol-gel process and composite-hydroxide-mediated approach, to study the impact of sample preparation on the crystal structure and electric properties. Other samples, BSTx¼0.02, 0.05, 0.10 and STO are all prepared by conventional solid-state method.

2.1.1. Synthesis of samples BST x¼0e0.10 by conventional solid- state method

The highly puriﬁed powders of BaCO₃, SrCO₃ and TiO₂ were mixed in stoichiometric ratios, followed by ground andﬁred at 1150C for 10 h in air. The resultant powders were pelletized and

sintered at 1400C for 6 h in air with intermediate grindings.

2.1.2. Synthesis of BST x¼0.08 by sol-gel process

Titanium tetraisopropoxide (98%, 12.5 ml) was added to ethylene glycol (30 g) in a 250 ml beaker under magnetic stirring, and the mixture was stirred for 10 min to obtain a clear transparent solution. To this solution was added dried citric acid (99.5%, 30.3 g).

The contents were stirred at room temperature for about 4 h until all the citric acid dissolved and a clear solution was obtained. Next, 7.0444 g Sr(NO3)2 and 0.7565 g Ba(NO3)2 were added to the ethylene glycoletitanium tetraisopropoxideecitric acid solution.

The mixture was stirred on a magnetic stirrer for 5 h, and a clear transparent solution was obtained. This pale yellow solution was stirred further at 55C for 2 h. The solution was then kept in an oven at 150C for 20 h to evaporate the solvent and to promote polymerization. Consequently, the solution became a black viscous resin. This resin was charred in an electrically heated furnace for 5 h at 550C and then cooled to room temperature. The resin turned into a black mass, which is referred to as the precursor.

White BST powder was obtained by heating this precursor after grinding to powder at 900C for 10 h. The product was compacted into pellets at a pressure of 26Mpa and then sintered at 1100C for 6 h.

2.1.3. Synthesis of BST x¼0.08 by the composite-hydroxide- mediated approach

The synthesis is performed in the following steps: (1)An amount of 12 g mixture of hydroxides (NaOH/KOH¼51.5:48.5) was placed in a 25 ml Teﬂon vessel with a cover. (2) A mixture of 0.1 g TiO2, 0.17 g SrCO3and 0.02 g BaCO3was used as raw material for reaction, and was placed on the top of hydroxide in the vessel. The vessel was put in a furnace, which was preheated to 200C. (3) After the hydroxides in the vessel were totally molten, the hydroxide solution was stirred by shaking the covered vessel to ensure the uniformity of the mixed reactants. (4) After reacting for 48 h, the vessel was taken out and cooled to the room temperature. Deionized water was added to the solid product. The product was ﬁltered and washed by deionized water and hot water to remove hydroxide on the surface of particles. (5) The synthesized product was compacted into pellets under 26Mpa pressure and then sintered at 1100C for 6 h.

2.2. Structure and electrical property characterization

The sintered pellet samples were crashed intoﬁne powders, and the crystalline structures of these samples were checked by the powder X-ray Diffraction (XRD) using a D8 Advance Bruker ma- chine with Cu Ka radiation at room temperature in the angle range of 20<2q<70.

In order to test the electrical property of the samples, the thin plate-like samples (with thickness down to 0.2 mm) were coated with top and bottom Au electrodes and the measurement was performed in the cryogenic vacuum chamber of physical properties measurement system (PPMS) (Quantum Design, Inc.). Dielectric constant (ε⁰andε'') was measured by HP4294A impedance analyzer attached to PPMS. Polarization (P) was measured by pyroelectric current method detected by Keithley 6514 electrometer. For the cooling procedure, each sample wasfirst cooled down to 2 K under a poling electric field (E¼10 kV/cm), followed by a short-circuit procedure of sufficiently long duration until the current (in-situ monitored by a Keithley 6514) was below 1.0 pA. Then, the T- dependence of polarizationPwas obtained by integrating the pyroelectric current through warming the samples at a rate of 2 K/

min, while different warming rates were used to check the reli- ability of the pyroelectric current data.

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3. Results and discussion

3.1. Comparison of structural and electrical properties of BST x¼0.08 ceramic sample by three synthesis routes

In the preceding sections, the wide-temperature range dielectric response, multiple peaks in ε׳(T) andI(T) curves are highlighted, and the goal here is to exclude the possibility that these observed novel phenomena are caused by the bad sample quality. The XRD patterns of BST x¼0.08 with different synthesis methods are shown in Fig. 1. Fig. 1(a) is the measured XRD q^-2q ^{spectra in} 20ºe70 range for nominal BST x¼0.08 samples with different synthesis methods respectively, and they are all identiﬁed as single pseudo-cubic phase without secondary phase. It implies that all the samples are chemically homogeneous at macro scale. The peak positions (diffraction angle 2q) shift to the higher angle for the samples fabricated by sol-gel process and composite-hydroxide- mediated approach compared to the sample prepared by conventional solid state method, as shown inFig. 1(b) and (c), which can be attributed to the different effective Ba^2þ/Sr^2þratios in the samples by the three synthesis methods.

Although it is well acceptable that the BST system is a kind of solid solution at the whole doping range, However, BaxSr1-xTiO3at low doping concentration here, chemically homogeneous at macro level, is not necessarily homogeneous at micro level too. Thus, we consult to the elemental mapping by high resolution scanning electric microscopy. Unfortunately, because of the high degree electrical isolation of the sample, we can't get the elements distribution of the sample at nanometer scale, and only get a homogeneous elements mapping at macro level (data not shown here).

Since BTO and STO have different crystal lattice constants, different clusters with different chemical composition should have different crystal lattice constants. Their TEM morphology images show the same crystal structure and crystal lattice constant (data not shown here), implying that at the micro level, the samples are very likely homogeneous too. So far, we can say that the BaxSr1-xTiO3(x0.10) ceramics samples here are chemically homogeneous solid solutions, as extensive documents reported.

Then, we are going to discuss the inﬂuence of different synthesis methods on the sample physical properties. Here, we are interested in the dielectric and ferroelectric behaviors. Fig. 2 depicts the temperature dependence of pyroelectric currentI(a), real part of the dielectric constant (b) and dielectric loss tgd(c) for BSTx¼0.08 samples by three different synthesis routes. The most interesting thing is that all theI(T) curves of the three nominal BSTx¼0.08 samples present multiple peaks, although the pyroelectric currents generated by the three samples display different peak intensity and different peak positions, for the grain size and real Sr²^þ/Ba²^þstoi- chiometric ratio of the three samples are different.

Generally, the pyroelectric current peaks appearing in theI(t) curves indicate phase transitions or thermal activation of possible trapped charges in the ceramic polycrystalline sample upon T increasing. In order to exclude the contributions of thermal activation and separate the current due to the ferroelectric phase transitions, take the sample BST x¼0.08 made by conventional solid-state method as an example, we have collected the pyroelectric current data at different heating rates of 2 K/min and 4 K/

min after poling electricﬁeld of 10 kV/cm during cooling, and the data at 4 K/min after poling electric ﬁeld of 10 kV/cm during cooling, which is shown by the insets ofFig. 2(b). It is seen that the Fig. 1.(a) Measured XRDq-2qspectra for nominal BSTx¼0.08 samples with different

synthesis methods. The local (110)(b) and (220) (c) reﬂections are taken from (a).

Fig. 2.Tdependence of pyroelectric currents (a), the real part of dielectric constants (b) and dielectric loss tgd(c) for BSTx¼0.08 samples by the three synthesis routes. The insets showTdependence of pyroelectric current of BST x¼0.08 fabricated by solid states method at warming ramp rate of 2 K/min (black solid line) and 4 K/min (red solid line) after poling electricfield of 10 kV/cm, and the data at 4 K/min warming ramp rate after poling electricfield of10 kV/cm (blue solid line). (For interpretation of the references to colour in thisfigure legend, the reader is referred to the Web version of this article.)

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current value for the heating rate 4 K/min is about 2 times of that of 2 K/min, and there is a small acceptable shift(z1 K) in the peak position ofI(T) curve for the heating rate of 2 K/min and 4 K/min.

Moreover, the pyroelectric current can be fully reversed by the negative external electric ﬁeld (E¼±10³kV/m, V¼4 K/min). All these pyroelectric data have conﬁrmed one thing: the pyroelectric current is the pure signal of ferroelectric property of the BST system.

In the following part, the results of the dielectric measurements are discussed. One can observe that the change trend of dielectric constant versusTfor the three samples is similar: the curves grow rapidly at the high-Tside, while decline relatively slowly at the low- Tside, and show a diffused (wide-Trange) dielectric response. The sample made by solid state method displays theﬁrst feature of dielectric anomaly most prominently, while the second feature of dielectric anomaly appears most noticeable by the sample fabricated by composite-hydroxide-mediated approach, and all the three samples show obviously the third feature. Meanwhile, there is a rough one-to-one correspondence between dielectric loss peaks and pyroelectric current peaks for all of the three BST x¼0.08 samples. It's well-known that for the ferroelectrics, the dielectric loss peaks is always related to theﬂop of the electric dipole moments or structural phase transition (relative displacement). Thus, both those pyroelectric and dielectric responses should be the directly manifestations of ferroelectric property of BST system.

3.2. Dopant Ba²^þinduced phase transitions in BaxSr1-xTiO3with x0.10

Based above discussion, one can get the information that Ba_xSr_1-

xTiO3withx0.10 is chemically homogeneous, and the multiple peaks appearing in the Tdependence of dielectric constant and pyroelectric current, as well as the diffused dielectric response, is the reflection of the ferroelectric nature of the BST system.Fig. 3 gives us the real part of dielectric constant versusTof all the BST (0x0.10) samples at different frequencies, f¼1 kHz, 10 kHz, 100 kHz.Fig. 3(a) presents the measuredε⁰(T) data with different frequencies for the namely chemically pure form STO ceramics sample we have fabricated. One can see not only a dielectric platform below 10 K, but also a relaxed dielectric anomaly. The former is the response of high intensity of quantumfluctuation. As known to all, it's just the about same strength of the long range FE order and large quantumfluctuations (QFs) at the lowTin the perfect STO, leads to a dielectric platform below 10 K. However, for the polycrystalline STO ceramics here, defects inevitably appear in it, and lead to the consequence that ferroelectric order is stronger than quantum fluctuations by a narrow margin. The relaxed dielectric peak are a dynamical response of the unstable ferroelectricity in the imperfect STO ceramics.

While, the ε⁰-T curves of doped samples, x¼0.02, and 0.05(Fig. 3(b)), 0.08 and 0.1 (Fig. 3(c)), are all diffused, but frequency independent. Those dielectric features are different from the dispersive dielectric response of the relaxor ferroelectrics. In the typical relaxor ferroelectrics crystals and ceramics, such as complex perovskite Pb(Mg_1/3Nb_2/3)O₃ [PMN] and (Pb_1-xBa_x)(Zr_yTi_1-y)O₃ [PBZT], tungsten bronze (Sr_1-xBa_x)Nb₂O₆ [SBN], other systems Ba(Ti1-xSnx)O3, the quenched compositional disorder is often inhomogeneous, e.g. small regions of the ordered state with local compositions over about 10 nm length scale are embedded in a disordered matrix. These polar nanoregions (PNRs) can be regarded as quenched phaseﬂuctuations. Different PNRs in a macroscopic sample are assumed to transform at different temperatures and with different relaxation time. Thus the temperature dependence of dielectric permittivity gets diffused and is frequency dependent.

Contrasting the dielectric behavior of the relaxor ferroelectrics with that of the BST (x0.1) solid solution, one can deduce that for the chemically homogeneous BST (x0.1)ceramics, BTO PNRs (all the unit cells in each PNR are spontaneously polarized in one and the same direction) begin to appear below a certain temperatureTBand are uniformly embedded in the paraelectric STO surroundings, growing when temperature below a certain temperature which depends onxvalue. Besides, the number of unit cells in each PNR is almost equal and accordingly, the magnitude of PNR dipole moment is nearly equal too. Roughly uniform distribution of the ferroelectric structure (PNRs) gives rise to frequent independent dielectric response. Relative short correlation length of the ferroelectric phase, which is interrupted by vast areas of paraelectric phase (matrix), leads to the diffused phase transition. As for the strong quantum ﬂuctuations make the ferroelectric correlation length short and dielectric constant versusTresponse rounded, we have made statements in our early work [37]. And consistent with our view, A. V. Sotnikov reported multiple dielectric peaks phenomena in BaxSr1-xTiO3at low doping concentration and attributed the multiple dielectric peaks to multiple polarization process [20].

Thus we have good reason to believe that multipeaks observed in ε⁰-TandI-Tcurves here also can be attribute to different polarization processes in the Ba_xSr_1-xTiO₃(x0.10) solid solution.

Fig. 3. Variation of the real part of dielectric constant with temperature at 1 kHz, 10 kHz, 100 kHz frequencies for BSTx¼0, 0.02, 0.05, 0.08 and 0.10.

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The stable ferroelectric ordering in BST (x¼0.02, 0.05, 0.08 and 0.10) also can be seen in their pyroelectricI-Tcurves(seen inFig. 4).

BSTx¼0.02 has shown us a signiﬁcant pyroelectric current and the polarization value at 2 K measured by this pyroelectric method is 3.474m^C/cm²(data not shown), which is far greater than the low

limit polarization value 0.06m^C/cm²^{that the}^P-Eloop can detect. So, BSTx¼0.02 is bound to haveP-Eloop below the phase transition temperature, though we have not characterized the P-E loops.

Obviously, the performance of BSTx¼0.02 here is very different from that of BSTx¼0.02 V. V. Lemanov reported [16], which shows evident diffused dielectric permittivity, but noP-Eloop, and was identiﬁed as dipole glass. With increasingx,the peak position of the BSTε⁰-Tand pyroelectric currentI-Tspectra for small Ba concentration shift to the high temperature, implying that the biggerx, the higher ferroelectricity. Therefore, ferroelectricity in BSTx¼0.02, 0.05, 0.08 and 0.1 has grown very stable. For the further demon- stration analysis, we consult to the pyroelectric behavior of BaxSrl- xTiO3series ceramics with differentx(x0.10).

The namely chemically pure STO ceramics sample here shows a pyroelectric current, and this pyroelectric current peak position remains unchanged as STO is doped with BTO (seen inFig. 4(a~e)).

Those tell us that this pyroelectric current peak comes from the STO itself. As known to all, the perfect STO crystal is a quantum paraelectric and has no ferroelectricity even thoughTis down to the lowest limit available so far. However, the STO ceramic sample here, certainly has some defects. These defects suppress the quantum ﬂuctuations to some extent, thus the ferroelectricity emerges from the quantumﬂuctuations.

Compared to theI-Tcurve of the nominal pure STO,I-Tcurves for the doped BST present more than one pyroelectric current peaks withT increase, and all these pyroelectric current peaks are the ferroelectric characteristic of the BST system which has been confirmed. We can classify the multiple current peaks into two groups according to the relationship between the criticalTwhere the pyroelectric current gets nonzero and the doping concentration x. Thefirst group current peaks have nothing to do withx, such as the peak belowT1(z18 K) andT2(z37 K), while the second group current peaks are closely related to the doping concentrationx, whose criticalTdenoted byT₃andT₄for example. Obviously, the first group current peaks is independent of the dopant Ba²^þ, and we attribute them to the ferroelectric phase transitions of STO merged into the high quantumfluctuations, but enhanced by Ba²^þdoping, as (Sr1-1.5xBix)TiO3 has shown [21e23]. Meanwhile, the current peaks intimately associating with the dopant Ba^2þconcentration, is well attributable to the ferroelectric order caused by the indirect dipoles-highly polarizable host lattice-dipoles interaction.

The current peak starting atT1for all the samples comes from the same source as STO does. For BSTx¼0.05, 0.08 and 0.10, there is another current peak starting at about 37 K which does not shift withx. For STO, relevant studies give the conﬁrmation of ferroelectric phase transition occurring at 37 K via ultrasonic techniques, thermal conductivity studies, dielectric permittivity and hysteresis measurements, which originate directly from the freezing behavior of the soft phonon mode at the center of Brillouin zone, and the Ti^4þ ions move relatively to the O²ions along thecaxis [21]. However, the displacement is smaller than the lattice vibration displacement, and the weak ferroelectric phase merged in the lattice vibration for the perfect STO crystal [38]. The substitution of Sr²^þby a small amount of Ba²^þhelp the weak ferroelectric phase transition at 37 K stand out here.T3andT4growing withx, conﬁrms the enhanced ferroelectricity of BST withx. Besides this, the ferroelectric polarization obtained by integrating from the pyroelectric current, also increases with the doping concentrationx. It is worth noting that there are twoxdependent pyroelectric current peaks, labeled byT3

andT₄, implying phase transitions between two polar structures and from paraelectric state to ferroelectric state respectively.

Theoretically, V. B. Shirokov produced a phenomenological concentration phase diagram of Ba_xSr_1-xTiO₃[27]. In the phase diagram at small Ba concentration, there are a tricritical point and two multiphase points one of which is associated with two polar states Fig. 4.Pyroelectric currentIand the real part (ε⁰) of the dielectric constant and

evaluatedPas a function ofT, respectively, for a series of BST,x¼0.00, 0.02, 0.05, 0.08, 0.10.

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Ima2 andCcspace group. Those tell us that, BST solid solution with small Ba concentration may undergo several structural phase transitions, and probably presents different polar crystal structures under certain experimental condition. To gain the further insights into the structural origin of ferroelectric phase transition of BST (x0.1), variable-temperature powder X-ray diffraction measurements on BST x¼0.08, as an example, were performed (Fig. 5).

There is no structural transition at all to the ferroelectric phase in the BSTx¼0.08 at macroscopic scale indicated by X-ray diffraction.

A macroscopically isotropic ferroelectric phase exists below the diffused phase transition temperature T_m. Although long-range order of polar phase does not develop, but relatively short-range ferroelectric order should be frozen in local region. The local ferroelectric symmetry breaking is responsible for the pyroelectric current in BST (x0.1) and those local ferroelectric structures may have phase transitions at certain temperature.

Comparing theε⁰(T) andI(T) curves of an established sample, as shown inFig. 4(a~e), one can observe that nonzeroIfor BST solution (x¼0.00e0.10) persists to the temperature 20e30 K higher than theε⁰(T) peak positions. Itﬁts in well with the pinning effect of BTO doped in the STO matrix. So actually, let's take a look at the ferroelectric polarization generationﬁrst. Small amounts of Ba²^þ ions substitution of Sr^2þions in the matrix STO brings about lattice distortions, coming with increased ferroelastic strain that couples with the ferroelectric distortions or hybridizing with the valence states, leading indirectly to changes in the Ti-O interactions [27].

Regarding the weaker covalency between Ba and O than that between Sr and O might enhance the hybridization of Ti with O, which in turn, weakens the short-range repulsive forces and softens the ferroelectric soft mode locally, consequently, is responsible for the Ba-doping-induced ferroelectricity in STO. Unlike the prototype ferroelectric BaTiO3,Pfor BST solid solutions does not become zero untilTis more than 20e30 K higher than theε⁰(T) peak positions. To understand this result, we need to quote in full the pyroelectric test procedure. The sample isfirst subjected to poling in the cooling run down toT¼2 K under the electricfield about 10 kV/cm from the temperature 150 K (at least 60 K higher than the phase transition temperature), and the dipoles are aligned along the direction of the polingfield. Then, at the lowest temperatureT¼2 K, the electrodes

have been shortened for up to 60 min in order to release any charges due to the change of large electricfields, trapped charges, etc., in the system. Finally, the pyroelectric currentIwas recorded in the warming sequence at afixed warming rate. The measuredI(T) data were used to obtainP(T) by a simple integration procedure.P value decreases with increasingT. But the reorientation of the polar clusters is pinned by the randomfields induced by the ionic size misfit of Ba²^þand Sr²^þ, and the disappearance of thePis precluded until the temperature is much higher thanTc.

4. Conclusions

In this work, we have studied the diffused dielectric response, and multi-peaks both in theε׳׳(T) and pyro-electric current I(T) curves for BST(x0.10) ceramics samples. It is confirmed that these features are not due to chemical compositionfluctuations, but to the local ferroelectric phase transitions in the BST system as temperature decreases. The local ferroelectric phase transitions include phase transitions of the matrix STO itself enhanced by Ba^2þdoping and defects in ceramics, which is suppressed by high quantum fluctuations in the perfect pure SrTiO3, and the local ferroelectric phase transitions of ordered polar microdomains related to Ba^2þ dopants in BST(x0.10). However, there is no structural phase transitions at all throughout the temperature range 10 Ke100 K.

Acknowledgments

Weﬁrst thank Professor J.-M Liu for his academic guidance and support in this work. This work was supported by the Natural Science Foundation of China (Grants No. 11504090) and the Joint Fund of the National Nature Science foundation of China and Henan province personnel training (Grants No. 162300410089).

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