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Dielectric Properties in the Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 Systems

Yeon Jung Kim

Center for Innovative Engineering Education, Dankook University, Yongin 16890, Korea
Correspondence to: E-mail: yjkim80@dankook.ac.kr
Received July 16, 2017; Revised July 25, 2017; Accepted July 26, 2017.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract

The dielectric constant and loss of poling/non-poling was measured in the Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 samples. The addition of La3+ to the Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 did not cause a large change in grain size. But the addition of La3+ did show transition temperature, which shifted toward low temperature in the Pb[(Mg1/3Ta2/3)Zr]O3 systems. In addition, the dielectric and pyroelectric properties (ɛ~20000, p~0.03 C/m2K) of this system using La3+ have been greatly improved. Pyroelectrics Pb0.97La0.02(Mg1/3Ta2/3)0.66Zr0.34]O3 system was found to have a relatively high ferroelectric FOMs (FV~0.035 m2/C, FD~0.52 × 10−4 Pa−1/2) at room temperature. Spontaneous polarization showed a value of 0.27~0.35 C/m2 in the composition added to La3+. The piezoelectric constant (d33=350~490 pC/N) and electromechanical coupling factor (kP=0.25~0.35) are obtained in Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 compositions with La3+ dopant.

Keywords : Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3, Ferroelectric, Relaxor, Pyroelectric
I. Introduction

Relaxor ferroelectrics crystallize to the general formula Pb(B1,B2)O3, where B1 is a low valence cation and B2 is a high valence cation, can undergo at least short-range ordering of (B1B2)-site cations. Lead magnesium tantalate Pb(Mg1/3Ta2/3)O3, a relaxor ferroelectric with a broad anomaly in dielectric constant centered around a maximum at −98°C [1]. The anti-ferroelectricity of lead zirconate PbZrO3 with a Curie temperature of 230°C is indicated by a “missed” incommensurate state [2]. Anti-ferroelectric is a material in which neighbor spontaneous polarization dipoles are anti-parallel to each other. When Pb(Mg1/3Ta2/3)O3 and PbZrO3 form a solid solution, the phase transition temperature and electrical characteristics depend on the composition. It was previously reported that solid solutions of Pb[(Mg1/3Ta2/3)Zr]O3 have the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases at PbZrO3=0.29~0.35 [3]. The substitution of small amount of La3+ to Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 systems to decreases the temperature of normal phase transition. However, the origin of these properties is not completely clear.

In this study, Pb[(Mg1/3Ta2/3)0.66Zr0.34]O3 of the MPB composition range was selected for Pb[(Mg1/3Ta2/3)Zr]O3 system. The MPB composition of Pb[(Mg1/3Ta2/3)Zr]O3 is an intense research, but the pyroelectric properties of the material depend on small doponts, such as Bi, Sn, Mn and La. Pb[(Mg1/3Ta2/3)0.66Zr0.34]O3 type polycrystalline systems were prepared by doping with La2O3. The purpose of this study was to develop materials with superior dielectric properties, higher pyroelectric properties and higher pyroelectric FOM at room temperature.

II. Experimental results and Discussion

1. Experiments and observations

Solid solution samples of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 of x=0~0.03 mole% was prepared using the columbite precursor method [4]. The most common problem when manufacturing complex perovskites of Pb-type is to optain pyrochlore phase (non-cubic) instead of perovskite. The samples were sintered at 1350°C for 4 hours at a heating rate of 300°C/h. All sintered pellets were analyzed using XRD. Prior to capacitance and pyroelectric current, the samples were poling by applying a DC electric field (20 kV/cm) for 30 minutes at room temperature. Dielectric constant is routinely calculated from the well-known equation follows $ɛ=Cdɛ0A$, where ɛ is the dielectric constant, C is the capacitance of sample, d is the thickness of sample, ɛo is the permittivity of free space and A is the area of sample. Pyroelectric current was measured by the static Byer-Roundy method [5] when the samples was heated at a rate of 4°C/min through the phase transition region. Piezoelectric properties were measured using modified resonance/anti-resonance method and piezo-d33 meter [6].

X-ray powder diffraction was used to identify the perovskite phase produced. Fig. 1 shows the x-ray diffraction pattern of pure and La-modified Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 at room temperature. The x-ray diffraction pattern of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 represents the perovskite (110) phase, and the pyrochlore (222) phase of the Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 system slightly increases as the La3+ content increases. This phenomenon is reported to be due to the substitution of La3+ ion in the Pb2+-site of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 ceramics as reported in 0.75(Pb1-3x/2Lax)(Mg1/3Nb2/3)O3-0.25(Pb1-3x/2Lax)TiO3 ceramics [7]. When doping La3+ to Pb2+, charge imbalance must be considered using one of two methods. Pyrochlore phase is a non-cubic, isometric structure with excess tantalum, titanium, oxygen and its material properties, similar to the perovskite Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 compositions. Pyrochlore phases have also been considered detrimental to the dielectric properties of ferroelectric relaxor materials in addition to other factors such as impurities and inter-granular phases. There, the optimal heat treatment condition in this experiment was about 1350°C. Fig. 2 shows the micrograph of a scanning electron microscope for a composition Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 (x=0, 0.02). The grain size was partially increased by the addition of La2O3 but no significant change was observed. The sintered sample has a grain size of 3.5 to 6.5 μm and a high density.

2. Dielectric properties

The temperature dependence of dielectric constant and loss at various frequencies (0.1 to 100 kHz) for the composition Pb0.97La0.02[(Mg1/3Ta2/3)0.66Zr0.34]O3 is shown in Fig. 3(a, b). This dielectric constant and dielectric loss data clearly show that the phase transition occurs from the paraelectric to the ferroelectric state. A relatively strong dielectric dispersion is observed at temperatures near the TC, which is characteristic of the relaxors. Of course, all relaxors are known to be very inhomogeneous. The higher dielectric constant of the sample reflects good stoichiometry. The phase transition of relaxor ferroelectric affects the physical environments such as thermal agitation, grain size and grain boundaries. And the dielectric constant of relaxor decreases with increasing frequency and the phase transition temperature is biased towards higher temperature. The Pb0.97La0.02[(Mg1/3Ta2/3)0.66Zr0.34]O3 is known as a rhombohedral/tetragonal structure in a ferroelectric state. Fig. 3 shows that the phase transition mechanism of Pb0.97La0.02 [(Mg1/3Ta2/3) 0.66Zr0.34]O3 differs from the Curie-Weiss law. Relaxors can be separated into long-range ferroelectric orders from the TC. The dielectric constant of relaxor near the Curie region is governed by the modified Curie-Weiss quadratic equation $1ɛ=1ɛm+(T-Tm)γ2ɛmδ2$, where ɛ is the dielectric constant, ɛm is the maximum dielectric constant, Tm is the dielectric constant maxima temperature, δ is the diffuseness parameter and γ is the critical exponent. In this experiment, the frequency dispersion and diffusion of the transition increases as the amount of La2O3 in the composition increases. This phenomena is similar to that of the Pb1-xLax [(Mg1+x/3Nb2-x/3)0.65Ti0.35(1-x)/4]O3 with the same Pb-type perovskite [8].

Fig. 4(a, b, c, d) shows the temperature dependence of the dielectric constant and loss of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 at 1 kHz in a series of different La3+ concentrations, compare the non-poling (Fig. 4(a, c)) and poling (Fig. 4(b, d)) samples. Most complex perovskite ferroelectrics have several advantages. Transitions generally exhibit large thermal hysteresis. Well-sintered ceramics have high dielectric constant but low dielectric loss. When the soft doping ions enter the lattice structure, the effect of the modified La2O3 is due to attributed to the certain of Pb2+ vacancies in the A(B′B″)O3 perovskite lattice. Since the Pb2+ ions occupy the A-site of the perovskite lattice, soft doping ions play a role in generating A-site vacancies. Therefore, since the valence of the crystal ion is higher than that the valence of the Pb2+ ion, an extra positive charge is introduced into the lattice. When two A-sites are occupied by two cations having a valence of +3, Pb vacancies are generated in the lattice to maintain electrical neutrality [9]. In other words, it is considered that A-site vacancy occurs due to the successive reaction of Mg1+x/3Ta2-x/3O4-x/2 and LaxPb1-x(Mg1+x/3Ta2-x/3)O3 during the fabrication of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3. Therefore, the dielectric constant of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 was the highest. It can be related to possible local variations of the La3+ concentration. In Fig. 4(a, c), the maximum value of the dielectric constant increases rapidly at the beginning and decreases as the La3+ content increases, but the frequency dispersion behavior is still observed. The maximum value of the dielectric constant is slightly increased by addition of La3+ up to 2 mole%, but decreased by further addition of La3+. The dielectric loss increased initially and slightly decreased with further addition of La3+. The increase in the amount of La2O3 strongly affects the TC, which is approximately linearly decreased. This is a attributed to the change in c/a in the crystal lattice of the Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 compositions. It has been found that the behavior of the additive effect on dielectric properties is very similar to that of the Pb[(Mg1/3Nb2/3)Zr]O3 composition. La3+ doping caused a downward shift of TC~12°C/mole% in Pb[(Mg1/3Ta2/3)Zr]O3 polycrystalline ceramics.

3. Pyroelectric properties

The pyroelectric coefficient is routinely calculated from well-known equation follows $ip=ApdTdt$, where iP is the pyroelectric current, A is the area of sample, p is the pyroelectric coefficient, and dT/dt is the heating rate. Fig. 5 and 6 show the pyroelectric coefficient and the spontaneous polarization for the composition of Pb1-3x/2 Lax[(Mg1/3Ta2/3) 0.66Zr0.34]O3 for a series of different La3+ concentrations. The peak of the pyroelectric coefficient initially increases and rapidly decreases with increasing temperature. The composition with maximum pyroelectric coefficient and spontaneous polarization may be similar to the composition observed in the dielectric constant vs. temperature study. The pyroelectric coefficient increases with the mole% of La3+, but if La3+ is larger than ~0.03, the pyroelectric coefficient decreases. The activation barrier of Pb-perovskite is dictated by the strength of the Pb-O-B″ bonds, which must be destroyed to relax the material. This can be achieved by substitution Pb2+ cations at the A-site using large B′ cations and pulling the Pb2+ cations at the small B″ cations [10]. In the phenomenological theory of ferroelectricity, there is an energy barrier between the unpolarized state and the polarized state at the first order phase transition temperature. At the atomic level, most ferroelectric perovskites ferroelectrically activated B cations occupy an off-center position above the Curie temperature. In this regard, the dielectric response of the relaxor is similar to that exhibited by many amorphous materials, including many of the weak/strong bonds, isolated electron pairs, etc., which are thought to cause low-lying excitations of correlated polarity. The orientation or polarization fluctuation of the dipole in the cluster affects not only the position of the adjacent cation but also the polarization of adjacent region [11,12,13].

Fig. 6 shows the spontaneous polarization decreases gradually with increasing temperature, but does not disappear at the transition temperature of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3. This is typical of a relaxor ferroelectric. Spontaneous polarization indicates that another dipole is destroyed when the temperature increases. As the La3+ concentration increases, the peak temperature decreases rapidly. The spontaneous polarization showed a value of 0.27~0.35 C/m2 in the composition added to La3+. This value is excellent as the the spontaneous polarization value of ferroelectric relaxor. In this composition, La3+ ions are replaced by Pb2+ ions on the A-site sub-lattice. Since La3+ is trivalent and Pb2+ is divalent, La3+ behaves as a donor dopant. These phenomena can result in the distribution of La3+ (1.03 pm) and Pb2+ (1.19 pm) ions with different ionic radii at the Pb-site. But they cannot be fully explained at this time. The addition of La3+ ions to Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 resulted in cation vacancies in the lattice for retention of electronegativity, lowering the peak temperature and increasing the pyroelectric coefficient at about room temperature. With the substitution of La3+, the peak temperature of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 can be adjusted to suit room temperature pyroelectric applications. Recently, a pyroelectric effect of a ferroelectric has attracted attention for application to infrared detectors. The properties required for such large pyroelectric coefficient, low dielectric constant and tanδ values. The pyroelectric response of the material is generally related to the application of the device by the FOMs $FV=pcvɛrɛ0$ and $FD=pɛrɛ0 tan δCv$, where CV is the volume specific heat, ɛr is the dielectric constant, ɛo is the permittivity of free space, and tanδ is the dielectric loss. The pyroelectric FOMs (FV~=0.012~0.035 m2/C, FD=0.15 × 10−4~0.52 × 10−4 Pa−1/2) of these ceramics is similar to the published values for Pb-perovskites [14,15]. The piezoelectric constant (d33~490 pC/N) and electromechanical coupling factor (kP~0.35) are obtained in the Pb0.97La0.02[(Mg1/3Ta2/3)0.66Zr0.34]O3 composition with 0.02 mole% of La.

III. Conclusions

Most compositions show diffuse phase transition with strong frequency dispersion. The frequency dispersion and diffusion of the transition increase as the amount of La2O3 in the composition increases. The maximum values of the dielectric constant and the pyroelectric coefficient are slightly increased by addition of La3+ up to 2 mole% and then decreased by further addition of La3+ ion. La3+ ion doping resulted in a TC~12°C/mole% downward shift in the Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 polycrystalline solid solution. The composition near Pb0.97La0.02[(Mg1/3Ta2/3)0.66Zr0.34]O3 was confirmed to have a high pyroelectric FOM near room temperature. The pyroelectric FOM of these ceramics at room temperature is similar to the Pb-perovskites. The spontaneous polarization showed a value of ~0.35 C/m2 in the composition added to 0.03 mole% of La3+. In the Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 system, the piezoelectric constant (d33=350~490 pC/N) and electromechanical coupling factor (kP=0.25~0.35) are obtained.

Figures
Fig. 1. X-ray diffraction patterns of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 systems.

Fig. 2. SEM images of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 systems. (a) 0.00La, (b) 0.01La, (c) 0.02La and (4) 0.03La.

Fig. 3. (a) Dielectric constant and (b) dielectric loss versus temperature of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 at various frequencies.

Fig. 4. Dielectric properties of poling(b, d)/non-poling(a, c) Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 with varying La3+ concentrations.

Fig. 5. Pyroelectric coefficient of Pb1-3x/2Lax[(Mg1/3Ta2/3)0.66Zr0.34]O3 with varying La3+ concentrations.

Fig. 6. Spontaneous polarization of Pb1-3x/2Lax[(Mg1/3Ta2/3) 0.66Zr0.34]O3 with varying La3+ concentrations.

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