Highly oriented 0.90[PbZr0.53Ti0.47]0.10[La0.80Sc0.20]O3-δ (PLZTS) thin films deposited on La0.67Sr0.33MnO3 (LSMO) coated MgO (100) substrates were grown by pulsed laser deposition technique. Temperature dependent dielectric measurements on metal-ferroelectric-metal Pt/PLZTS/LSMO thin film capacitors were carried out at several frequencies which exhibit high dielectric constants (450–580) with the diffuse peak around 400 K, and the diffusivity parameter γ was obtained as 1.96 for 100 kHz data. The slim polarization-electric field hysteresis loop was observed with less remanent polarization (∼7–10 μC/cm2) indicating its relaxor behavior. Temperature dependent Raman spectra measured between 80 and 550 K show softening of the symmetric E(LO2) band that disappeared at 300 K, corroborating the tetragonal-cubic phase transition. From the analysis of the measured hysteresis loops, the recovered energy density Ure ∼ 19 J/cm3 with the efficiency η ∼ 66% was estimated, suggesting its possible application in energy density capacitors.

For the past several years, dielectric capacitors with high energy storage densities have attracted several researchers because of their potential application in capacitors for modern electronics and electrical power systems.1–3 In addition, they can be a great choice for several other electronic applications, such as power inverters and pulsed power devices including lasers, wind power generators, medical defibrillators, etc.1–3 Research on dielectric materials, to store high energy density, has been focused on linear dielectrics (LDs), ferroelectrics (FEs), relaxor ferroelectrics (RFs), and antiferroelectric materials (AFs). Among these, RFs and AFs are more commonly used dielectric capacitors due to their higher energy storage densities than those of LDs and FEs.4,5 Relaxor ferroelectrics have been extensively studied because of their unique physical properties such as high dielectric constant, high electric breakdown fields, temperature- and frequency-dependent diffuse dielectric permittivity maximum, self-assembled polar nanoregions (PNRs), and a giant piezoelectric effect.6,7 Relaxors have the characteristic feature of possessing PNRs dispersed in a nonpolar matrix manifested often in a slim polarization-electric field (P-E) loop and broad phonon spectra.8 PNRs are formed due to either chemical frustration or compositional induced disorder in oxide perovskites, and their behavior is solely governed by the evolution of polar nanoregions.8,9 The slim polarization-electric field (P-E) loop of relaxor ferroelectrics often yields a larger area to store the electrical energy and exhibits fast discharge capacity.10,11

Pb(Zr0.53Ti0.47)O3 (PZT) is known to be a technologically robust ABO3 type ferroelectric material.12–14 Its ferroelectric properties can be controlled systematically by suitable substitution of the cations either on the A-site (Pb) and/or the B-site (Zr/Ti) to innovate materials to improve energy storage performances. In several theoretical studies, it is argued that PZT doped with larger ionic radii such as La3+, Nd3+, and Ta5+ (donor dopant) and lower ionic radii cations such as Sc3+, Yb3+, and Fe3+ (acceptor dopant), enters into A- and B-sites, respectively, resulting in strengthening the domain wall mobility and thereby enhancing its electronic properties.13,15,16 It is observed that proper substitution of rare earth elements in PZT often leads to enhancement of the recoverable energy density, high energy efficiency, and piezoelectric strain.17 Interestingly, Pb1−xLax(ZryTi1−y)O3 (PLZT), a La3+ doped ferroelectric, is observed to be a robust material for high energy storage application, exhibiting relaxor or antiferroelectric behavior.2,18–20 La3+ doping in PZT causes lattice distortion and enhances the tetragonality (c/a ratio) of the PLZT unit cell resulting in an increase in spontaneous polarization.21 Hao et al. have reported that for up to 108 (measured) charge-discharge cycles, a significantly higher polarization-fatigue endurance is reported for La3+ doped PZT.22 Remarkably, B-site doped Sc3+ is found to improve the endurance of PZT materials23 and a reduction in remanent polarization due to the off-centering of Sc3+ ions in BO6 octahedra.24 However, an insight into the energy storage behaviors, such as the recovered energy density Ure and the storage energy efficiency η, of these capacitors is yet to be gained. It is expected that the slim P-E loop of La3+- and Sc3+-substituted lead zirconate titanate (PLZTS) thin films, characteristic of relaxor ferroelectrics, could possibly improve its storage energy behavior. This will be examined in detail in the present paper. In this letter, we report on the studies on the rare earth doped elements such as La3+ and Sc3+ on PZT, a highly oriented thin film, grown on a MgO(100) substrate, by using pulse laser deposition technique. Structural and ferroelectric behavior were studied, and its high energy storage capacity behavior was examined for possible energy storage applications.

A 0.90[PbZr0.53Ti0.47]0.10[La0.80Sc0.20]O3-δ (PLZTS) target with a stoichiometric molecular formula was prepared by using a conventional solid-state reaction method as reported elsewhere.24–26 PLZTS/La0.67Sr0.33MnO3 (LSMO) thin films were grown on MgO (100) substrates using pulsed laser deposition (PLD) employing a KrF excimer laser (λ = 248 nm, f = 5 Hz) with optimized parameters as provided in the Table SI, similar to the procedure reported earlier.11,27 The thickness of the PLZTS thin films is ∼300 nm, measured using an XP-200 profilometer and filmetrics (F20). Energy dispersive x-ray analysis on as-grown thin films suggests around 15% compositional deviation from the target.

Phase stability and purity of the thin films were probed by employing a Rigaku Ultima III X-ray diffractometer (XRD) equipped with a CuKα radiation (λ = 1.5405 Å) source configured in Bragg-Brentano (θ–2θ) geometry and operating at 40 kV and 44 mA at room temperature. Single phase materials with (100) orientation of the grown thin films were found from the XRD diffraction results (Fig. SI). It can be mentioned here that the co-existence of a tetragonal or monoclinic or rhombohedral structure of PZT perovskite oxide at the morphotropic phase boundary is reported.28,29 However, with a rare earth Sc3+ cation dopant, reduction in tetragonality was observed for Pb0.85Sc0.10Zr0.53Ti0.47O3 (PSZT) ceramics.24 Furthermore, it has been reported that an increase in the concentration of La3+ in PZT at the morphotropic phase boundary led to a structural change from a rhombohedral to tetragonal structure.30 The atomic force microscopic (AFM) micrograph of PLZTS thin films observed in contact mode over a 3 μm × 3 μm × 20 nm z-scale area is shown in Fig. SI (inset), and the observed roughness Rq and Ra values are found to be between 5 and 10 nm. The higher value of surface roughness may be due to the high laser energy or growth temperature used for the growth of thin films.7 

The metal shadow mask of an area of 104μm2 was used to fabricate Pt square capacitors as a top electrode by using DC sputtering technique, and the bottom electrode was designed by a buffered conducting LSMO layer deposited upon MgO substrates to investigate the dielectric and ferroelectric properties. The schematic sandwich diagram of electrodes is shown in Fig. 5 (inset: labeled a). Dielectric measurements were carried out in the temperature range of 100–600 K at various frequencies (100 Hz and 1 MHz) by using a programable temperature controller (MMR K-20) and an impedance analyzer (HP 4294A).

Figure 1 shows the frequency dependent dielectric constant (ε′) of PLZTS in a wide temperature range of 100–600 K. One can notice a stable ε′ and low tan δ ≤ 0.05 in the measured frequency range 100 Hz–1 MHz. However, a strong frequency dispersion with a substantial decrease in the ε′ value and an increase in dielectric loss above 105 Hz are observed. This could be attributed to the semiconducting nature of the bottom electrode.31 The possible reason for the observed drop in the dielectric constant (ε′) at high temperatures (≥400 K) and high frequency (>105 Hz) is due to thermally activated hopping of oxygen vacancies. As is expected, in several perovskites, at high temperatures,32,33 the ionic conductivity increases due to thermal energy assisted oxygen movement enhancement, hence, the observed reduction in dielectric constant is understandable. Although one can expect such a reduction in ε′ due to the presence of secondary phases, it is not observed in our case. The substantial increase in tan δ at higher frequencies is associated with disordered materials.34 The temperature dependence of relative permittivity (1000/ε′ vs Temperature plot) measured at a frequency of 100 kHz is shown in Fig. 2. The broadening of the dielectric constant over a wide range of temperatures is argued to be due to diffused phase transition (DPT) in ferroelectric materials.34 The broadness in the dielectric profile is known to be the significant behavior of disordered materials with a diffuse phase transition.35 In fact, a wider distribution of relaxation time which essentially represents the dielectric peak is expected to be due to Sc3+ doping on the B-site of PLZT (Fig. 1). The DPT of ferroelectric materials can be explained by the well-known Curie-Weiss law above the Tm, the corresponding temperature of dielectric maxima (ε′max). The reciprocal of dielectric permittivity (ε) is related to the temperature as36,37

1ε=TTmCT>Tm,
(1)

where C is the Curie-Weiss constant, and the calculated values are shown in Table I. The higher value of ΔTm = TCWTm= 60 represents the degree of deviation from the Curie-Weiss law and provides the evidence for the compositional induced diffuse phase transition behavior.11,38 It can be mentioned here that Tcw denotes the temperature from which the dielectric constant ε′ begins to deviate. The reciprocal of ε′ begins to deviate from the Curie-Weiss law at Tcw which is quite higher than the Tm.39 The DPT can be explained by a modified Curie-Weiss law as given in Eq. (2),40,41 where C′ is a Curie-Weiss like constant and γ gives the DPT character

1ε1εm=(TTm)γC1γ2.
(2)
FIG. 1.

Variation of dielectric constant (ɛ′) with the frequency of PLZTS thin films [inset: variation of the dissipation factor (tan δ) with several frequencies].

FIG. 1.

Variation of dielectric constant (ɛ′) with the frequency of PLZTS thin films [inset: variation of the dissipation factor (tan δ) with several frequencies].

Close modal
FIG. 2.

Reciprocal of dielectric constant (1000/ɛ′) of PLZTS thin films measured at 100 kHz frequency [Inset: ln(1ε1εm)vslnTTm]. The numbers in the parenthesis are standard error in the least significant digit.

FIG. 2.

Reciprocal of dielectric constant (1000/ɛ′) of PLZTS thin films measured at 100 kHz frequency [Inset: ln(1ε1εm)vslnTTm]. The numbers in the parenthesis are standard error in the least significant digit.

Close modal
TABLE I.

Summary of dielectric parameters of PLZTS thin films measured at 100 kHz.

Parameter:εmTm (K)C (K)Tcw (K)ΔTm (K)γ
Values 517 480 2.25(8) × 105 540 60 1.96(9) 
Parameter:εmTm (K)C (K)Tcw (K)ΔTm (K)γ
Values 517 480 2.25(8) × 105 540 60 1.96(9) 

For γ = 1, a normal Curie-Weiss law is obtained and γ = 2 describes a complete diffuse phase transition. The γ value lies in between 1 and 2 corresponding to the incomplete diffuse phase transition.34,42Figure 2 (inset) shows the plot of ln(1ε1εm)vslnTTm at 100 kHz frequency for PLZTS thin films where the black dots are experimental values and the red solid line is obtained from the linear fitting of data. The calculated value of γ is 1.96(9) and 1.95(7) at 100 kHz and 10 kHz (Fig. SII), respectively, which clearly indicate that the PLZTS thin films exhibit diffuse phase transition.

The phonon spectra of PLZTS measured in the environment are shown in Fig. SIII, and individual band frequencies are assigned by comparing with the earlier report.43 PLZTS stabilizes in the tetragonal phase and belongs to the point group C4v. The irreducible representation of optical phonons is Γopt = 3A1 + 4E + B1, where A1 and E modes are Raman and IR active, and B1 mode is only Raman active. One can find only four prominent peaks in the spectrum [Fig. 3(a)]; however, by analyzing the spectrum using a damped harmonic oscillator model, six modes could be obtained in the wave number range 60–1000 cm−1. The presence of A1 (TO) bands in the spectrum indicates the existence of ferroelectric ordering24,43 in the compound. These bands are found to be broad as compared to the classical ferroelectric PbTiO3(PT)44 in the environment. The broadening of bands could be due to substitutional disorder due to the occupancy of cation sites A by La and B by Zr/Ti/Sc cations. This is expected due to the fluctuation in the bond strength of BO6 octahedra and the Pb/La-O bond, and hence, the observed spectrum is the statistical sum of all those expected possible frequencies. Thus, the observed broadening of the Raman bands as compared to pure ferroelectric PT is understandable. Figure 3(a) shows the temperature dependent Raman spectra in the temperature range 80–550 K. Upon increasing the temperature, the spectra exhibit a gradual change: the bands broaden further and their intensity reduces. One can notice that above 300 K, the prominent symmetric E(LO2) band disappears completely as shown in Fig. 3(b) for clarity, and the spectra become flat with an absence of any Raman features. The softening of this band with temperature is noticed, and the first order temperature coefficient is found to be −0.017 cm−1 K−1. Upon increasing the temperature, the tetragonal distortion reduces, and the PLZTS system is expected to transform into the cubic phase. Similar phase transition to the cubic phase has been reported in ferroelectrics like PZT43 and PSZT.24 The irreducible representation of optical phonons for the cubic phase (Oh point group) can be Γopt = 3F1u + F2u, where the F1u mode is only IR active, whereas F2u mode is an inactive (silent) one. Therefore, the absence of Raman active modes is expected in the cubic phase. In this context, since no Raman active modes are observed above 300 K [Fig. 3(a)], the system probably transformed into the cubic phase above this temperature. The plot on dielectric behavior measured at various frequencies shows similar anomalous behavior around 300 K (Fig. 2). Although one could expect the appearance of sharp peaks45,46 riding over the broad spectrum in the high temperature cubic relaxor phase, they were not observed in the present case. This could be due to the excessive broadening of Raman bands and their insufficient intensities. On the other hand, our dielectric results do suggest diffuse phase transition (relaxor) behavior at high temperatures.

FIG. 3.

(a) Temperature dependent Raman spectra of PLZTS in the temperature range 80–550 K. The arrow mark indicates the temperature dependence of the E(LO2) optical band. (b) Enlarged view of the softening of the E(LO2) band with temperature.

FIG. 3.

(a) Temperature dependent Raman spectra of PLZTS in the temperature range 80–550 K. The arrow mark indicates the temperature dependence of the E(LO2) optical band. (b) Enlarged view of the softening of the E(LO2) band with temperature.

Close modal

We observed slim loop hysteresis curves (P-E loops) by using a Radiant tester set up at room temperature for various applied voltages (25–65 V) as shown in Fig. 4. The Pr and Ec values of the observed P-E loop are ∼4 μC/cm2 and ∼12 kV/cm, respectively, at 0.8 MV/cm, and as expected, these values are found to be smaller than the ferroelectric PZT system.12,47 The asymmetries observed in positive and negative branches of the hysteresis loop of Pt/PLZTS/LSMO could be due to different work functions of the top and bottom electrodes. Choi et al. reported an asymmetric hysteresis loop of LaCoO3 (LCO)/PZT/(La,Sr)CoO3 (LSCO), due to different work functions of the top and bottom electrodes.48 The observed difference in positive and negative Ec values in Pt/PZT/Au is attributed to the internal bias field generated between dissimilar electrodes.12,49 In practice, the energy storage capacity and efficiency (η) of dielectric capacitors are estimated by carrying out the analysis of the polarization-electric field (P-E) loop, as shown in the inset (b) of Fig. 4.

FIG. 4.

P-E hysteresis loops of PLZTS thin films measured at various applied voltages at 10 kHz frequency [inset: (a) schematic sketch of thin film capacitors with conducting top and bottom electrodes and (b) charging and discharging of storage energy at a maximum applied voltage of 65 V].

FIG. 4.

P-E hysteresis loops of PLZTS thin films measured at various applied voltages at 10 kHz frequency [inset: (a) schematic sketch of thin film capacitors with conducting top and bottom electrodes and (b) charging and discharging of storage energy at a maximum applied voltage of 65 V].

Close modal

The recoverable energy density (Ure) per unit volume of a dielectric material can be obtained by using Eq. (3). In other words, Ure is the released energy when the stored energy discharge is from Emax to zero17,25,50

Ure=PmaxPrEdP;0EEmax,
(3)

where E, Pr, and Pmax are the applied electric field that causes the variation in electric polarization (P), Pr is the remanent polarization, and Pmax is the maximum polarization under the applied electric field Emax. For practical applications, larger energy storage efficiency (η) is an important factor, and is often considered along with the higher value of Ure. The energy storage efficiency η can be defined as the ratio of discharging, Ure (output) energy to charging, Ure (input) energy

η=UreUst×100%,
(4)

where Ust=Ure+Uloss is the stored energy when the applied field increases from zero to Emax. The lost energy density Uloss is calculated by the numerical integration of the closed area of the P-E hysteresis loops.10,51 Figure SIV illustrates a unipolar P-E loop, the electric field dependence of the recovered energy density (Ure), loss energy density (Uloss), total stored charged density (Ust,), and energy storage efficiency (η). The area under the curve filled with green and orange colors resemble the Ure and Uloss, respectively. The sum of green and orange areas represents Ust. The stored energy of the PLZTS dielectric capacitor can be calculated from the integration of the unipolar polarization-electric field (P-E) loop as shown in Fig. 4 (inset: labeled as b) for the maximum applied voltage.

We obtained Ure ∼ 19 J/cm3 with high energy efficiency ∼66% at 2.3 MV/cm and 10 kHz. The estimated Ure and η for Pt/PLZTS/LSMO thin films for different electric fields are shown in Table SII, and shown in Fig. 5 for clarity. These estimated results are similar to those reported by Brown et al. for PLZT thin films.19 For Pt/BaTiO3 (BTO)/Ba1–xSrxTiO3 (BST) grown on MgO (001) by PLD, the Ure was reported to lie between 6 and 12 J/cm3 for an applied field of 0.8–1.66 MV cm−1 with an efficiency of 59%–67%.10 Nguyen et al. have reported enhanced recoverable high energy density in PLD fabricated 10% La3+ doped epitaxial PLZT thin films on a SrRuO3 (SRO)/SrTiO3 (STO)/Si substrate with a Ure value of 13.7 J/cm3 and a quite high energy efficiency of 88.2% at 1 kV/cm.51 PLZT/LaNiO3 (LNO)/Ni thick film capacitors prepared by the sol-gel method have an energy density of ∼22 J/cm3 and an energy storage efficiency of ∼77% with DC dielectric breakdown strength of ∼1.6 MV/cm.2 Since our thin films are of ∼300 nm thickness, the accumulation of free charge at the interface could be a possible reason for the reduction of energy density. Note that comparatively thicker films20,52 show larger energy storage density due to negligible interfacial microstructural defect. The Ure of the (001) oriented PZT thin films was observed as 8.4 J/cm3 at 0.80 MV/cm with an efficiency of ∼76%.53 

FIG. 5.

The recoverable energy density (Ure) and energy density efficiency η as a function of the applied electric field in the Pt/PLZTS/LSMO capacitor.

FIG. 5.

The recoverable energy density (Ure) and energy density efficiency η as a function of the applied electric field in the Pt/PLZTS/LSMO capacitor.

Close modal

We synthesized highly oriented (100) PLZTS thin films on the MgO substrate. The phase purity of the samples was analyzed employing XRD. The diffuse phase transition of the materials is evident from the analysis of dielectric permittivity (ε′) in a wider frequency range and temperature. Ferroelectric measurement on films reveals the thin hysteresis loop for the capacitors. We obtained the recoverable energy density (Ure) of 19 J/cm3 with an energy efficiency (η) of ∼66% at an applied electric field of 2.3 MV/cm. The present estimated energy density and efficiency of PLZTS thin films hint their use for several energy storage applications.

See the supplementary material for the optimized parameters for thin film growth, XRD curves, AFM images, the Curie-Weiss plot at 10 kHz, Raman spectrum of PLZTS recorded at room temperature and mode assignments, and a schematic diagram of unipolar P-E loop for PLZTS thin films.

This work was financially supported by the Department of Defense, USA (DoD Grant No. FA9550-16-1-0295).

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Supplementary Material