Energy density and storage capacity of La3+ and Sc3+ co-substituted Pb(Zr0.53Ti0.47)O3 thin films

We studied the energy density and storage capacity properties of rare-earth modified lead zirconate titanate thin films. Highly oriented thin films of (PbZr0.53Ti0.47)(1−y)(LaxSc1−x)yO3 wherein; [for y = 0 and x =0 viz PL0] and, [for y = 0.1 and x = 0.2, 0.4, 0.6 and 0.8 viz PL2, PL4, PL6 and PL8 respectively] abbreviated as PL10x have synthesized on MgO (100) substrate by the pulsed laser deposition technique. The higher proportion of lanthanum increased the broadening of dielectric permittivity and dielectric maxima that shifted to higher temperatures with increasing frequency, signifying the relaxor-type behavior of these films. The value of the relaxation parameter varies from γ = 1.69 for PL6 and 1.95 for PL8 that was estimated from the linear fit of the modified Curie-Weiss law indicating the relaxor nature satisfying Vogel-Fulcher relation. Furthermore, we achieved enhanced spontaneous polarization of the fabricated thin films. Slim loop hysteresis was observed on tuning lanthanum and scandium and the estimated recovered energy density (Ure) is 51.15 J cm−3 and 26.54 J cm−3 with efficiency (η) of 47.38% and 65.88% respectively for PL6 and PL8 thin films. The high dielectric permittivity, high breakdown strength, and enhanced energy storage density of thin films could make it promising materials for memory, power electronics, and energy storage applications.


Introduction
Lead-zirconate titanate Pb(Zr x Ti 1−x )O 3 (PZT) thin film capacitors have been extensively studied ABO 3 perovskite for many years due to its potential applications in numerous fields such as memory devices [1,2], high energy storage [3], piezoelectric devices [4], sensors and actuators [5] and ultrasonic transducers [6], etc. Promising features of PZT, such as high dielectric permittivity (ε r ), high spontaneous polarization (P s ), ultrahigh strain (d 33 ), high piezoelectric response make its extensive ranges of utilization [5,7]. Moreover, it has compositional specialty (Zr/Ti) and its properties can be tuned with the desired substitution of cations on either A site (Pb) or/and B-site (Zr/Ti). One can tailor these properties of PZT as a ferroelectric thin film capacitor with the higher concentration of Ti [8,9], however, the antiferroelectric (AFE) phase was observed for the lower value of Ti5 [10,11]. The cation substitution in PZT improves a domain wall mobility depending upon the size of ions, the larger ions such as La 3+ , Nd 3+ , Sr 2+ and Ta 5+ favor A-site, while lower ionic radii such as Sc 3+ , Yb 3+ , and Fe 3+ occupy B-site [9,11,12].
La 3+ doped Pb 0.92 Zr 0.52 Ti 0.48 O 3 (PLZT) on A-site increases thermal stability of thin films which could be suitable for energy storage applications [13], and attributes easy orientation and mobility of domain walls [14] increases spontaneous polarization (P s ) of PbZr 0.52 Ti 0.48 O 3 [15], consequently storing higher energy. Our earlier studies of Sc 3+ doped on B-site, PbZr 0.53 Ti 0.47 O 3 (PZTS) showed an increase of P s and exhibited improved energy density of the thin-film capacitor [16]. Role of Sc 3+ doping is reported to improve the relaxor behavior in several perovskites such as Pb 0.78 Ba 0.22 Sc 0.5 Ta 0.5 O3 [17] and PbSc 0.5 Nb (1−x)/2 Ta x /2O 3 (PSNT) with 0x1 [18]. Also, mixed doped in PZT system plays a significant role in charge balance between the A-site, B-site, and oxygen vacancies [19]. Dalakoti et al have studied the substitution of Sr 2+ and Zn 2+ together in PZT and reported a significant increase in polarization [9].
In this study, we report a systematic study of La 3+ and Sc 3+ substituted thin films on PZT considering the significant role of mixed doped cations, for the charge balance between A-and B-sites and oxygen vacancy. Thin films were grown on MgO (100) substrate applying pulse laser deposition (PLD) technique on a thin buffer layer of La 0.67 Sr 0.33 MnO 3 (LSMO) of various compositions. The respective targets of different compositions were made by a solid-state reaction method [20]. We measured the microstructure, ferroelectric properties, dielectric properties of thin films, and analyzed their suitability in electronic applications such as memory and energy storage devices.
We measured the orientation and phase purity of thin films by x-ray diffraction (XRD; CuK α radiation with wavelength λ=1.5405 Å) at room temperature. The atomic force microscopy (AFM) micrograph of PL10x thin films was recorded in contact mode over an area of 3 μm × 3 μm and 20 nm z-scale. For electrical and dielectric measurements, we deposited Pt at the top of thin films (top electrode) by DC sputtering technique (Power 20 W, helium pressure 100 mTorr and time 8 min with a base vacuum of 10 −6 Torr) using a metal shadow mask of an area ∼10 −8 m 2 . Then we used a profilometer to find out the exact area of Pt electrodes and obtained an average area ∼1.5×10 −8 m 2 which was used for all the calculations of dielectric and ferroelectric parameters. The dielectric and ferroelectric properties of thin films (PL10x) with the bottom electrode (LSMO) and top electrode (Pt) were studied. Thus, PL10x is considered as LSMO/PL10x/Pt capacitors throughout this manuscript in dielectric and ferroelectric chapters. The impedance analyzer (Model: HP4294A) and MMR Technologies K-20 programmable temperature controller (K-20) were used to record frequency-dependent capacitance (C) and loss tangent (dissipation factor) in the wide range of temperature 100-650 K at frequencies ranges 100 Hz-1 MHz. Then relative dielectric permittivity (ε′) was calculated using equation (1) [24].   [27]. In addition, the PZT system at MPB revealed a higher tetragonal symmetry over rhombohedral phase [28]. This is due to smaller ionic radius of La 3+ than that of Pb 2+ which induces lattice distortion and shrinkage in volume accompanied with reduction in a-axis and c-axis [25,29] . However, it has also been reported that PZT over morphotropic phase boundary (MPB) region the tetragonal and rhombohedral phases with space group P4mm and R3c coexists at the room temperature for [30]. We used XRD data to analyze the peak broadening in terms of the crystalline size and lattice strain due to dislocation. The instrumental broadening (β) was corrected, corresponding to each diffraction peak of thin films using the relation [31]: The average nanocrystalline size was calculated using Debye-Scherrer formula: The average nanocrystalline size of thin films sample is calculated using Debye-Scherrer equation where D=crystalline size in nanometer, K=Scherrer constant (0.9), and λ=wavelength of the Cu Kα radiation (1.5406 Å) and β is the peak width at half-maximum intensity. The strain-induced (L s ) in thin films due to crystal imperfection and distortion was calculated using the formula: From this equation and the Scherrer equation, it is clear that the peak width from crystallite size varies as 1/cosθ, whereas the strain varies as tanθ. Williamson and Hall (W-H) proposed a method of deconvoluting size and strain broadening by looking at the peak width as a function of the diffracting angle 2θ and obtained the mathematical equation [32,33].
It can be rearranged as, The lattice planes corresponding to peaks (1 0 0), (2 0 0), (3 0 0) for the respective thin films were deconvoluted with the Gaussian model to calculate β and θ. The linear fitting of βcosθ (radian) along y-axis and sinθ (radian) along x-axis is shown in figure 1(b) and the fitted parameters are shown in , and PL8 respectively. Thin films without doping i.e., PL0 has low R a that increased on lanthanum doping contents yielding roughness of ∼8.69 nm for PL8 thin film, and it may be due to higher ionic radius of lanthanum (1.061 Å) to scandium (ionic radii 0.745 Å). We observed XRD peak broadening due to La 3+ and Sc 3+ in PZT thin films, and higher incorporation of La 3+ concentration resulted in higher roughness as evident from AFM images. A decrease in crystal size causes higher peak broadening which consequently increases the surface roughness of thin films [34,35]. One can notice distinct surface roughness of respective films in 3D images as shown in the inset of figures 2(a)-(e).

Dielectrics behavior
The frequency (f) dependence of the real component of relative dielectric permittivity (ε′) at various temperatures from 100-650 K for all compositions of thin films are shown in figures 3(a)-(e) and their respective dielectric loss tangent (tanδ) in right y-axis. We observed stable ε′ with high value and low tanδ in a wide range of frequencies in good agreement as reported in ferroelectric thin films of PbZr 0.52 Ti 0.48 O 3 [36]. We obtained room A comparison of temperature-dependence of the relative permittivity (ε′ versus temperature) measured at frequency 1 kHz is shown in figures 4(a), (b). One can notice that PL0, PL6, and PL8 have dielectric maxima at ε m ′∼575 K, 525 K, and 450 K respectively denoted as T m. It is expected that such a peak for PL2 and PL4 existed  above 650 K, out limiting temperature for measurements. Furthermore, we observed ε′ as diffused over a wide range of temperatures on PL6 and PL8 which is in agreement with the earlier research on Lanthanum doped lead zirconate titanate thin films [37]. In ferroelectric materials, such type of behavior is pronounced as diffused phase transition (DPT) [38], known as disordered ferroelectric materials [39].
The ferroelectric behavior with the DPT phenomenon of the dielectric materials can be explained by Curie-Weiss law above T m [40]. The relationship for ε′ with the temperature above T m is given by equation (7).
Where; C is Curie-Weiss constant and T 0 is Curie-Weiss temperature (T c ) for second-order phase transition and less than T m for the first-order phase transition [41]. Figures 5(a)-(c) show the reciprocal of ε′ with temperature for PL0, PL6 and PL8 thin films. As we fitted using equation (2), we observed T 0 is above T m for all thin films. We noticed the value of T 0 is reduced for the higher doping on lanthanum. We observed a degree of deviation, D = -T T cw CW m =15, 50, and 70 for PL0, PL6 and PL8 respectively due to compositional induced diffuse phase transition behavior [21]. Where T cw represents the temperature from which ε′ begins to deviate. Some of the parameters calculated are shown in table 3.
In addition, the modified Curie-Weiss law can also explain the DPT behavior of the relaxor materials related by equation (8) [42]. Where, γ provides DPT behavior and C′ is Curie-Weiss like constant.   denotes normal ferroelectric material whereas 2 represents complete phase transition behavior of relaxor materials.
The fitting with an equation (8) is shown in figures 5(a)-(c) inset, at 1 kHz frequency for PL0, PL6 and PL8 thin films. We obtained γ as 1.19±0.05, 1.69±0.05 and 1.95±0.16 indicating that PL6 and PL8 thin films exhibit incomplete diffuse phase transition. Thus, on increasing Lanthanum concentration such behavior is dominant than undoped or lower doped of lanthanum. It also indicates that dopants with La 3+ Sc 3+ increase the DPT of the materials.
As we observed the DPT of PL6 and PL8 thin films, where ε′ m decreases and T m shifts towards higher temperature with increasing frequency. Furthermore, we analyzed the frequency dependency of T m by using the Vogel-Fulcher relation given in equation (9) [43]. = - Where; f 0 is the pre-exponential factor, T VF is the characteristic Vogel-Fulcher freezing temperature, E a is the activation energy and k B is Boltzmann's constant [44,45]. The reasonably well fitted non-linear curve shown in figure 6, yielded f 0 ∼10 6 Hz, T VF =477 K and 411 K and factor, E a =0.031 eV and 0.054 eV respectively and are physically acceptable values [46]. These are in good agreement with the earlier reports on Pb(Zr 0.53 Ti 0.47 ) 0.60 (Fe 0.5 Ta 0.5 ) 0.40 O 3 [47]. The earlier report on Pb(Zr 0.53 Ti 0.47 ) 0.90 Sc 0.10 O 3 had shown E a =0.037 eV and f 0 =1.538×10 6 Hz [16]. Thus, analysis of Vogel-Fulcher relation, further supports PL6 and PL8 thin films behave disorder ferroelectric, typically called relaxor ferroelectric materials. In addition, one can notice a comparatively slim polarization electric field (P-E) hysteresis loop of those thin films which suggest higher content of lanthanum on PZT strengthen the spontaneous polarization. Figures 7(a)-(e) show the Cole-Cole plots of the temperature-dependent dielectric permittivity of the real (ε′) and imaginary (ε″) part of PL10x thin films at frequency range 10 2 -10 6 Hz. We observed some characteristics of ε′ versus ε″ response for PL10x thin films in the temperature ranges 100-650 K. (i) The ε′ radius of the semicircular arc shows high variation with temperature, (ii) With increasing temperature the bulk permittivity contribution (ε′ radius, when ε″=0) in the Cole-Cole plot increases, since the intercept of the semicircular arc gives an estimation of sample resistance, this indicates that the resistance increases from PL0 to PL6 and again decreases for PL8. The incorporation of the La 3+ and Sc 3+ ions could be responsible for change in its conduction properties [36,48]. These dopants are mainly of two types: La 3+ is a donor dopant that produces a soft PZT and Sc 3+ is an acceptor dopant that produces a hard PZT. In addition, the decrease in the resistance for PL8 is due to higher doping of Lanthanum compared to scandium may facilitate the domain wall motion [49,50]. (iii) the samples exhibit multi-dispersive relaxation time on increasing the temperature. Two intercepts  between the real axis ε′ and the circular arc, assign for the static dielectric constant, ε s (largest value ε′ radius, when ε″=0) and the optical dielectric constant, ε ∞ (smallest value ε′ radius, when ε″=0) were observed in all samples which increased on doping compounds, this large increase is due to the variation of the domain wall motion that affects the resistance of the compound. These observed changes in the shape of Cole-Cole plots of dopant films could be due to different strains, that are produced by the different sizes of the doping ions that produce a variation of the dielectric and conductivity properties of the materials [51,52],.

Ferroelectric properties
The energy storage performance of ferroelectric materials is one of the key indicators for evaluating materials engineering applications. It can be estimated using unipolar P-E loops under the external applied electric field. According to the definition of energy-storage density using P-E hysteresis loops, the stored energy per unit volume (U st ) and the recovered energy per unit volume (U re ) are given by [53] Where; E is the applied electric field, P is the displacement charge density for ferroelectric materials, and P r is the remnant polarization, P m is the maximum polarization. Evidently, based on equations (10) and (11), the value of U st and U re can be easily obtained by numerical integration of the area between the polarization axis and the curves of the P-E loops. To estimate these values, we analyzed the positive branch of P-E curve and calculated the energy storage efficiency (η) as Figures 8(a)-(e) shows temperature-dependent (100-500 K) P-E hysteresis loops of PL10x thin film measured under an applied electric field of 0.67 MV cm −1 electric field at 1 kHz frequency. Reproducible and stable ferroelectric loops with slight changes in polarization and coercive field were observed for all thin films. From the comparative graph of PL10x thin films as shown in figure 8(f), we observed typical ferroelectric loops with enhanced polarization and reduced coercive field for PL2 and PL4 thin films. Furthermore, PL6 and PL8 show slim loop hysteresis with high ΔP (P m -P r ). These changes in parameters of the hysteresis loop were due to the substitution of scandium and lanthanum in PbZr 0.53 Ti O.47 (PZT). We estimated the energy storage capacity at 300 K for the same applied electric field (0.67 MV cm −1 ) for PL10x thin films, achieving enhanced U re , U st and η due to doping elements. The estimated U re value estimated was the greatest ∼12.24 with η=53.4% for PL6. But we had better η=66.67% with U re =4.82 J cm −3 for PL8 thin films. All the calculated parameters are summarized in table 4. Considering energy storage capacity and efficiency, we further, analyzed the positive branch of P-E loop for possible energy storage applications. Figures 9(a), (b) show the room temperature unipolar P-E hysteresis loops of PL6 and PL8 thin films measured under various applied electric fields at 2 kHz of frequency. The slim loop of these thin-film capacitors revealed relaxor ferroelectric behavior in PL6 and PL8 in line with dielectric results. We observed slight asymmetries at positive and negative branches in the hysteresis loop of PL6 and PL8 films that could be due to different work functions of top (Pt) and bottom (LSMO) electrodes [54,55]. In addition to the layer interface effect which acts as pinning centered, defect-related oxygen vacancy and impurities might be also responsible for the observed asymmetry hysteresis loop [56,57]. Near the breakdown electric field (∼2.67 MV cm −1 ) at 2 kHz frequency, the enhanced polarization P r ∼46.6 μC cm −2 and P m =125.4 μC cm −2 , consequently yielded D = = P 2.69, m P P m r and reduced E c ∼205 kV cm −1 were obtained for PL6 films. In the similar applied electric field, for PL8 films E c was further reduced to 12 kV cm −1 and P r ∼8.4 μC cm −2 and P m ∼39.2 μC cm −2 , achieved ΔP m =4.66, which resulted in slimmer P-E loop strengthening η. Thinner P-E loops were achieved in our films than undoped PbZr 0.53 Ti 0.47 O 3 (PZT) [58], PbZr 0.40 Ti 0.60 O 3 [59] films which were expected. It is obvious that higher ΔP m would better to achieve enhanced η.
We estimated U re , U st and η for PL6 and PL8 films from their respective P-E loops. It turns out to be U re =51.15 J cm −3 U st =107.95 and η=47.38% at an applied electric field of 2.67 MV cm −1 and frequency 2 kHz for PL6 thin films which is in good agreement with e e = ¢ U , where E BD is electric breakdown strength  Parameter:

Conclusion
The XRD patterns of (PbZr 0.53 Ti 0.47 ) (1−y) (La x Sc 1−x ) y O 3 thin films revealed highly oriented (100) films with the perovskite phase. The dielectric data revealed an increase of dielectric permittivity when lanthanum content increased by 6%, however, a further increase of lanthanum (8%) resulted in the reduction of dielectric constant. We further noticed the role of increasing lanthanum contents (decreasing scandium) resulting in broadening of dielectric permittivity with temperature and ferroelectric phase transition from 575 K to 450 K. It clearly indicated that analysis of modified Curie-Weiss law for the thin films PL6 and PL8 behave relaxor-ferroelectric which further supported by Vogel-Fulcher relation. Cole-Cole plot analysis showed the samples exhibit multidispersive relaxation time on increasing the temperature. In addition, a large increase in the resistance of the samples was observed when the amount of the lanthanum dopant was increased because the lanthanum modifies the domain wall motion in the samples. The polarization electric field (P-E) plot exhibits the role of lanthanum and scandium causes a substantial reduction in coercive fields. Furthermore, enhanced polarization was achieved on PL2, PL4 and PL6 thin films. In addition, PL6 and PL8 thin films show the slim loop P-E curve, which indicates a relaxor-ferroelectric nature. We achieved U re =51.15 J cm −3 with η=47.38% for PL6 under 2.67 MV cm −1 at 2 kHz frequency and η=65.88% with U re =26.54 J cm −3 for PL8 thin films. The study confirms the idea that, by tuning La 3+ and Sc 3+ , the dielectric and ferroelectric functional properties of thin films can be controlled for better memory, power electronics, and energy storage devices.