Amphoteric Doping Effect of Ho³⁺ on the Performance of Medium-Temperature-Sintered PLZT Energy Storage Ceramics

Abstract

The development of dielectric capacitors with high energy-storage density and ultrafast discharge capability is essential for next-generation pulsed power systems. In this work, (Pb, La, Ho, Zr, Ti)O₃ (PLZTH) ceramics were fabricated via medium-temperature sintering (950–1100 °C) combined with Ho³⁺ doping to systematically tailor their energy-storage properties. This processing strategy not only mitigates Pb volatilization but also enhances compatibility with base-metal electrodes such as Ni and Cu. In addition, Ho³⁺ ions exhibit amphoteric doping behavior, which contributes to the enhancement of relaxor characteristics and grain refinement. H4 ceramic delivers an outstanding recoverable energy-storage density (W_rec) of 0.91 J/cm³ and a high energy efficiency (η) of 87% under 216 kV/cm, along with a power density (P_D) of 28.8 MW/cm³ and an ultrafast discharge time (t_0.9) of only 4.97 ns at 180 kV/cm. This study not only proposes a viable route toward high-performance medium-temperature-sintered PLZT ceramics but also elucidates the effective mechanism of Ho³⁺ amphoteric doping in modulating the relaxor state and properties of perovskite-based ceramics.

1. Introduction

Dielectric energy storage ceramics exhibit broad application prospects in pulsed power systems, electric vehicles, and renewable energy storage due to their high power density, rapid charge–discharge capabilities, and excellent thermal stability [1,2]. Within the broad family of perovskite oxides, lead-free ceramics such as barium titanate (BT), sodium bismuth titanate (BNT), silver niobate (AgNbO₃), and sodium niobate (NaNbO₃) have been intensively investigated owing to their environmental benignity. However, their practical application is generally constrained by intrinsically low breakdown strengths and relatively broad P-E loops, which in turn limit both the recoverable energy density and storage efficiency [3,4,5]. By contrast, lanthanum-modified lead zirconate titanate (PLZT) ceramics combine outstanding ferroelectric/antiferroelectric properties with a high dielectric permittivity, and their phase transition characteristics can be flexibly tailored through rare-earth substitution or Zr/Ti ratio adjustment. Such tunability enables the realization of slim P-E loops together with high energy conversion efficiencies, rendering PLZT one of the most competitive and scalable candidates for advanced dielectric energy-storage applications [6,7]. However, conventional PLZT ceramics typically require sintering temperatures exceeding 1200 °C, which not only results in substantial energy consumption but also induces compositional deviations from Pb volatilization at high temperatures, leading to increased lattice defects and degraded performance [8]. Furthermore, such elevated sintering temperatures restrict compatibility with low-cost electrode materials (e.g., Ni and Cu), impeding the cost-effective fabrication of multilayer ceramic capacitors. Consequently, the development of medium-temperature sintered (950–1100 °C) PLZT ceramics holds significant engineering and economic implications for reducing production costs, minimizing environmental pollution, and enabling co-firing with base metal electrodes [9,10].

Rare-earth doping, owing to its unique ionic radii, flexible valence states, and electronic configurations, can induce lattice distortion and defect engineering through A-site or B-site substitution, thereby enabling fine control over polarization response and domain-wall motion [11]. As a typical rare earth amphoteric dopant, Ho³⁺ possesses a moderate ionic radius and stable +3 valence, allowing it to occupy A-sites as donor defects or B-sites as acceptor defects; this amphoteric mechanism can further enhance PLZT ceramics’ energy storage properties by modulating polarization responses, domain structure evolutions, and carrier mobilities [12]. Nevertheless, current studies primarily concentrate on Ho³⁺ doping’s effects on the ferroelectric properties of bismuth ferrite [13]. and bismuth titanate ceramics [14], with limited systematic investigations into its amphoteric role in regulating the phase structures, microstructures, and energy storage performances of medium-temperature sintered PLZT for energy storage applications.

This study systematically explores the impacts of Ho³⁺ doping on the phase compositions, microstructures, dielectric properties, and energy storage characteristics of PLZT ceramics through the integration of Ho³⁺ as an amphoteric dopant with medium-temperature sintering. It particularly elucidates how Ho³⁺ site occupancy optimizes the energy storage performance of medium-temperature sintered PLZT ceramics by tuning lattice distortions, defect dipoles, and dynamic domain responses. This work seeks to furnish novel theoretical and experimental foundations for developing high-performance, cost-effective energy storage ceramic materials.

2. Experimental Procedures

A series of Ho³⁺-doped PLZT ceramics with the chemical formula (Pb_0.97-xLa_0.03Ho_xZr_0.95Ti_0.05)O₃ (where x = 0.01, 0.02, 0.03, and 0.04, denoted as H1–H4, respectively) were synthesized via the conventional solid-state reaction method. During the synthesis, the Mg_0.8Mo_0.2O_1.4 composite oxide was incorporated as an effective sintering aid to substantially lower the densification temperature and mitigate Pb volatilization, thereby facilitating a medium-temperature sintering process. The raw materials (purity ≥ 99.9%), including Pb₃O₄, La₂O₃, ZrO₂, TiO₂ and Ho₂O₃, were weighed according to the stoichiometric ratios, mixed with anhydrous ethanol, and wet-milled in a ball mill for 12 h. After drying, the mixture was pre-sintered at 680 °C for 4 h, followed by another ball milling for 24 h. The dried powders were then pressed into disk pellets with a diameter of 10 mm and sintered at 1030 °C for 2 h.

The crystalline phase compositions of the samples were determined using an X-ray diffractometer (Bruker D8, Bruker, Billerica, MA, USA). Microstructures were examined with a field-emission scanning electron microscope (SU5000, Hitachi, Tokyo, Japan). Raman spectra were acquired using a micro-Raman spectrometer (LabRAM HR Evolution, Horiba Scientific, Kyoto, Japan) with the excitation source of 532 nm. P-E hysteresis loops were characterized employing a ferroelectric analyzer (TF2000, aixACCT, Aachen, Germany). For this purpose, ceramic disks with a thickness of 0.2–0.3 mm were prepared, in which a top electrode of silver paste with a diameter of 3 mm was deposited, while the bottom side was coated with a larger 6 mm electrode to act as the ground contact. The practical charge–discharge performances of the ceramics were assessed on a commercial testing platform (CFD-001, Gogo Instruments Technology, Shanghai, China). In this case, thinner specimens with a thickness of ~0.14 mm were employed, and symmetrical silver electrodes with a diameter of 3 mm were applied to both surfaces. X-ray photoelectron spectroscopy (XPS) measurements were performed using a spectrometer(Thermo Fisher, Thermo Scientific K-Alpha, MA, USA).Finally, domain morphologies, piezoelectric response amplitudes, and phase signals were investigated via piezoresponse force microscopy (MFP-3D Origin+, Asylum Research, CA, USA).

3. Results and Discussion

3.1. X-Ray Diffraction Studies

Figure 1a presents the XRD patterns of H1–H4 ceramics along with local magnified views of the key diffraction angle regions (Figure 1b,d). All samples display a pure perovskite phase structure. As illustrated in Figure 1c,d, at 2θ ≈ 37.4° and 43.4°, H1 ceramic, which has the lowest Ho³⁺ content, shows a faint superlattice diffraction peak (indicated by arrows). The presence of this peak provides direct evidence for long-range ordering in the crystal structure [15]. As the Ho³⁺ content increases, the intensity of this superlattice peak diminishes and eventually disappears, indicating the breakdown of long-range ordering in the crystal structure. Furthermore, at 2θ ≈ 44° (Figure 1d), the (200) diffraction peak of H1 exhibits a tetragonal splitting, which directly reflects the lattice distortion caused by the long-range ordered structure [16]. As the Ho³⁺ content increases, the degree of peak splitting gradually reduces, and eventually, in H4, the peaks merge into a single symmetric peak, signifying the transition of the overall crystalline structure into a high-symmetry pseudocubic phase. The XRD patterns of all samples were subjected to Rietveld refinement using GSAS-II software. The refinement was performed based on a tetragonal P4mm space group structural model (PDF#70-4264). Figure 1e,f display the Rietveld-refined XRD patterns of H1 and H4, with the evolution of key lattice parameters extracted from the refinement summarized in

Table 1. Lattice parameters of H1–H4 ceramics.

Ceramic	a (Å)	c (Å)	V (Å3)	Rwp	Rp	GOF
H1	4.13156 (13)	4.11551 (22)	70.251 (3)	9.85	6.47	1.65
H2	4.13276 (4)	4.13279 (5)	70.5869 (21)	10.15	7.02	1.62
H3	4.13094 (11)	4.11786 (19)	70.2700 (31)	10.48	7.04	1.69
H4	4.12939 (14)	4.12429 (23)	70.327 (6)	7.46	5.54	1.19

. The tetragonality exhibits a complex, non-monotonic variation with increasing Ho³⁺ content and reaches a value closest to the ideal cubic structure at x = 0.02 (H2), suggesting that this composition lies near a phase boundary induced by composition. Additionally, the unit cell volume did not show the theoretically anticipated systematic shrinkage with increasing Ho³⁺ content, but instead, it displayed non-monotonic fluctuations. This behavior is attributed to the lattice contraction effect caused by A-site substitution being counteracted by the local lattice expansion induced by partial Ho³⁺ occupation of the B-site.

3.2. Microstructural Characterization Studies

The cross-sectional SEM images and grain size distributions of H1–H4 ceramics are presented in Figure 2a–d. All samples consist of densely packed grains with clear boundaries, indicating high sintering density. The average grain sizes of H1, H2, H3, and H4 are 0.625 μm, 0.573 μm, 0.566 μm, and 0.549 μm, respectively, showing a systematic decrease with increasing Ho³⁺ content. Such grain refinement is attributed to the effective suppression of abnormal grain growth by the medium-temperature sintering process, together with the grain-boundary migration pinning effect induced by Ho³⁺ doping [17]. The reduced grain size lowers the probability of large-scale defect formation and restricts the mean free path of charge carriers, thereby suppressing avalanche breakdown under high electric fields. Consequently, the breakdown strength (E_b) is enhanced, enabling the ceramics to sustain higher operating fields and resulting in an increased W_rec [18,19].

An energy-dispersive X-ray spectroscopy (EDS) mapping analysis was performed on H4 to evaluate the chemical homogeneity and the spatial distribution of the doped elements (Figure 3). The SEM image of the analyzed region is presented in Figure 3a. The corresponding elemental mapping results (Figure 3b–e) reveal that the major constituent elements, including Pb, La, Zr, and Ti, are uniformly distributed within the microstructure. As shown in Figure 3f, although Ho is present only as a trace dopant, its signal remains detectable and exhibits a homogeneous distribution similar to that of the primary elements. Importantly, no evidence of Ho enrichment or local segregation was found across the entire scanned area.

3.3. Raman Spectroscopic Studies

The room-temperature Raman spectra of H1–H4 ceramics in the wavenumber range of 100–1000 cm⁻¹ are shown in Figure 4, providing insight into the structural evolution induced by Ho³⁺ doping at the scale of local atomic vibrations. To quantitatively analyze the vibrational modes, all spectra were deconvoluted, as illustrated in Figure 4a. The deconvolution and mode assignment of H1, which possesses the highest degree of long-range order, are presented in Figure 4b. The Raman spectra of lead-based perovskites can be divided into three distinct regions. The low-frequency peak at 145 cm⁻¹ arises mainly from lattice vibrations associated with A-site Pb²⁺ (E(TO₁)). The strong and broad bands at 290 cm⁻¹ and 495 cm⁻¹ are assigned to the bending vibration of BO₆ octahedra (A₁(TO₁)+E(TO₂)) and the twisting vibration (A₁(TO₂)), respectively. In the high-frequency region, the bands at 710 cm⁻¹ and 883 cm⁻¹ correspond to B-O bond stretching vibrations (A₁(TO₃)+E(LO₄)) and second-order Raman scattering, respectively [20,21]. Amphoteric doping of Ho³⁺ inevitably introduces point defects to maintain charge neutrality. As shown in Figure 4a, the gradual disappearance of the 145 cm⁻¹ peak with increasing Ho³⁺ content indicates the collapse of long-range translational symmetry, which may be associated with weakened A-site ordering caused by $\mathrm{H}{\mathrm{o}}_{\mathrm{B}}^{\cdots }-{\mathit{V}}_{\mathrm{Pb}}^{″}$ defect dipoles. In addition, the formation of defect dipoles generates local and random electric and stress fields in the lattice, which strongly scatter phonons and shorten their lifetimes. This results in a pronounced broadening of all Raman peaks, a phenomenon closely related to the incorporation of $\mathrm{H}{\mathrm{o}}_{\mathrm{A}}^{\cdots }-{\mathit{V}}_{\mathrm{O}}^{··}$ dipoles.

3.4. XPS Studies

The X-ray photoelectron spectroscopy (XPS) results of H1 and H4 are presented in Figure 5. As shown in Figure 5a,d, the Ho 4d spectra of both H1 and H4 were deconvoluted into two spin–orbit components, Ho 4d_5/2 near 161 eV and Ho 4d_3/2 near 166 eV. These binding energies are in good agreement with the reference values of Ho³⁺ in Ho₂O₃, confirming that the incorporated holmium ions predominantly exist in the ceramics in a stable +3 oxidation state [22,23]. In addition, the Pb 4f spectra of H1 and H4 (Figure 5b,e) exhibit the characteristic Pb²⁺ spin–orbit doublet (Pb 4f_7/2 ≈ 138 eV, Pb 4f_5/2 ≈ 143 eV), without noticeable chemical shifts between H1 and H4, suggesting that the oxidation state and chemical environment of the A-site cations remain unchanged.

The O 1s spectra (Figure 5c,f) can be fitted into three components. The low-binding-energy peak (~529.5 eV) is assigned to lattice oxygen (O²⁻) in the perovskite framework, the medium-binding-energy peak (~531.0 eV) is associated with oxygen atoms adjacent to oxygen vacancies (O_vac), and the high-binding-energy peak (~532.2 eV) is attributed to adsorbed oxygen (O_ads) at grain boundaries [24]. Importantly, the relative fraction of the O_vac peak increases from 66.12% in H1 to 70.31% in H4, indicating a significant rise in oxygen vacancy concentration with increasing Ho³⁺ content. According to defect chemistry, substitution of Ho³⁺ for A-site Pb²⁺ (donor doping) is primarily compensated by lead vacancies ($V_{\mathrm{Pb}}^{″}$) and should not generate substantial oxygen vacancies. In contrast, substitution of Ho³⁺ for B-site Zr⁴⁺/Ti⁴⁺ (acceptor doping) requires charge compensation through the formation of oxygen vacancies ($V_{\mathrm{O}}^{··}$). Therefore, the increased oxygen vacancy fraction in highly doped samples provides strong evidence that a portion of the Ho³⁺ ions occupy the B-site.

3.5. Ferroelectric Studies

At the breakdown electric field and a frequency of 10 Hz, the bipolar P-E loops of H1–H4 ceramics are shown in Figure 6a. Unlike the double hysteresis loops or broad hysteretic loops typically observed in long-range-ordered materials, all samples exhibit extremely slim P-E loops with low remanent polarization (P_r) and coercive field (E_c) (H4: P_r = 0.4 μC/cm², E_c = 7.9 kV/cm). With increasing Ho³⁺ content, the maximum polarization (P_max) decreases from 8.4 μC/cm² for H1 to 7.3 μC/cm² for H3, and then rises to 8.9 μC/cm² for H4. This recovery in P_max is primarily attributed to the substantial enhancement of the breakdown E_b, which increases from 153 kV/cm in H1 to 216 kV/cm in H4.

Owing to their slim P-E loops and minimal energy loss, all samples demonstrate excellent η values above 85%, as shown in Figure 6b. H2 exhibits the highest efficiency of 90.4%. W_rec increases monotonically with Ho³⁺ content, with H4—benefiting from its highest E_b and largest P_max—achieving an outstanding W_rec of 0.91 J/cm³.

Figure 7 summarizes the reported energy storage performances of representative Pb-based (anti)ferroelectric ceramics in recent years [25,26,27,28,29,30,31,32,33]. As can be seen, most systems exhibit a trade-off between W_rec and η. For instance, conventional PLZT and PLBZST ceramics typically show η values in the range of 60–75%, and although some compositions achieve W_rec values approaching 1 J/cm³, their energy conversion efficiency remains limited. Conversely, systems such as PSZT and PMN-PT demonstrate relatively higher efficiencies, but their W_rec usually does not exceed 0.6 J/cm³.

In contrast, the PLZTH ceramics developed in this work achieve a W_rec of 0.91 J/cm³ and an η of 87% at a thickness of 0.2–0.3 mm, thereby striking a more favorable balance between energy density and efficiency. Compared with other reported PLZT-based ceramics, PLZTH maintains a moderate energy storage density while exhibiting a remarkable advantage in energy conversion efficiency. This indicates that Ho³⁺ doping not only sustains a relatively high W_rec but also effectively suppresses energy dissipation during polarization switching, leading to reduced energy loss under fast charge–discharge conditions.

3.6. Charge–Discharge Behavior Studies

To assess the suitability of PLZTH ceramics sintered at intermediate temperatures for practical pulsed power applications, pulsed charge–discharge measurements were performed on H1 and H4. In the underdamped mode (Figure 8a,e), both samples exhibited a linear increase in the first peak current density with the applied field, and the discharge process was completed after 2–4 oscillations. Such oscillatory behavior originates from the intrinsic RLC characteristics of the testing circuit, where the oscillation period follows $T\approx 2\pi \sqrt{LC}$. Here, L represents the equivalent inductance of the circuit and C is the capacitance of the sample; since all measurements were carried out on the same platform, L can be regarded as constant. The similarity of oscillation profiles at different fields indicates that the capacitance of the ceramics remains stable during nanosecond-scale discharge. Beyond W_rec, current density (C_D) and P_D are also critical metrics for evaluating pulsed capacitor dielectrics [34,35]. As shown in Figure 8d,h, both C_D and P_D increase monotonically with the applied field, reaching 148.1 A/cm² and 7.4 MW/cm³ for H1 at 100 kV/cm and 320.3 A/cm² and 28.8 MW/cm³ for H4 at 180 kV/cm.

In the overdamped mode with a load resistance of 205 Ω (Figure 8b,f), all discharge current traces exhibited a rapid rise followed by exponential decay. The peak discharge currents increased progressively with the applied field, reaching 6.9 A for H1 and 8.5 A for H4. Integration of the current-time profiles yielded the discharge energy density (W_d), as shown in Figure 8c,g. W_d increased linearly with field strength, attaining 0.18 J/cm³ for H1 at 100 kV/cm and 0.43 J/cm³ for H4 at 180 kV/cm. These values correspond to 75% and 73% of the recoverable energy densities obtained from the quasi-static P-E loops (H1: 0.24 J/cm³; H4: 0.59 J/cm³). The slight energy loss is attributed to intrinsic dielectric dissipation under high-frequency conditions during nanosecond-scale discharge, where polar nanoregions cannot fully follow abrupt field reversals, and conduction and interfacial polarization losses convert part of the stored energy into heat [36,37,38].

Throughout the entire test range, t_0.9 remained below 20 ns, with values of 18.85 ns for H1 at 100 kV/cm and as low as 4.97 ns for H4 at 180 kV/cm, demonstrating an ultrafast discharge response. The enhanced charge transport and energy release efficiency induced by Ho³⁺ doping further highlights the promise of medium-temperature sintered PLZTH ceramics as advanced dielectric candidates for pulsed power capacitors.

3.7. PFM Imaging Studies

To reveal the local polarization characteristics and domain structures of PLZTH ceramics at the nanoscale, piezoresponse force microscopy (PFM) characterization was performed on the H1 sample, which exhibits the highest long-range order, as shown in Figure 9. Figure 9a presents PFM topography, where the piezoelectric response distribution is uniform, and the crystal structure and grain size correspond well with the SEM results. In Figure 9b, PFM amplitude image reveals variations in amplitude intensity across different grains and grain boundaries, indicating spatial heterogeneity in the local piezoelectric response. Notably, the PFM phase image in Figure 9c clearly demonstrates the existence of local polarization orientations and domain structures, confirming the presence of nanoscale polarization domains within the material. These polarization domains are likely induced by localized compositional fluctuations or internal stress fields caused by Ho³⁺ doping, which contribute to the stabilization of nanoscale domains and influence the overall relaxation behavior [39,40,41]. With increasing Ho³⁺ content, these domain structures may evolve into a pseudocubic phase with shorter-range order, further optimizing the dielectric and energy storage properties of the material.

4. Conclusion

In this work, a series of Ho³⁺-doped PLZT ceramics (H1–H4) were fabricated via medium-temperature solid-state sintering, and their structural evolution and energy-storage properties were systematically studied. Through the relaxor phase transition induced by Ho³⁺ amphoteric doping, H4 delivered outstanding energy-storage performance (W_rec = 0.91 J/cm³, η = 87%), along with a power density of 28.8 MW/cm³ and an ultrafast t_0.9 of 4.97 ns at 180 kV/cm. These results demonstrate that the synergistic strategy of medium-temperature sintering and Ho³⁺ amphoteric doping not only effectively mitigates Pb volatilization but also enhances processing compatibility with low-cost internal electrodes such as Ni and Cu, thereby offering a promising material platform for high-performance multilayer ceramic capacitors (MLCCs) and advanced pulsed power systems.