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Effect of Pr³⁺ doping on the energy storage and electrocaloric performances of [(Bi 0.5 Na 0.5 ) 0.94 Ba 0.06 ] 0.975 Sr 0.025 TiO 3 antiferroelectric ceramic

This study demonstrates that doping the antiferroelectric ceramic [(Bi₀.₅Na₀.₅)₀.₉₄Ba₀.₀₆]₀.₉₇₅Sr₀.₀₂₅TiO₃ with 1% Pr³⁺ significantly enhances its energy storage density and electrocaloric cooling performance near room temperature without altering its structural phase boundary, positioning it as a promising lead-free candidate for multifunctional electronic applications.

Original authors: N. Zorgui, Z. Abdelkafi, N. Abdelmoula, H. Khemakhem

Published 2026-09-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: N. Zorgui, Z. Abdelkafi, N. Abdelmoula, H. Khemakhem

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Modern electronics are shrinking, yet they generate heat that must be removed to keep functioning. While traditional cooling relies on fans and liquid coolants, a quieter, more compact solution lies in the behavior of certain solid materials that change temperature when an electric field is applied or removed. This phenomenon, known as the electrocaloric effect, offers a path toward solid-state cooling that could replace bulky mechanical systems. Alongside cooling, these same materials can store electrical energy, acting as tiny, efficient batteries that charge and discharge rapidly. The challenge for scientists is finding materials that are not only effective at these tasks but also free of toxic lead, a common ingredient in older high-performance ceramics that poses environmental risks.

In a recent study, researchers from the University of Sfax in Tunisia explored a specific lead-free ceramic to see if a tiny tweak in its chemical makeup could improve both its cooling ability and its energy storage capacity. The material they started with is a complex mixture of bismuth, sodium, barium, strontium, and titanium. It belongs to a class of materials called antiferroelectrics, where the internal electric charges are arranged in opposing pairs that cancel each other out, creating a stable state. When a strong electric field is applied, these opposing pairs can be forced to align, a switch that releases or absorbs heat and stores energy. The team wanted to know if adding a small amount of praseodymium, a rare-earth element, to this mix would make the material more responsive and efficient.

The researchers prepared two versions of the ceramic: one with the standard recipe and another with just one percent of the bismuth atoms replaced by praseodymium. They fired the materials in a furnace to create solid discs and then subjected them to rigorous testing. Using X-ray diffraction, they confirmed that the crystal structure remained stable and pure, with the praseodymium fitting neatly into the lattice without disrupting the delicate balance that defines the material's properties. This was a crucial finding, as it showed that the addition of the new element did not break the material's fundamental architecture.

When the team measured how the material responded to electric fields at room temperature, the results were clear. The praseodymium-doped sample proved superior in almost every metric. In terms of energy storage, the doped material could hold and release more energy with less waste. Specifically, at a temperature of 310 Kelvin, the undoped sample stored about 229 millijoules of energy per cubic centimeter with an efficiency of 72 percent. The doped version, however, increased that storage to roughly 276 millijoules per cubic centimeter and boosted efficiency to 77 percent. This improvement suggests that the praseodymium made it easier for the internal electric charges to switch states, allowing the material to capture more energy during the charging process and release more of it during discharge.

The same doping also enhanced the material's ability to cool. When an electric field was applied and then removed, the doped ceramic experienced a slightly larger change in temperature compared to the undoped version. At 310 Kelvin under a field of 40 kilovolts per centimeter, the temperature change rose from 0.087 Kelvin in the pure sample to 0.095 Kelvin in the doped one. While these numbers seem small, they represent a meaningful gain in performance for solid-state cooling applications, particularly when the material is used near room temperature. The researchers noted that this improvement was linked to the material's ability to undergo a smoother transition between its different internal states when the electric field was applied.

The study also looked at how these properties changed as the temperature rose. Both materials performed even better as they approached a specific threshold where their internal structure becomes less ordered, a point known as the depolarization temperature. For the doped material, this threshold was lower, occurring at 366 Kelvin compared to 396 Kelvin for the undoped version. This shift allowed the doped ceramic to maintain high efficiency and energy storage capacity at temperatures that are more practical for everyday electronic devices. At these higher temperatures, the doped sample achieved an energy storage density of over 313 millijoules per cubic centimeter with an efficiency of 94 percent, outperforming many other lead-free ceramics reported in scientific literature.

The researchers concluded that the addition of praseodymium did not alter the fundamental phase boundary of the material but instead fine-tuned its behavior to be more responsive. By creating a slightly more disordered internal structure, the praseodymium allowed the electric dipoles to switch more easily, which in turn improved both the energy storage and the cooling capabilities. These findings suggest that this specific lead-free ceramic, when doped with a trace amount of praseodymium, is a strong candidate for use in next-generation microelectronics cooling and high-capacity energy storage systems, offering a path forward for devices that are both powerful and environmentally safe.

Technical Summary: Effect of Pr³⁺ Doping on Energy Storage and Electrocaloric Performances of [(Bi₀.₅Na₀.₅)₀.₉₄Ba₀.₀₆]₀.₉₇₅Sr₀.₀₂₅TiO₃ Antiferroelectric Ceramic

Problem Statement
The development of lead-free materials for solid-state cooling and high-density energy storage is a critical objective in modern electronics. Antiferroelectric (AFE) materials are particularly promising due to their unique field-induced phase transitions, which offer high energy storage density and significant electrocaloric effects (ECE). However, optimizing these properties near room temperature (RT) without compromising structural stability or efficiency remains a challenge. Previous work on the lead-free system [(Bi₀.₅Na₀.₅)₀.₉₄Ba₀.₀₆]₀.₉₇₅Sr₀.₀₂₅TiO₃ (BNBTS25:00) indicated that Pr³⁺ doping could tune dielectric and photoluminescence properties, shifting the depolarization temperature ( $T_d$ ) closer to RT. This study investigates whether low-level Pr³⁺ doping can further enhance the energy storage and electrocaloric performances of BNBTS25:00, specifically targeting applications in microelectronic device cooling and energy storage.

Methodology
The researchers synthesized two ceramic compositions, undoped BNBTS25:00 and 1% Pr³⁺-doped BNBTS25:10 (where Pr substitutes Bi), using a conventional solid-state reaction method. Raw oxides and carbonates were mixed, calcined at 900–950°C, pressed into pellets, and sintered at 1150–1200°C.

Structural characterization was performed via X-ray diffraction (XRD) with Rietveld refinement to determine phase purity and crystal structure. Electrical properties were evaluated by measuring polarization-electric field ( $P-E$ ) hysteresis loops at various temperatures (298–400 K) and electric fields (up to 60 kV/cm). From these loops, the authors calculated:

Energy Storage: Recoverable energy density ( $W_{rec}$ ), total stored energy ( $W_{tot}$ ), energy loss ( $W_{loss}$ ), and efficiency ( $\eta$ ).
Electrocaloric Effect: Adiabatic temperature change ( $\Delta T$ ) and electrocaloric responsivity ( $\xi$ ), derived using the thermodynamic Maxwell relation and assuming a constant specific heat capacity.

Key Contributions and Results

Structural Stability:
XRD analysis confirmed that both samples possess a single-phase perovskite structure. Rietveld refinement revealed a mixture of cubic and tetragonal phases ($P4bm/R3c$), indicating the presence of a morphotropic phase boundary (MPB). Crucially, the introduction of 1% Pr³⁺ did not alter the MPB, suggesting structural stability upon doping.
Ferroelectric Behavior and Domain Dynamics:
At temperatures below the depolarization temperature ( $T < T_d$ ), both compositions exhibited slim $P-E$ loops characteristic of weak ferroelectricity with low coercive fields ( $E_c$ ). Pr³⁺ doping reduced the remanent polarization ( $P_r$ ) and $E_c$ (e.g., at 310 K, $P_r$ decreased from 1.19 to 0.82 $\mu$ C/cm² and $E_c$ from 4.69 to 2.96 kV/cm). The authors attribute this to the creation of local disorder and the fragmentation of large ferroelectric domains into smaller polar nanoregions (PNRs), enhancing relaxor behavior.
Enhanced Energy Storage:
Pr³⁺ doping significantly improved energy storage metrics at room temperature (310 K).
- Undoped (BNBTS25:00): $W_{rec} = 229.06$ mJ/cm³, $\eta = 72\%$ .
- Doped (BNBTS25:10): $W_{rec} = 275.83$ mJ/cm³, $\eta = 77\%$ .
  The improvement is linked to the destabilization of the ferroelectric phase and stabilization of the AFE order, evidenced by the shift of $T_d$ from 396 K to 366 K. At elevated temperatures near $T_d$ , both samples showed further increases in $W_{rec}$ and $\eta$ (reaching 313.40 mJ/cm³ and 94% for the doped sample at 360 K), attributed to the high breakdown strength of the AFE phase.
Improved Electrocaloric Response:
The electrocaloric performance was also enhanced by doping, particularly under high electric fields.
- At 310 K and 40 kV/cm, the undoped sample showed $\Delta T = 0.087$ K and $\xi = 0.022$ K·mm/kV.
- The doped sample exhibited $\Delta T = 0.095$ K and $\xi = 0.023$ K·mm/kV.
  The authors suggest this improvement stems from an easier reorientation of electric dipoles and a larger entropy change in the doped material. While the doped sample showed slightly lower $\Delta T$ at very high temperatures (340 K) due to weakened polar order, the overall performance near RT and slightly above was superior.

Significance and Claims
The paper claims that the 1% Pr³⁺-doped BNBTS25:10 ceramic represents a significant advancement over the undoped parent composition and other reported lead-free BaTiO₃-based systems operating near room temperature. The study highlights that Pr³⁺ doping successfully enhances multifunctional properties without disrupting the structural MPB.

The authors position these materials as promising candidates for:

Energy Storage: Offering high recoverable energy density and efficiency suitable for electric vehicles and industrial automation.
Electrocaloric Cooling: Providing efficient solid-state cooling for microelectronic devices, particularly at operating temperatures relevant to such applications.

The findings are presented as a validation of Pr³⁺ doping as an effective strategy to tune the depolarization temperature and optimize the trade-off between energy storage density and efficiency in lead-free antiferroelectric relaxors.

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