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Effects of Ca2+ doping on the Structure and electric properties of Bi4Ti3O2 - BaBi4Ti4O15 based ceramics

This study demonstrates that A-site Ca²⁺ doping in Bi₄Ti₃O₁₂–BaBi₄Ti₄O₁₅-based intergrowth ceramics significantly enhances orthorhombic lattice distortion and raises the Curie temperature to 708 °C, with the optimal composition (BBITKC-0.7C) exhibiting excellent high-temperature piezoelectric performance and thermal stability suitable for demanding applications.

Original authors: Chong Zhao, Na Tu, Chaoqun Chen, Yan Hong, Yunjing Chen, Chao Chen, Xing Nie, Xiangping Jiang

Published 2026-08-10

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CC BY 4.0

Original authors: Chong Zhao, Na Tu, Chaoqun Chen, Yan Hong, Yunjing Chen, Chao Chen, Xing Nie, Xiangping Jiang

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

Imagine a world where machines need to "feel" things—like the pressure of a jet engine's heat or the vibration of a nuclear reactor—but the tools they use to sense these things are made of materials that melt, melt down, or simply forget how to work when things get too hot. For decades, scientists have been hunting for a special kind of "smart rock" called a piezoelectric ceramic. Think of these rocks as tiny, invisible springs that squeeze electricity out of themselves when you squish them, or squish themselves when you zap them with electricity. The problem is that most of these smart springs are like chocolate left in the sun: they work great at room temperature, but if you heat them up past a certain point (usually around 400°C), they lose their memory and stop working. This is a huge headache for engineers trying to build sensors for rockets, deep-earth drilling, or nuclear power plants, where temperatures can get scorching. To fix this, researchers are trying to build a new kind of smart spring that can handle the heat without melting its memory.

This paper is about a team of scientists from Jingdezhen Ceramic University who decided to play a game of "molecular Tetris" to solve this heat problem. They started with a specific type of smart rock made of layers of bismuth and titanium, which is already known for being tough, but they wanted to make it even tougher. Their secret weapon? Calcium. You might know calcium as the stuff that makes your bones strong, but in this experiment, the scientists used it as a tiny, heavy-duty replacement for a different ingredient called Barium. Imagine the rock's internal structure as a multi-story apartment building. The Barium ions are like large, bulky tenants living in the main rooms. The scientists swapped some of these large tenants for smaller, more energetic Calcium ions. The theory was that these smaller tenants would squeeze the building's structure tighter, making it more distorted and, crucially, much harder to melt down. They wanted to see if this tiny swap could raise the temperature at which the rock "forgets" its electrical memory (a point called the Curie temperature) and keep the rock working in the furnace.

The team created a series of these new rocks, gradually swapping more and more Barium for Calcium, and then put them through a rigorous workout. They used high-powered X-rays and lasers to peek inside the rocks, checking if the Calcium actually moved into the right spots and how it changed the shape of the crystal "apartments." They also heated the rocks up to extreme temperatures to see if they could still generate electricity. The results were quite promising. The Calcium did exactly what they hoped: it squeezed the crystal structure, making it more distorted (like a slightly squashed box), which pushed the "forgetting" temperature way up. The best version of their new rock, with a specific amount of Calcium, could handle a Curie temperature of 708°C. That is a massive jump from the original version, which started to forget things at 535°C.

But getting hot is only half the battle; the rock also needs to stay strong while it's hot. The scientists tested how well the rock could still "feel" pressure after being heated to 550°C. The original rock would have lost most of its ability to work by then, but the new Calcium-doped rock was a champion. Even after being roasted at 550°C, it kept about 87.6% of its original strength. It also showed a very high electrical resistance, meaning it didn't leak electricity even when it was glowing hot. The researchers found that the Calcium didn't create any new "holes" or defects that would let electricity escape, which is why the rock stayed so stable. While the rock didn't become a super-powerful generator (it still produces a modest amount of electricity), its ability to survive the heat without losing its mind makes it a very strong candidate for the next generation of high-temperature sensors. The paper suggests that by carefully tuning how much Calcium is added, they found a "sweet spot" where the rock is both heat-resistant and electrically active, offering a real path forward for sensors that can survive in the most extreme environments on Earth and beyond.

Technical Summary: Effects of Ca²⁺ Doping on the Structure and Electric Properties of Bi₄Ti₃O₁₂-BaBi₄Ti₄O₁₅ Based Ceramics

Problem Statement
High-temperature piezoelectric sensors are increasingly required for aerospace and nuclear applications, yet commercial piezoelectric materials typically fail to operate above 400 °C. Bismuth layer-structured ferroelectrics (BLSFs) offer a lead-free alternative with high Curie temperatures ( $T_c$ ). Specifically, the Bi₄Ti₃O₁₂-BaBi₄Ti₄O₁₅ (BIT-BBT) system, a 3-4 layer intergrowth structure, possesses high spontaneous polarization. However, its practical application is hindered by poor ferroelectric and piezoelectric properties caused by oxygen vacancies (from Bi₂O₃ volatilization) and antisite defects (Ba²⁺ entering (Bi₂O₂)²⁺ layers) during high-temperature sintering. While previous ionic doping strategies (e.g., Ta⁵⁺, Ce⁴⁺, K⁺/Ce⁴⁺ co-substitution) have improved piezoelectric coefficients ( $d_{33}$ ), the resulting Curie temperatures generally remain below 550 °C. The authors identify a need to further elevate $T_c$ while maintaining electrical performance, noting that $T_c$ in stable BLSF compositions is inversely correlated with the tolerance factor.

Methodology
The study synthesized a series of intergrowth bismuth-layered piezoelectric ceramics with the formula Bi₄Ti₃O₁₂-Ba₀.₈₅₋ₓ(K₁/₂Ce₁/₂)₀.₁₅CaₓBi₄Ti₄O₁₅ (BBITKC-xCa, where $0.00 \le x \le 0.85$ ) using a conventional solid-state reaction method. The synthesis involved ball-milling precursors (Bi₂O₃, TiO₂, BaCO₃, K₂CO₃, CeO₂, CaCO₃), calcination at 800 °C, and sintering between 1060–1130 °C.

The structural and electrical properties were characterized through:

Structural Analysis: X-ray diffraction (XRD) for phase identification and lattice parameters; Raman spectroscopy for phonon mode analysis and lattice distortion; X-ray photoelectron spectroscopy (XPS) for valence states and oxygen vacancy concentration.
Morphology: Scanning electron microscopy (SEM) to observe grain growth and orientation.
Electrical Characterization: Dielectric measurements (permittivity and loss vs. temperature) to determine $T_c$ and relaxation behavior; P-E hysteresis loops for ferroelectric properties; DC resistivity measurements for conduction mechanisms; and piezoelectric coefficient ( $d_{33}$ ) measurements before and after thermal depoling.

Key Contributions and Results
The primary contribution of this work is the successful substitution of A-site Ba²⁺ with smaller Ca²⁺ ions to modulate the tolerance factor and enhance thermal stability without introducing significant charge defects.

Structural Modifications:
- XRD and Raman analyses confirmed that Ca²⁺ successfully substitutes for Ba²⁺ in the perovskite-like layers, resulting in a single orthorhombic phase without secondary phases.
- The substitution of larger Ba²⁺ (1.64 Å) with smaller Ca²⁺ (1.34 Å) reduced the unit cell volume and interplanar spacing, evidenced by the shift of the (118) peak to higher angles.
- Ca²⁺ doping enhanced orthorhombic lattice distortion, indicated by the splitting of the (201) peak and increased intensity of specific Raman modes (v8) associated with [TiO₆] octahedral distortion.
- XPS analysis revealed that the ratio of oxygen vacancies to lattice oxygen ( $O_v/O_L$ ) decreased slightly with Ca doping, confirming that isovalent substitution does not generate additional oxygen vacancies.
Thermal and Electrical Performance:
- Curie Temperature ( $T_c$ ): A significant increase in $T_c$ was observed with increasing Ca content, rising from 535 °C ( $x=0$ ) to 708 °C ( $x=0.85$ ). This is attributed to the reduced tolerance factor and the electronic configuration differences between Ca²⁺ (4s) and Ba²⁺ (6s), which enhance lattice distortion.
- Piezoelectric Properties: The composition with $x=0.7$ (BBITKC-0.7Ca) exhibited optimal performance, with a piezoelectric coefficient $d_{33} = 23.5$ pC/N and a $T_c$ of 691 °C.
- Thermal Stability: The BBITKC-0.7Ca sample demonstrated excellent thermal stability. After depoling at 550 °C, the $d_{33}$ value retained 20.6 pC/N (87.6% of its room-temperature value).
- Resistivity: At 550 °C, the DC resistivity of BBITKC-0.7Ca was $1.92 \times 10^6$ $\Omega\cdot$ cm, significantly higher than the undoped sample ( $7.66 \times 10^5$ $\Omega\cdot$ cm). Activation energy analysis indicated that conduction is dominated by oxygen vacancies below 400 °C and intrinsic conduction above 400 °C.
- Relaxor Behavior: The dielectric relaxation exponent ( $\gamma$ ) increased with Ca doping up to $x=0.55$ , indicating strong relaxor behavior due to A-site disorder. At $x=0.85$ , $\gamma$ decreased, suggesting a recovery of ferroelectricity.

Significance
The paper claims that BBITKC-0.7Ca ceramics are highly promising for high-temperature applications due to their combination of high Curie temperature, improved piezoelectric activity, and excellent thermal stability of piezoelectric properties. By utilizing Ca²⁺ doping to reduce the tolerance factor, the study successfully overcomes the limitation of low $T_c$ in modified BIT-BBT systems while maintaining low dielectric loss and high resistivity at elevated temperatures. The work provides a viable pathway for developing lead-free piezoelectric sensors capable of operating in environments exceeding 550 °C.

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