Engineering A-Site NaNbO₃ Lead-Free Antiferroelectrics Achieve Distinct Double Hysteresis Loops

Title

J Am Ceram Soc：Engineering A-Site NaNbO₃ Lead-Free Antiferroelectrics Achieve Distinct Double Hysteresis Loops

Material System: (Na₁–₂ₓ-yCaₓAgᵧ)NbO₃ (100xCa-100yAg) Ceramics

Research Background

Antiferroelectrics (AFEs) have significant applications in energy storage capacitors due to their unique double hysteresis loop characteristics. Traditional PbZrO₃-based materials pose environmental hazards, while NaNbO₃ (NN) faces irreversible challenges in AFE-FE phase transitions as a lead-free alternative. Existing methods of co-doping A/B sites to control the tolerance factor (t) can achieve “squeezed” hysteresis loops, but residual polarization (Pr) and hysteresis remain high. This study proposes a new strategy to control both the tolerance factor and electronegativity difference solely through A-site (Ca²⁺/Ag⁺) doping.

Research Content

① Prepared (Na₁–₂ₓ-yCaₓAgᵧ)NbO₃ ceramics via solid-state method, with XRD and STEM confirming a pure perovskite structure.

② Ca²⁺ (x=0.06) doping alone achieved a squeezed hysteresis loop with Pr=16.9 μC/cm², while co-doping with Ag⁺ (y=0.4) resulted in a double loop with Pr=1.3 μC/cm².

③ Rietveld refinement showed A-site ion displacement decreased from 0.14-0.17Å to 0.04-0.10Å, stabilizing the AFE P phase.

④ Dielectric thermometry showed TP-R decreased from 290°C (x=0.02) to 190°C (x=0.06), remaining stable after Ag⁺ doping.

⑤ The 6Ca-40Ag composition achieved a double loop under 240kV/cm, with excellent stability across temperature (25-120°C) and frequency (1-100Hz).

Innovations

① Achieved the first clear double hysteresis loop in NN-based ceramics through A-site doping alone, breaking the limitations of traditional A/B site co-doping.

② Proposed a new co-doping mechanism of Ca²⁺ (decreasing t) and Ag⁺ (decreasing electronegativity difference), reducing Pr by 92%.

③ Revealed collective displacements along the [101] direction and anti-parallel dipole arrangements of the [010] modulation wave through atomic-scale STEM.

④ Achieved stable AFE characteristics under high electric fields (EF=225kV/cm) and low hysteresis (EA=85kV/cm).

⑤ Strain analysis showed Sm reached 0.5% with Sneg≈0, confirming a fully reversible phase transition.

Paper Summary

This study successfully achieved a typical double hysteresis loop (Pr=1.3μC/cm², EF=225kV/cm) in NaNbO₃-based ceramics through A-site Ca²⁺/Ag⁺ co-doping. Structural characterization indicates that this design reduced A-site ion displacement by 71%, stabilizing the AFE P phase of the Pbcm space group. Performance tests show that the 6Ca-40Ag composition maintains a stable double loop across a wide temperature range (25-120°C) and frequency range (1-100Hz), with a strain response Sm reaching 0.5% and negative strain close to zero, confirming a fully reversible AFE-FE phase transition. This work provides a new A-site engineering approach for developing lead-free antiferroelectrics.

Original Text

https://ceramics.onlinelibrary.wiley.com/doi/10.1111/jace.70207