NKBLT-x Ceramics: Tunable Dielectric & Energy Storage

• NKBLT-x ceramics are lead-free perovskite materials with a tunable single-phase rhombohedral structure achieved through targeted K⁺/La³⁺ substitutions.
• They exhibit enhanced dielectric stability and reduced coercive fields due to suppressed octahedral tilting and refined grain sizes from sol-gel self-combustion synthesis.
• Energy storage efficiency dramatically increases with x, reaching up to ~87%, making these ceramics promising for high-temperature capacitor and energy storage applications.

NKBLT-x ceramics denote the lead-free solid solution system (Na₀.₅₋ₓKₓBi₀.₅₋ₓLaₓ)TiO₃, engineered to optimize structural, dielectric, ferroelectric, and energy storage characteristics through targeted compositional modifications. These ceramics employ simultaneous replacement of Na⁺ and Bi³⁺ with K⁺ and La³⁺ at the A-site within the perovskite lattice, yielding a tunable single-phase rhombohedral structure (space group R3c) across 0 ≤ x ≤ 0.12. The system is developed via sol-gel self-combustion synthesis and is characterized by decreased anti-phase octahedral tilting and enhanced thermal stability, rendering them suitable candidates for high-temperature capacitor and energy storage applications (Verma et al., 2018).

1. Crystal Structure and Compositional Modulation

NKBLT-x ceramics are confirmed by synchrotron X-ray Rietveld refinement to retain a pure rhombohedral perovskite framework (R3c) for all studied substitutions. Lattice modifications are induced by increasing x: the lattice parameter "a" expands while "c" contracts, causing the c/a ratio to decrease from 2.47 (x = 0) to 2.44 (x = 0.12), indicative of decreasing structural distortion. Anti-phase TiO₆ octahedral tilt—quantified by the angle θₜ = 4√3·ε—drops from 8.47° (x = 0) to 5.89° (x = 0.12), reflecting suppressed tilting due to ion substitution. A-site ionic radius calculations using

$$r_A = (1 - x)\left(r_{\mathrm{Na}^+} + r_{\mathrm{Bi}^{3+}}\right) + x\left(r_{\mathrm{K}^+} + r_{\mathrm{La}^{3+}}\right)$$

corroborate this lattice expansion, directly linking structural tuning to substitutional chemistry.

2. Dielectric Properties and Thermal Stability

Dielectric analysis of NKBLT-x reveals two diffuse temperature-dependent anomalies across all compositions, associated with successive ferroelectric-to-antiferroelectric and antiferroelectric-to-paraelectric transitions. With increasing x, both the depolarization temperature ($T_a$) and temperature of maximal dielectric constant ($T_m$) shift toward lower values, attributed to reduced off-centering of A-site ions resulting from K⁺/La³⁺ substitution. Notably, the thermal stability of the dielectric constant increases considerably. The parent Na₀.₅Bi₀.₅TiO₃ exhibits a stable region (Δε/ε ≤ ±10%) from ∼280°C–380°C, while compositions such as x = 0.06 display an expanded stable dielectric window (∼180°C–340°C) with elevated mid-range values (€ₑₘᵢd ∼ 2508). Even for higher substitutions (x = 0.12), the stable dielectric constant ($\sim$1608) is maintained above the parent phase, marking NKBLT-x ceramics as robust candidates for high-temperature electronics.

3. Ferroelectric Domain Dynamics and Grain Effects

Ferroelectric measurements utilizing polarization-electric field (P-E) hysteresis loops show a systematic reduction in coercive field (E_c) from the parent toward higher substituted compositions. This decrease is directly correlated with grain refinement, confirmed by FESEM analysis: average grain size drops from ∼14 μm (x = 0) to ∼0.94 μm (x = 0.12). Diminished grain size supports enhanced domain wall mobility and facilitates domain switching, leading to lower coercive fields and favorable energy-saving behaviors. A proportionality between domain size (d) and grain size (t):

$d \propto t$

highlights the mechanistic impact of microstructural control on polarization response. Remnant polarization (P_r) peaks at intermediate $x$ (up to x = 0.06) before tapering at higher doping, a trend accompanied by increasingly slender P-E loops beneficial for low-loss switching applications.

4. Energy Storage Efficiency and Quantitative Performance

Energy storage efficiency (η) of NKBLT-x ceramics demonstrates near-exponential improvement with progressive K⁺/La³⁺ content, rising from ∼17% (x = 0) to ∼87% (x = 0.12). Recoverable energy density (W_s) is calculated from P-E loop integration,

$W_s = \oint P dE$

and total efficiency as

$$\eta = \left( \frac{W_s}{W_\mathrm{supplied}} \right) \times 100\%$$

where $W_\mathrm{supplied}$ includes both stored and dissipated energy. These metrics underscore exceptional energy storage capability coupled with thermally stable dielectric profiles, essential for capacitor and high-reliability energy storage systems in environments demanding robust performance across large temperature excursions. Applications include MLCCs, aerospace, automotive, and oil-gas drilling electronics where temperature resilience is paramount.

5. Comparative Context and Implications for Lead-Free Ceramics

Relative to other lead-free systems—such as (Bi₀.₅Na₀.₅)TiO₃ (BNT), (K₀.₅Na₀.₅)NbO₃ (KNN), and KNLNT-x analogs—NKBLT-x ceramics distinguished themselves by:

• Maintaining a single-phase rhombohedral structure with suppressed octahedral tilting rather than complex multiphase boundaries.
• Realizing a broad, thermally stable dielectric window unmatched by KNN and BNT, whose dielectric and ferroelectric properties are more composition and temperature dependent (Mitra et al., 2014, Sahoo et al., 16 Oct 2025).
• Achieving significant grain refinement and reduced coercive fields (down to ∼0.94 μm grain size and low E_c), which facilitate efficient domain switching.
• Delivering high energy storage efficiency, exceeding values typically reported for undoped NBT or modified BNT-BT systems (Verma et al., 2018).

6. Design Strategies and Future Optimization Directions

The synthesis approach (sol-gel self-combustion) enables substantial microstructural control, directly impacting domain dynamics and electrical response. The demonstrated correlation between compositional tuning, structural distortion reduction, grain size control, and functional property enhancement in NKBLT-x ceramics forms the basis for rational design of future lead-free dielectric and energy storage ceramics. Further optimization may exploit multinary A-site and B-site substitutions to target morphotropic boundary-like effects observed in other systems, thereby potentially amplifying piezoelectric coefficients while retaining thermal and electrical stability. A plausible implication is that combining strategies from recent MPB-engineered systems could produce variants of NKBLT-x with improved piezoelectric responses rivaling traditional lead-based materials (Sahoo et al., 16 Oct 2025).

7. Application Prospects and Research Trajectory

NKBLT-x ceramics, by virtue of their high dielectric constants, wide thermal stability, low loss factors, and high energy storage efficiency, present a technically viable pathway toward lead-free capacitors, energy storage modules, and other high-temperature electronic components. Their tunable structural and microstructural characteristics facilitate adaptation to diverse operational requirements in harsh environments. The pronounced improvements achieved through compositional modulation emphasize the importance of finely controlled synthesis and targeted substitution, informing continued efforts in the development of environmentally sustainable advanced ceramics for next-generation electronic applications.