Fabrication and Dielectric Performance of Spherical and Rod‑Shaped PbTiO3 Powders via Molten‑Salt Synthesis

Abstract

By reacting PbC₂O₄ and TiO₂ in eutectic NaCl‑KCl, we successfully prepared both spherical and rod‑like PbTiO₃ (PTO) powders using molten salt synthesis (MSS) and template MSS, respectively. X‑ray diffraction confirms a pure tetragonal phase for all samples. Varying the molar ratio of PbC₂O₄:TiO₂:NaCl:KCl from 1:1:10:10 to 1:1:60:60 had negligible impact on the spherical morphology when synthesized at 950 °C for 5 h. In contrast, rod‑shaped PTO (diameter 480 nm–1.50 µm, length up to 10 µm) was achieved at 800 °C for 5 h using rod‑like anatase TiO₂ templates and a 1:1:60:60 molar ratio. Energy‑dispersive spectroscopy shows stoichiometric Pb:Ti ratios of ~1:1. Molten salt content critically influences rod formation: insufficient salt prevents rod synthesis, while extended reaction times favor spherical particles. Dielectric measurements at room temperature reveal that spherical PTO powders exhibit a high dielectric constant (~340 at 10⁶ Hz) with low loss (tan δ ≈ 0.06), outperforming rod‑like powders (ε_r ≈ 140, tan δ ≈ 0.08). These findings position spherical PTO powders as promising candidates for multilayer capacitors and resonators.

Introduction

Perovskite oxides (ABO₃) are cornerstone materials for ferroelectric, magnetic, and optoelectronic applications. PbTiO₃ (PTO) stands out with the largest tetragonal distortion (c/a ≈ 1.064) and highest spontaneous polarization among ferroelectric perovskites, yielding excellent dielectric, piezoelectric, and pyroelectric behavior. Conventional solid‑state routes to high‑density PTO ceramics suffer from grain growth, microcracking, and compositional instability, prompting exploration of alternative synthesis strategies. Molten salt synthesis (MSS) offers a low‑temperature, short‑time pathway to pure perovskite phases by dissolving reactants in a molten chloride medium, facilitating ion transport and nucleation. Prior MSS studies have yielded spherical PTO nanoparticles, yet the production of one‑dimensional (1D) nanostructures remains limited due to the isotropic growth tendency of the perovskite lattice. Recent template MSS approaches have shown promise in directing anisotropic growth, but comprehensive dielectric characterization of the resulting morphologies is lacking. Here, we report the controlled synthesis of both spherical and rod‑like PTO powders by MSS and template MSS, respectively, and systematically evaluate their dielectric performance.

Materials and Methods

Materials

Analytical‑grade PbC₂O₄ and TiO₂ (anatase/rutile mix) were sourced from Sigma‑Aldrich. NaCl, KCl, K₂CO₃, AgNO₃, and HCl were obtained from Shanghai Chemical Reagent Co., Ltd.

Spherical PTO by MSS

Mixtures of PbC₂O₄, TiO₂, NaCl, and KCl were ground for 30 min, then calcined at 950 °C for 5 h in alumina crucibles. Post‑heat treatment involved water washing until chloride ions were undetectable (AgNO₃ test) followed by drying at 120 °C for 4 h.

Rod‑like PTO by Template MSS

Rod‑like anatase TiO₂ templates were fabricated by converting K₂Ti₄O₉ (prepared from K₂CO₃ and TiO₂) with HCl leaching and annealing at 800 °C. These templates were mixed with PbC₂O₄ and NaCl‑KCl (1:1:20:20 or 1:1:60:60) and heated at 800 °C for 1–10 h. Final products were washed, dried, and sintered at 1150 °C for 2 h to densify pellets for dielectric testing.

Characterization

X‑ray diffraction (Rigaku D/Max‑RA, Cu Kα) assessed phase purity; SEM (Hitachi S‑3400 N II, 30 kV) examined morphology; EDS verified composition. Dielectric constants and losses were measured on silver‑plated pellets (12 mm × 1 mm) using an Agilent 4192 A impedance analyzer from 10 Hz to 1 MHz.

Results and Discussion

Spherical PTO: Phase and Morphology

All XRD patterns indexed to tetragonal PTO (P4mm) with lattice constants a ≈ 0.390 nm, c ≈ 0.408 nm (c/a ≈ 1.045). Morphology remained spherical across all salt ratios (1:1:10:10 to 1:1:60:60) after 950 °C/5 h treatment. The molten salt primarily acts as a medium for PbO diffusion onto TiO₂ surfaces, enabling nucleation of spherical PTO via a solution‑precipitation mechanism.

Rod‑like PTO: Template MSS Effectiveness

TiO₂ templates annealed at 800 °C yielded pure anatase rods (diameter 480–570 nm, length ~12 µm). When used in MSS with 1:1:60:60 molar ratio and 800 °C/5 h, rod‑shaped PTO formed with diameters 480 nm–1.50 µm and lengths up to 10 µm. Increasing molten salt content and reaction time enhanced rod volume fraction up to ~97 %. Low salt conditions or prolonged heating (10 h) favored spherical particles, indicating that high salt concentration promotes anisotropic growth by allowing PbO to diffuse slowly, enabling re‑assembly of nanocrystallites into rods.

Dielectric Performance

Sphere‑like PTO exhibited a pronounced frequency‑dependent dielectric constant: high values (~3000 at < 10³ Hz) dropping to ~340 at 10⁶ Hz, with low loss (tan δ ≈ 0.06). Rod‑like PTO showed a flatter response: ε_r ≈ 140–150 across the same frequency range and tan δ ≈ 0.07–0.08. The superior dielectric properties of spheres are attributed to larger grain sizes (~2 µm) and reduced grain‑boundary polarization. These values surpass those reported for sol‑gel or hydrothermal PTO nanoparticles, underscoring the advantage of the MSS route.

Conclusions

We demonstrated that MSS and template MSS can produce high‑purity, tetragonal PbTiO₃ powders with controlled morphologies. Spherical powders retain their shape regardless of molten salt quantity, whereas rod‑like powders require high salt concentrations and optimal reaction times. Dielectric measurements confirm that spherical PTO offers the best performance, making it a strong candidate for multilayer capacitor and resonator applications.