Microstructure and Properties of KSr₂Nb₅O₁₅ Ceramics with Excess K⁺

KSr₂Nb₅O₁₅-xK (KSN-xK, x = 0 mol.%, 4 mol.%, 8 mol.%, 12 mol.%, 16 mol.%, and 20 mol.%) lead-free ferroelectric ceramics have been prepared by a buried sintering method using needle-like KSN particles synthesized by molten salt synthesis, and their microstructure, dielectric properties, and infrared transmittance investigated. The results suggest that the KSN-xK ceramics had simplex tungsten bronze structure for x ≤ 12 mol.%, but K₂Nb₈O₂₁ secondary phase appeared at higher x. Excess K⁺ compensated the shortage of A-site ions in KSN crystallites, alleviated lattice distortion, and drove the KSN component closer to stoichiometric ratio, all of which increased the Curie temperature. The dielectric relaxor behavior of the ceramics was enhanced as the excess K⁺ content was increased. The dielectric constant, dielectric tunability, and infrared transmittance initially increased then decreased with increasing x. The specimen with 12 mol.% excess K⁺ showed optimum electrical properties, including maximum infrared transmittance of ∼60%. This work confirms that A-site vacancies in KSN can be compensated by excess K⁺, and that this effect can be used to adjust the local composition, alleviate structural distortion of the oxygen octahedron, enhance the Curie temperature, etc.