Synthesis and dielectric properties of layered-perovskite KCa₂Na_n-3Nb_nO_3n+1 ceramics

KCa₂NaNb₄O₁₃ (4KCNNO)¹ ceramics are well-sintered at 1325 °C without any secondary phase. However, K₂O evaporation occurs in KCa₂Na₂Nb₅O₁₆ (5KCNNO)² ceramics calcined at temperatures higher than 1200 °C, resulting in the formation of the KCa₂Na₃Nb₆O₁₉ (6KCNNO)³ secondary phase. Excess-K₂O was added to the 5KCNNO specimen to synthesize pure 5KCNNO ceramics by compensating for the evaporated K₂O. The 5KCNNO ceramic containing 20 mol% excess-K₂O sintered well at 1325 °C without any secondary phase. However, the 6KCNNO secondary phase also developed in specimens containing a large excess of K₂O, owing to the reaction between excess-K₂O and the 5KCNNO phase. Dielectric constant (ɛ_r) of the 4KCNNO ceramic is 62 with a dielectric loss (tan δ) of 0.6% at 100 kHz and the 5KCNNO ceramic showed increased ɛ_r of 70 with tan δ of 0.3% at 100 kHz. The increase in the ɛ_r value is rationalized by the inc_reased number of NbO₆ octahedral layers between the K⁺ layers. In addition, the structural and dielectric properties of the 5KCNNO ceramic are more important than those of the 4KCNNO ceramic from the application point of view, because the ε_r value of the 5KCNNO ceramic is larger than that of the 4KCNNO ceramic.