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Preparation of Phase-Pure K0.5Na0.5NbO3 Fine Powders by a Solid-State Reaction at 625°C from a Precursor Comprising Nb2O5 and K, Na Acetates



Phase-pure K0.5Na0.5NbO3 (KNN) fine powders were synthesized via a solid-state route from a homogeneous solid mixture. A colloidal dispersion comprising a mixed ethanol solution of potassium and sodium acetates and Nb2O5 fine particles was attrition milled and dried carefully to avoid water absorption. Two-step calcination in air at 450°C and 625°C, each for 3 h, resulted in the phase-pure KNN powders. The volume-based median diameter of the final product was ca. 0.8 μm. Starting from the same precursors without dissolving the acetates, the phase-pure KNN was never achieved even when the two calcination temperatures were increased to 550°C and 700°C, in spite of the same milling conditions. Key issues of eliminating second phases were (i) starting from a wet-milled mixture with a single solution containing both of the A-site species, and (ii) repeated wet milling of the reaction mixture to disintegrate reaction-induced agglomerates. These enabled rapid nuclei growth from chemically interacted precursor prior to calcination, and short diffusion path due to repeated deagglomeration, excluding formation of off-stoichiometric second phases. All these items were confirmed by different analytical tools, among others, thermo-gravimetry and differential thermal analysis (TG-DTA), particle size analyses, and XPS at various reaction stages. On the heating stage microscope, a shrinkage onset was observed at 850°C, that is, 150 K lower than that of conventionally prepared KNN, that is, via a solid-state synthesis from carbonates by a two-step calcination at 800°C and 750°C, for 4 h each. No second phase was observed after sintering up to 1100°C.