Facile In Situ Formation of Potassium Sodium Niobate (KNN) Using The Hexaniobate Polyoxometalate

Lead-based piezoceramics are the dominant materials used in electronic devices, despite the known toxicity of lead. Developing safer piezoelectric materials has inspired the pursuit of lead-free piezoceramics, however some challenges remain in accessing these materials reproducibly. Here we demonstrate a simple and robust method for synthesis of the lead-free piezoceramic material, potassium sodium niobate (K_xNa_1-xNbO₃, KNN) via an aqueous route. Stochiometric KNN (K_0.5Na_0.5NbO₃) was prepared, by combining alkali-nitrate salts (NaNO₃ and KNO₃) with the hexaniobate ([H_xNb₆O₁₉]^8-x, Nb₆) species in water, followed by heating at elevated temperatures for at least one hour. Ex situ heating of the amorphous alkali-Nb₆ precursor reveals stoichiometric control and phase uniformity are possible in making KNN, versus a solid-state route. In situ heating in a transmission electron microscope (TEM), with selected area electron diffraction (SAED), facilitates monitoring the real-time transformation of the amorphous alkali-Nb₆ precursor, to yield monoclinic KNN, in agreement with ex situ results. Therefore, an aqueous route via hexaniobate is an attractive alternative approach for developing lead-free piezoceramic materials.