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

Abstract

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 (KxNa1-xNbO3, KNN) via an aqueous route. Stochiometric KNN (K0.5Na0.5NbO3) was prepared, by combining alkali-nitrate salts (NaNO3 and KNO3) with the hexaniobate ([HxNb6O19]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.