Using simple aqueous precursors for a green synthetic pathway to potassium sodium niobate thin films

Abstract

Potassium sodium niobate (KNN) is a leading candidate for certain applications within the ongoing move towards lead-free ferroelectric and piezoelectric materials. One common synthetic route for synthesizing KNN utilizes sol-gel chemistry, which typically suffers from issues with low film quality (due to bulky leaving groups), air sensitivity, and hazardous waste. Here we describe a green synthetic pathway to KNN thin films using polyoxometalate precursors in the form of X8Nb6O19 (X=Na, K, H) salts. These hexaniobate clusters are soluble in water and stable in air, allowing for simple deposition with a benign precursor. The alkali content is tunable based on control of pH and subsequent protonation of the hexaniobate cluster. The aqueous precursor solutions create dense polycrystalline KNN films on polycrystalline sapphire substrates. In addition, high quality KNN thin films are observed to grow epitaxially on (001) SrTiO3 substrates. Overall, this synthetic approach demonstrates a flexible pathway to Nb-based thin films with an emphasis on the sustainable chemical practices that are at the heart of modern chemistry and the quest for lead-free ferroelectrics.