Solid-state synthesis of undoped and Sr-doped K0.5Na0.5NbO3

The solid-state synthesis of undoped K0.5Na0.5NbO3 (KNN) and KNN doped with 1, 2 and 6 mol% Sr, from potassium, sodium and strontium carbonates with niobium pentoxide, was studied using thermal analysis and in situ high-temperature X-ray diffraction (HT-XRD). The thermogravimetry and the differential thermal analyses with evolved-gas analyses showed that the carbonates, which were previously reacted with the moisture in the air to form hydrogen carbonates, partly decomposed when heated to 200 A degrees C. In the temperature interval where the reaction was observed, i.e., between 200 and 750 A degrees C, all the samples exhibited the main mass loss in two steps. The first step starts at around 400 A degrees C and finishes at 540 A degrees C, and the second step has an onset at 540 A degrees C and finishes with the end of the reaction between 630 and 675 A degrees C, depending on the particle size distribution of the Nb2O5 precursor. According to the HT-XRD analysis, the perovskite phase is formed at 450 A degrees C for all the samples, regardless of the Sr content. The formation of a polyniobate phase with a tetragonal tungsten bronze structure was detected by HT-XRD in the KNN with the largest amount of Sr dopant, i.e., 6 mol% of Sr, at 600 A degrees C.