Synthesis of Sodium Potassium Niobate: A Diffusion Couples Study

Abstract

The sequence of reactions during the synthesis of sodium niobate, potassium niobate, and sodium potassium niobate from alkaline carbonates and niobium oxide has been studied by diffusion couples in the temperature range between 500° and 700°C for up to 48 h. The reactions proceed by coupled diffusion of alkaline and oxygen ions into niobium oxide. The first phase to form at the interface Na2CO3/Nb2O5 is Na2Nb4O11 at 500°C. The perovskite phase forms only after heating at 700°C at the boundary between Na2Nb4O11 and Na2CO3. In the K2CO3/Nb2O5 diffusion couple, the sequence of phases after heating at 600°C is Nb2O5/K6Nb10.88O30/K4Nb6O17/KNbO3/K2CO3. In the (K2CO3+Na2CO3)/Nb2O5 diffusion couple the (K,Na)NbO3 solid solution forms via the intermediate phase (K,Na)2Nb4O11 at 600°C. The order of magnitude of the parabolic reaction rate constant for the diffusion‐controlled reaction at 600°C is about 10−15 m2/s for the (K2CO3+Na2CO3)/Nb2O5 and the K2CO3/Nb2O5 systems, which is about one order of magnitude less than that for Na2CO3/Nb2O5 (10−14 m2/s). The reaction rate in the ternary system is determined by the diffusion of the slower species, i.e., the potassium ions.