Thermodynamic stability of lead-free alkali niobate and tantalate perovskites

Lead-free niobates and tantalates currently form some of the most promising groups of ferroelectrics, piezoelectrics and related materials, with important applications for the next generation of lead-free sensors, actuators and microelectromechanical systems (MEMs). In view of their importance, the enthalpies of formation from binary oxide components at 25 °C, measured by high temperature oxide melt solution calorimetry of a set of alkali tantalates and niobates with perovskite-like structures, LiTaO₃, LiNbO₃, NaTaO₃, NaNbO₃ and KNbO₃, are reported to be -93.74 ± 1.77, -93.44 ± 1.48, -147.35 ± 2.46, -141.63 ± 2.27 and -207.12 ± 1.74 kJ mol^-1 for LiTaO₃, LiNbO₃, NaTaO₃, NaNbO₃ and KNbO₃, respectively. The surface energies of nanocrystalline perovskites of these alkali tantalates and niobates were experimentally determined for the first time by calorimetry. The energies of the hydrated surface are 1.04 ± 0.34, 1.21 ± 0.78, 1.58 ± 0.29, 2.16 ± 0.57 and 2.95 ± 0.59 J m^-2 for LiTaO₃, LiNbO₃, NaTaO₃, NaNbO₃ and KNbO₃, respectively. The stability of the lead-free perovskites of I-V type is discussed based on their tolerance factor and acid-base chemistry. The formation enthalpy becomes more exothermic (higher thermodynamic stability) and the surface energy increases (greater destabilization for a given particle size) with the increase in the ionic radius of the A-site cations (Li, Na and K) and with increase in the tolerance factor. These correlations provide key insights into how lead-free niobates and tantalates behave during synthesis and processing; i.e. they explain, for example, why KNbO₃ and KTaO₃ nanoparticles are thermodynamically more reactive than their Li and Na counterparts. This understanding will facilitate the development of optimized processing techniques and applications.