Thermodynamic stability of lead-free alkali niobate and tantalate perovskites

Abstract

Lead free niobates and tantalates currently form one 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.kJ/mol 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 surface energy increases (greater destabilization for a given particle size) with increase in ionic radius of the A-site cation (Li, Na and K) and with increasing 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 will be thermodynamically more reactive than their Li and Na counterparts. This understanding will facilitate the development of optimized processing techniques and applications.

Chemical stability of KNbO3, NaNbO3, and K0.5Na0.5NbO3 in aqueous medium

“…Tolerance factor, t, of KN and NN were reported as 1.03 and 0.94, respectively, by Sahu et al 51 and 0.98 and 0.85, respectively, by Li et al 57 It was stated that the high tolerance factor reflects high surface energy of perovskite structure as well. 51,58,59 In conclusion, these studies support that by having a slightly higher tolerance factor, KN exhibits less stability in the aqueous environment than NN. 51 Hence, the structure potentially tends to lower the surface energy by simply leaching A site cation to form less soluble and stableB-rich oxide-like surface.…”

Formation and Dehydration Enthalpy of Potassium Hexaniobate

“…The enthalpies of formation of K 4 Nb 6 O 17 ·3H 2 O and K 4 Nb 6 O 17 from component oxides and elements were obtained using suitable thermodynamic cycles (Table ). The enthalpy for drop solution of K 2 O and Nb 2 O 5 in 3Na 2 O·4MoO 3 at 702°C was taken from our previously published work . For calculation of enthalpies of formation from the elements, the enthalpies of formation of binary oxides from Robie et al were used.…”

“…Among various photocatalysts, TiO 2 has been widely researched due to its high activity, stability, and high photoinduction under ultraviolet (UV) light . In recent years, a variety of metal oxides, including titanates, niobates, and tantalates with wide band gaps, have also been found to exhibit such properties …”

“…10 In recent years, a variety of metal oxides, including titanates, niobates, and tantalates with wide band gaps, have also been found to exhibit such properties. [11][12][13][14] K 4 Nb 6 O 17 has recently been a focus of research attention. Its structure consists of layers of (NbO 6 ) 7À octahedra linked by corner/edge oxygen atoms.…”

Alkali Niobate and Tantalate Perovskites as Alternative Photocatalysts

“…Sets of alkali tantalate (LiTaO 3 , NaTaO 3 , KTaO 3 ) and niobate (LiNbO 3 , NaNbO 3 , KNbO 3 ) powders with perovskite‐like structures were prepared by a solid‐state reaction; a more detailed procedure is described elsewhere . It is known that the PCA of metal‐oxide semiconductors depends on a number of parameters, such as surface area, particle size, particle‐shape anisotropy, crystallinity, local structure, light absorption, and diffusion rates of charge carriers; therefore, to reduce the number of variables only single‐phase catalysts with a certain degree of crystallinity were examined herein.…”