Pyrochlore and Perovskite Potassium Tantalate: Enthalpies of Formation and Phase Transformation

Alkali niobates and tantalates are currently important lead-free functional oxides. The formation and decomposition energetics of potassium tantalum oxide compounds (K₂O-Ta₂O₅) were measured by high-temperature oxide melt solution calorimetry. The enthalpies of formation from oxides of KTaO₃ perovskite and defect pyrochlores with K/Ta ratio of less than 1 stoichiometry - K_0.873Ta_2.226O₆, K_1.128Ta_2.175O₆, and K_1.291Ta_2.142O₆ - were experimentally determined, and the values are (-203.63±2.92) kJmol^-1 for KTaO₃ perovskite, and (-339.54±5.03) kJmol^-1, (-369.71±4.84) kJmol^-1, and (-364.78±4.24) kJmol^-1, respectively, for non-stoichiometric pyrochlores. That of stoichiometric defect K₂Ta₂O₆ pyrochlore, by extrapolation, is (-409.87±6.89) kJmol^-1. Thus, the enthalpy of the stoichiometric pyrochlore and perovskite at K/Ta=1 stoichiometry are equal in energy within experimental error. By providing data on the thermodynamic stability of each phase, this work supplies knowledge on the phase-formation process and phase stability within the K₂O-Ta₂O₅ system, thus assisting in the synthesis of materials with reproducible properties based on controlled processing. Additionally, the relation of stoichiometric and non-stoichiometric pyrochlore with perovskite structure in potassium tantalum oxide system is discussed.