Pyrochlore and Perovskite Potassium Tantalate: Enthalpies of Formation and Phase Transformation |
| |
| Authors: | Sebastian Zlotnik Dr Sulata K Sahu Prof Alexandra Navrotsky Prof Paula M Vilarinho |
| |
| Affiliation: | 1. Department of Materials and Ceramic Engineering, Centre for Research in Ceramics and Composite Materials, CICECO, University of Aveiro, 3810‐193 Aveiro (Portugal), Fax: (+351) 234 370204;2. Peter?A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, CA 95616 (USA) |
| |
| Abstract: | 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—K0.873Ta2.226O6, K1.128Ta2.175O6, and K1.291Ta2.142O6—were experimentally determined, and the values are (?203.63±2.92) kJ mol?1 for KTaO3 perovskite, and (?339.54±5.03) kJ mol?1, (?369.71±4.84) kJ mol?1, and (?364.78±4.24) kJ mol?1, respectively, for non‐stoichiometric pyrochlores. That of stoichiometric defect K2Ta2O6 pyrochlore, by extrapolation, is (?409.87±6.89) kJ mol?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. |
| |
| Keywords: | calorimetry perovskite phases potassium tantalum thermochemistry |
|
|
