Zlotnik Sebastian, Sahu Sulata K, Navrotsky Alexandra, Vilarinho Paula M
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.
Chemistry. 2015 Mar 23;21(13):5231-7. doi: 10.1002/chem.201405666. Epub 2015 Feb 16.
Alkali niobates and tantalates are currently important lead-free functional oxides. The formation and decomposition energetics of potassium tantalum oxide compounds (K2 O-Ta2 O5 ) 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-K0.873 Ta2.226 O6 , K1.128 Ta2.175 O6 , and K1.291 Ta2.142 O6 -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 K2 Ta2 O6 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 K2 O-Ta2 O5 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.
碱金属铌酸盐和钽酸盐是目前重要的无铅功能氧化物。通过高温氧化物熔体溶液量热法测量了钽酸钾化合物（K₂O - Ta₂O₅）的形成和分解能量。实验测定了KTaO₃钙钛矿以及K/Ta比小于1化学计量比的缺陷烧绿石——K₀.₈₇₃Ta₂.₂₂₆O₆、K₁.₁₂₈Ta₂.₁₇₅O₆和K₁.₂₉₁Ta₂.₁₄₂O₆——从氧化物形成时的焓，其值对于KTaO₃钙钛矿为(-203.63 ± 2.92) kJ·mol⁻¹，对于非化学计量比的烧绿石分别为(-339.54 ± 5.03) kJ·mol⁻¹、(-369.71 ± 4.84) kJ·mol⁻¹和(-364.78 ± 4.24) kJ·mol⁻¹。通过外推法得到化学计量比缺陷K₂Ta₂O₆烧绿石的焓为(-409.87 ± 6.89) kJ·mol⁻¹。因此，在实验误差范围内，化学计量比的烧绿石和钙钛矿在K/Ta = 1化学计量比时的焓在能量上相等。通过提供各相热力学稳定性的数据，这项工作提供了关于K₂O - Ta₂O₅体系内相形成过程和相稳定性的知识，从而有助于基于可控加工合成具有可重复性能的材料。此外，还讨论了钽酸钾体系中化学计量比和非化学计量比烧绿石与钙钛矿结构的关系。
