Thermodynamic properties of complex oxides in the Sm-Ba-Cu-O system
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I.
INTRODUCTION
THE knowledge of thermodynamic properties, including phase relations, phase stability, decomposition pressure of a complex oxide, etc., becomes more and more important in the R-Ba-Cu-O system (R: most of the rare earths) due to the superconducting properties of RBa2Cu3O6.51d.[1] These thermodynamic data can be obtained by performing electromotive force (EMF) measurements on the solid-state electrochemical cell[2] and by the calorimetric method. Numerous studies by several groups have focused on the YBa-Cu-O system.[3–11] The Sm-Ba-Cu-O system is also a high Tc superconducting one with a Tc of about 90 K.[12,13] Phase relations and thermodynamic data related to this system are very limited. The only EMF measurements on Sm2CuO4 were made by Tretyakov et al.[14] and Idemoto et al.[15] using a stabilized zirconia as an electrolyte. In the previous article,[16] subsolidus phase relations in the Sm-BaCu-O system were determined at 950 7C in air. Six compounds, BaCuO2, Sm2CuO4, Sm2BaO4, Sm2BaCuO5, SmBa2Cu3O6.51d, and Sm1.4Ba1.6Cu3O6.71d, were isolated and indexed. The purpose of the present article is to determine the standard Gibbs energies of formation of these complex oxides by performing EMF measurements. Their heat capacities were determined by an adiabatic scanning calorimeter from 2150 7C to 800 7C. The standard enthalpies of formation and standard entropies as well as decomposition pressures of complex oxides were calculated. Particularly, thermodynamic stabilities of member complex oxides were discussed. II.
EXPERIMENTAL
A. Reagent and Sample Preparation Sm2O3 (99.9 pct), BaF2 (AR) (AR: analytical reagent) ZrO2 (AR), BaCO3 (AR), and CuO (AR) were used as start-
XIANRAN XING, Associate Professor, ZHIYU QIAO, Professor, and SHOUKUN WEI, Professor and Member, Chinese Academy of Sciences, are with Department of Physical Chemistry, University of Science and Technology, Beijing 100 083, People’s Republic of China. Manuscript submitted January 4, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
ing materials and baked for dehydration. Cu2O (AR) was reheated in purified argon for 4 hours at 1323 K to remove the excess oxygen. Six complex oxides, BaCuO2, Sm2CuO4, Sm2BaO4, Sm2BaCuO5, SmBa2Cu3O6.51d, and Sm1.4Ba1.6Cu3O6.71d, were synthesized by the solid-state reaction method.[3] BaZrO3 was synthesized by mixing equimolar amounts of BaCO3 and ZrO2, compacted into a small cylindric pellet and sintered at 1273 K for 48 hours in air atmosphere. X-ray diffraction patterns of the products agree well with the JCPDS card.[17] Pellets of optical quality CaF2 single crystal, approximately 10-mm diameter and 2-mm thickness, were employed as electrolyte. In cell (I), a magnesia-stabilized zirconia tube, with one end closed and dimension 12-mm o.d. and 1.5-mm wall thickness, was used as the solid electrolyte to prevent the two electrodes from interfering through the atmosphere. The arrangement of cell (I) was demonstrated in Figure 1(a) and described in detail in Reference 18. B. Electrode Test and reference elect
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