Thermochemistry of BaSm₂O₄ and thermodynamic assessment of the BaO–Sm₂O₃ system

Abstract

The BaO–Sm2O3 system is of interest for the optimization of synthesis of electroceramics. The only systematic experimental study of phase equilibria in the system was performed more than 40 years ago. The reported experimental values of the enthalpy of formation of BaSm2O4 are in conflict, and the reported compound Ba3Sm4O9 has never been confirmed. In this work we synthesized BaSm2O4 by solid‐state reaction and determined its heat capacity, enthalpy of formation, and phase transitions by differential scanning calorimetry, high‐temperature oxide melt solution calorimetry and ultra‐high‐temperature differential thermal analysis, respectively. We confirmed the existence of Ba3Sm4O9 and its apparent stability from 1873 to 2273 K by X‐ray diffraction on quenched laser‐melted samples but were not able to obtain single‐phase material for calorimetric measurements. The CALPHAD method was used to assess phase equilibria in the BaO–Sm2O3 system, using both available literature data and our new measurements. A self‐consistent thermodynamic database and the calculated phase diagram of the BaO–Sm2O3 system are provided. This work can be used to model and thus to understand the relationships among composition, temperature, and microstructure for multicomponent systems with BaO and Sm2O3.

Thermodynamic assessment of BaO–Ln2O3 (Ln = La, Pr, Eu, Gd, Er) systems

“…However, there are significant discrepancies in reported values. [7][8][9][10][11] As shown in Figure 1 and Table 1, the Gibbs energy of formation and the enthalpy of formation of BaSm 2 O 4 from the binary oxides determined by Xing et al 7 differ by a factor of two from those of Subasri and Sreedharan, 8 and even more from the results of Uspenskaya et al 9…”

“…20 Those of BaO in liquid and rock salt structures as well as in the hypothetical metastable phases were taken from our previous assessment. 11 The liquid, X SS , H SS , A SS , B SS , and BaO-based solid solution phases as well as the BaLa 2 O 4 or BaGd 2 O 4 and Ba 3 Gd 4 O 9 compounds were taken into account during the thermodynamic modeling and calculations on the BaO-La 2 O 3 and BaO-Gd 2 O 3 systems. As part of an ongoing program as well as for compatibility, the solution phases in the systems are thermodynamically described by the same substitutional solution model as in our previous work.…”

“…As part of an ongoing program as well as for compatibility, the solution phases in the systems are thermodynamically described by the same substitutional solution model as in our previous work. 11,12 Thus, the Gibbs energy for each solution phase was expressed by Redlich-Kister polynomials 23 as Equation (7).…”

“…These values were taken from the literature assessment. 11,20 The interaction parameters a 0 , b 0 , a 1 , and b 1 were to be optimized. Due to the very limited solubility of rare earth oxides in solid BaO, 6 the BaO-based solid solution was treated as the pure oxide.…”

“…The final thermodynamic database for the BaO-La 2 O 3 and BaO-Gd 2 O 3 system is given in Table 3. These parameters together with the Gibbs energies of BaO, La 2 O 3 , and Gd 2 O 3 from the literature 11,20 allow calculations of the thermodynamic properties and the phase diagrams of the BaO-La 2 O 3 and BaO-Gd 2 O 3 systems.…”