Heat capacities, standard entropies and Gibbs energies of Sr-, Rb- and Cs-substituted barium aluminotitanate hollandites. (February 2016) - Wu, Lili
Highlights: Heat capacities of Sr-, Rb- and Cs-hollandite were measured from T = (2 to 300) K. Quantum Design Physical Property Measurement System (PPMS) was used. Heat capacities were fitted to equations in three temperature ranges. Standard entropies and Gibbs energies were calculated. This work is relevant to nuclear waste containment using hollandite ceramics. Abstract: Heat capacities of Sr-, Rb-, and Cs-hollandite with the compositions Ba1.14 Sr0.10 Al2.38 Ti5.59 O16, Ba1.17 Rb0.19 Al2.46 Ti5.53 O16, and Ba1.18 Cs0.21 Al2.44 Ti5.53 O16 were measured from T = (2 to 300) K using a Quantum Design Physical Property Measurement System (PPMS). From the heat capacity results, the following thermodynamic parameters have been determined. The characteristic Debye temperatures Θ D over the temperature range (30 to 300) K of Sr-, Rb-, and Cs-hollandite are T = (178.2, 189.7, and 189.2) K, respectively, and their standard entropies at T = 298.15 K are (413.9 ± 8.3), (415.1 ± 8.3), and (419.6 ± 8.4) J · K −1 · mol −1 . Combined with previously reported formation enthalpies, their corresponding Gibbs energies of formation from oxides (Δf G ox °) are (−194.9 ± 11.4), (−195.0 ± 12.8), and (−201.1 ± 12.8) kJ · mol −1, and those from elements (Δf G el °) are (−7694.6 ± 12.5), (−7697.0 ± 13.9), and (−7697.1 ± 13.9) kJ · mol −1 at T = 298.15 K. The similarities among the obtained Δf G ox ° values suggest that the three substituted hollandites have similar thermodynamic stabilities at standard conditions, which is in agreement with the ease of Cs–Rb–Sr substitutions in the hollandite structure.