Thermodynamic properties of SrAl12O19 and SrAl4O7

Strontium aluminates are important compounds with interesting properties such as long-duration phosphorescence and elastico-deformation luminescence. They have potential application in flexible light emitting panels. Since there are serious discrepancies in available thermodynamic data for these compounds, a redetermination of their Gibbs energies of formation was undertaken using solid-state electrochemical cells incorporating single-crystal SrF2 as the electrolyte in the temperature range from 1000 to 1300 K. However, the measurements were restricted to SrAl12O19 and SrAl4O7 because of the formation of strontium oxyfluoride phase between SrAl2O4 and SrF2. For the reactions, SrO + 6 Al2O3 → SrAl12O19, ΔG o/J mol−1 (± 280) = −83386 − 25.744 (T/K), and SrO + 2Al2O3 → SrAl4O7, ΔG o/J mol−1 (± 240) = −80187 − 25.376 (T/K). The high entropy of SrAl4O7 and SrAl12O19 can be partly related to their complex structures. The results of this study are consistent with calorimetric data on enthalpy of formation of other Sr-rich aluminates and indicate only marginal stability for SrAl4O7 relative to its neighbours, SrAl12O19 and SrAl2O4. The thermodynamic data explain the difficulty in direct synthesis of phase pure SrAl4O7 and the formation of SrAl2O4 as the initial ternary phase when reacting SrO and Al2O3 or crystallizing from amorphous state, irrespective of composition.