Thermodynamic activity of Na2O in Na β-alumina

β-alumina samples prepared by sintering Na2O–xAl2O3 (5.5 < x < 19) at T > 1900 K were subjected to XRD at room temperature. For all compositions from 5.5 < x < 19, both β″-alumina and β-alumina were observed and α-, β″-, and β-alumina could be found at x > 8.5. The proportion of β″-alumina is observed to reach a maximum value at x = 8.5, which is near the β/α phase boundary and decreases to lower values for both x < 8.5 and x > 8.5. The c parameter for β-alumina is observed to increase from 22.45 Å at x = 5.5 to 22.75 Å at x = 8.5 and then is almost constant for x > 8.5. Emf measurements were made at higher temperature (973 K) using the following galvanic cell: (−) Na (reference) | Pt | Na β-alumina | Pt | Na2O–xAl2O3 (5.5 < x < 19) (+). Calculations from emf data show that the activity of Na2O in the Na2O–xAl2O3 binary system, decreases with increasing value of x for 5.5 < x < 8.5 and is constant for 8.5 < x < 19. Both lattice parameter and emf data are in agreement with the phase diagram proposed by Y. Le Cars and coworkers, which shows only a single phase β-alumina region within the composition range 5.5 < x < 8.5 and a two-phase β- and α-region from x > 8.5. These results suggest that β″-alumina is not a separate phase and provide new edvidence that β″-alumina can be considered as a region of stacking disorder within the β-alumina phase, as reported by D. Gratias and coworkers. Although XRD can identify the presence of both structures, thermodynamic evidence is provided to suggest that the microdomains of β″-alumina and β-alumina structures are part of one single β-alumina phase, formed syntaxially as long-range stacking disorder.