Phase formation in the system Na2O.Al2O3-CaO.Al2O3-Al2O3 at 1200 ° C in air

Phase relations in the system Na2O· Al2O3-CaO· Al2O3-Al2O3 at 1200°C in air were determined using the quenching method and high-temperature X-ray diffraction. The compound 2Na2O · 3CaO · 5Al2O3, known from the literature, was reformulated as Na2O · CaO · 2Al2O3. A new compound with the probable composition Na2O · 3CaO · 8Al2O3 was found. Cell parameters of both compounds were determined. The compound Na2O · CaO-2Al2O3 is tetragonal with a = 1.04348(24) and c= 0.72539(31) nm; it forms solid solutions with Na2O · Al2O3 up to 38 mol% Na2O at 1200°C. The compound Na2O · 3CaO · 8Al2O3 is hexagonal with) a= 0.98436(4) and c= 0.69415(4) nm. The compound CaO · 6Al2O3 is not initially formed from oxide components at 1200°C but behaves as an equilibrium phase when it is formed separately at higher temperatures. The very slow transformation kinetics between ß and ß"-Al2O3 make it very difficult to determine equilibrium phase relations in the high-Al2O3 part of the diagram. Conclusions as to lifetime processes in high-pressure sodium discharge lamps can be drawn from the phase diagram