Phase Formation in the System Na 2 O · Al 2 O 3 ‐CaO · A1 2 O 3 ‐Al 2 O 3 at 1200°C in Air

Phase relations in the system Na 2 O · Al 2 O 3 ‐CaO· Al 2 O 3 ‐Al 2 O 3 at 1200°C in air were determined using the quenching method and high‐temperature X‐ray diffraction. The compound 2Na 2 O · 3CaO · 5Al 2 O 3 , known from the literature, was reformulated as Na 2 O · CaO · 2Al 2 O 3 . A new compound with the probable composition Na 2 O · 3CaO · 8Al 2 O 3 was found. Cell parameters of both compounds were determined. The compound Na 2 O · CaO‐2Al 2 O 3 is tetragonal with a = 1.04348(24) and c = 0.72539(31) nm; it forms solid solutions with Na 2 O · Al 2 O 3 up to 38 mol% Na 2 O at 1200°C. The compound Na 2 O · 3CaO · 8Al 2 O 3 is hexagonal with) a = 0.98436(4) and c = 0.69415(4) nm. The compound CaO · 6Al 2 O 3 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 β and β ”‐Al 2 O 3 make it very difficult to determine equilibrium phase relations in the high‐Al 2 O 3 part of the diagram. Conclusions as to lifetime processes in high‐pressure sodium discharge lamps can be drawn from the phase diagram.