Abstract
The phase equilibria relationships in the 
system were established in air using DTA and melting point data obtained from compositions previously equilibrated by solid‐state reactions. In addition to the previously reported rapid, reversible 
polymorphic inversion at 1305°C (1), a second rapid reversible transition 
was found to occur at 1410°C prior to melting at 1620°C. 
is soluble in: (a) 
to a maximum of 1 or 2 m/o (mole per cent), (b) 
to a maximum of 21 m/o at 1075°C with 12 m/o sufficient to stabilize the β‐phase to room temperature, and (c) 
to a maximum of 31 m/o at 1125°C. A eutectoid reaction exists at 1075°C where 
solid solution, 
solid solution, and a ternary compound 
are in equilibrium. 
is soluble in 
to a maximum of about 3 m/o at 1050°C. The luminescence of Eu2+‐activated 
is very dependent on composition and the polymorph present at the reaction temperature. Small additions of 
to all 
polymorphs rapidly quench the luminescence. Although both high‐temperature forms of 
revert to the α‐modification upon cooling to room temperature, phosphors prepared by solid‐state reaction in the temperature stability range of 
possess superior brightness levels.