Abstract
The phase diagram for the system 
has been determined in detail. Although no intermediate compounds were found in the system, two extensive solid solution regions having the 
and 
structures were found to exist. 
is a newly discovered polymorph stable above 1390°C. The divalent tin‐activated luminescence of various phases in the system was studied. The most efficient phosphor was 
which had properties similar to those of 
. The performance of 
in fluorescent lamps was evaluated.